Observant hikers in the Sonoran Desert soon notice grasshoppers jumping near their feet or flitting between bushes and cacti. These are among the most frequently encountered and easily observed of the Sonoran Desert insects in the late summer.
Grasshoppers have a fairly simple body design. The rounded head capsule contains the compound eyes, chewing mouth parts, and the short thread-like antennae, which are always shorter than the body (hence the name “short-horned” grasshoppers, in contrast to another suborder, the katydids or “long-horned” grasshoppers). The middle thoracic segments and part of the abdomen are covered by a shield-like pronotum that extends from the first thoracic segment. The forewings are leathery and not used for flight. Instead they protect the delicate hind wings, which are folded accordion-like beneath the forewings until they are unfolded for flight. However, all immature stages and the adults of many species lack wings altogether and cannot fly.
The most noticeable feature of grasshoppers is their long, jumping hind legs, which enable them to leap well over 20 times their body length (imagine a 6-foot tall person jumping 120 feet!). However, while the powerful jumping muscles of the hind legs provide the force necessary for leaping, they cannot propel the grasshopper in these impressive leaps unaided. Most of the kinetic energy to do this comes not from the muscles, but from the semilunar crescent located in the knee of the hind leg. This crescent-shaped organ is made of elastic fibers that store energy in preparation for a jump; they release this energy explosively, propelling the grasshopper forward many times its body length.
Grasshoppers develop through incomplete metamorphosis. The nymphs appear similar to the adults except that they lack wings and have incomplete reproductive organs. The number of instars (larval stages between molts) through which a grasshopper develops before reaching adulthood is fixed in some species (typically 4 to 6). In others, it depends on growing conditions: the better the conditions, the fewer the immature stages.
Life History and Ecology
Most Sonoran Desert short-horned grasshoppers spend the winter months in the soil as eggs. These are laid in clutches from just a few to well over a hundred eggs, depending on the species. The eggs are enclosed in an egg pod made of a frothy material that protects them from parasites, desiccation and mechanical hazards. Grasshoppers hatch during 2 separate seasons in the Sonoran Desert. The smaller winter cohort (all members of a population hatching at about the same time) emerges from the egg stage after the winter rains, maturing to adults in April to May. The much larger summer cohort emerges after the monsoon rains, leading to a peak in adult abundance from late August to early October. The number of generations per year for many species depends largely on the duration and quantity of the summer monsoons. For example, the pallid-winged grasshopper (Trimerotropis pallidipennis) can produce two or three generations a year in a good rainy season, but only one or none at all in dry years.
Male grasshoppers attract females both visually and acoustically. Males of banded-winged grasshoppers (subfamily Oedipodinae) can typically be seen in the summer taking short flights, flashing their brightly-colored wings, snapping them together, or both, producing a distinct sound. These short flight noises are called crepitation; the sounds are usually species-specific. Males also attract females by stridulation (scraping the hind femora against the forewing); this too produces species-specific mating sounds. Not all sounds produced by grasshoppers function solely to attract females, however. While hiking in the Sonoran Desert, one is likely to hear short bursts of clicks emanating from a creosote bush. This incessant clicking comes from a male desert clicker (Ligurotettix coquilletti); it spends much of its life on a single creosote bush claimed as a territory, stridulating to warn away other males as well as to attract females.
|Suborder: Caelifera (short-horned grasshoppers)|
|Subfamilies and species discussed:|
|Acridinae (Slant-faced grasshoppers)|
|Cyrtacanthacridinae (spur-throated grasshoppers)|
|Schistocerca nitens (gray bird grasshopper)|
|Schistocerca shoshone (green bird grasshopper)|
|Gomphocerinae (tooth-legged grasshoppers)|
|Bootettix argentatus (creosote bush grasshopper)|
|Ligurotettix coquilletti (desert clicker)|
|Dactylotum variegatum (harlequin or rainbow grasshopper)|
|Oedipodinae (banded-winged grasshoppers)|
|Trimerotropis pallidipennis (pallid-winged grasshopper)|
|Taeniopoda eques (horse lubber)|
|Brachystola magna (plains lubber)|
|Suborder: Ensifera (long-horned grasshoppers)|
|Insara covilleae (creosote bush katydid)|
About 70 percent of herbivorous insects eat only one or a few species or genera of plants. In contrast, grasshoppers are generalist feeders, eating plants from an extremely broad range of families. Grasshoppers tend to grow better and produce more offspring when their diet consists of a mixture of plants. But different species go about this in very different ways. The gray bird grasshopper (Schistocerca nitens) and the green bird grasshopper (Schistocerca shoshone) are cryptically colored, avoiding predators by spending most of the day on a single host plant. (Bird grasshoppers are so-named because they are among the largest of the Sonoran grasshoppers, with the females reaching 6.5 cm [2½ inches] in length.) They obtain the necessary dietary mixtures after feeding for a long period on one host species, by eventually shifting to another. In contrast, the horse lubber (Taeniopoda eques) and the harlequin or rainbow grasshopper (Dactylotum variegatum) also eat a wide variety of plants, but switch frequently, taking a nibble here and a nibble there.
Not all grasshoppers are generalist plant-eaters, however. The creosote bush grasshopper (Bootettix argentatus) is the only one among the more than 8000 species of grasshoppers worldwide that eats a single species of plant—the creosote bush. Hiking in the desert scrub one is likely to come upon another exception, a large (8 cm [3¼ inches]), heavy-bodied, flightless grasshopper, the plains lubber (Brachystola magna). This short-winged grasshopper is a generalist herbivore, yet it has recently been shown to be predacious as well, pouncing on and eating other grasshoppers and insects.
Grasshoppers employ a wide range of mechanisms to keep from being eaten. The foremost of these is crypsis, matching the background in color or texture. This is most evident in the banded-winged grasshoppers. These ground-dwelling grasshoppers superbly match the color of the soil they live on. In some species, such as the pallid-winged grasshopper, populations living on red soil have a predominantly reddish color, those living on white soil are white, and those on dark or black soil are dark brown or black. These differences can be seen over distances of only a few hundred meters. Species of the genera Achurum and Mermiria in the subfamily Acridinae (commonly called the slant-faced grasshoppers because their faces are positioned obliquely to the rest of the body) live and feed on grasses. They are long and slender and typically have bands running the length of the body, mimicking grass stalks. The casual observer will be hard-pressed to notice one of these insects clinging to a stalk of grass.
The creosote bush grasshopper is another excellent example of crypsis. This species eats only the leaves of the creosote bush and spends all its time among them. It is olive green, with shiny, pearly spots mimicking the green leaves with their shiny, oily secretions. The creosote bush katydid (Insara covilleae), in a separate suborder of long-horned grasshoppers, also lives solely on creosote bushes; it too is olive green with pearly, shiny patches—a fine example of convergent evolution.
Banded-winged grasshoppers take a different approach to escaping predators. Their hind wings are often brightly colored with red, orange, yellow, or white bands, in sharp contrast to the often drab brown of the forewings. When startled by a predator (or hiker), they take to the air. The predator focuses attention on the brightly colored and flashy hind wing, only to have the grasshopper disappear from sight when it folds its wings, lands, and again cryptically blends into the background.
Rather than hiding, some grasshoppers actually advertise their presence to predators. A hiker in the Sonoran Desert is likely to come upon the horse lubber (Taeniopoda eques), a large (6.5 cm [2½ inches]), black heavy-bodied grasshopper with yellow or orange stripes and antennae, greenish veins on the forewings, and pinkish-red hind wings. The harlequin or rainbow grasshopper (Dactylotum variegatum) is smaller (3.5 cm [1¼ inches]), short-winged, and black with bright blue, red, yellow, and white markings. These species sequester toxins from the plants they eat, making them unpalatable for most predators. Their aposematic (warning) coloration informs potential predators that they are poisonous—stay away!
Katydid or Grasshopper?
“It’s green. It must be a Katydid!”
Although it’s true that katydids are often green, there are some other ways to tell them apart from grasshoppers. For one thing, their appearance is different. Katydids have long antennae and sword-like ovipositors; grasshoppers have short antennae and blunt ovipositors. (The ovipositor is the egg-laying structure at the hind end of the abdomen of the female.) Another difference is in their egg-laying behavior. Katydids lay their eggs in plants whereas grasshoppers lay theirs in the ground.
In biology, success is measured by the number of genes an individual passes on to the next generation. In order to reproduce, an individual must live long enough to obtain from the environment sufficient raw materials with which to replicate its genes and give them a new start in its offspring. The survival of herbivores, animals whose diet consists of plant materials, is constantly threatened by an omnipresent community of predators, which obtain their energy from eating animal protein.
Many vertebrate predators, particularly birds, like to eat insects, so natural selection has favored various schemes that help insects avoid being devoured. Stick and leaf insects belong to the family Phasmatidae, a group of predominantly tropical plant-eating insects closely related to cockroaches, grasshoppers, crickets, and mantids, and they are survivalists extraordinaire. Usually long and slender—some species grow up to 30 centimeters (12 inches) in length— walkingsticks bear remarkable resemblance in both structure and color to twigs and leaves of the woody plants they eat. Many of the stick mimics are wingless, but some have added “leaves” to their twig disguises in the form of shortened wings and elaborate legs that look like foliage.
The external skeletons of a number of these arthropods have spines that resemble the thorns of their host plants, and body segments frequently duplicate the plant’s internodal distance (the space between leaves). The cuticle, or outer covering, may even be structured and colored to approximate nodes and scars. Some species, notably Carausius morosus, are able to change color, like chameleons, to blend into the background.
Such tactics are called crypsis, a group of behaviors which includes camouflage—blending to escape detection—and defensive mimicry—looking like something unpalatable or non-nutritous. Walkingsticks do both and survive quite well. (Very few animals eat sticks.)
If stick insects moved quickly or abruptly, they would betray their almost perfect disguises. So, to enhance their cryptic appearance, walkingsticks move very slowly, if at all, during the day. Most species wisely restrict their activities to nighttime.
Yet, a walkingstick that remained still on a shaking plant would be much more conspicuous than one that moved in concert with the plant. So when a stick insect is disturbed, perhaps by a bird alighting nearby or a slight breeze causing the plant to tremble, it flexes its legs randomly, making its body quiver. This subtle behavior, called quaking, produces small, irregular movements not likely to be noticed by birds and other predators, which are programmed to detect the purposeful, highly coordinated movements of prey.
When crypsis fails, stick insects often invoke secondary defensive behaviors. Insectivorous birds usually give a tentative, investigative peck to any novel object that might be food; initial caution minimizes the possibility of injury to the beak. A pecked walkingstick responds by immediately releasing its hold on the plant and falling to the ground, where it remains motionless for a long time, perhaps the rest of the day. Some species even jump to the ground when pecked.
If grabbed by a predator, many phasmatids become rigid. The attacker may assume that is has found a stick and drop the insect. But what if the predator arrives at a different conclusion and tries to eat the insect?
The majority of walkingsticks have yet another line of defense—glands that release distasteful or noxious chemicals. Some species regurgitate a foul liquid or leak blood from their leg joints. If a predator tastes the liquid or blood before mortally injuring the stick insect, it will likely release it. Even if the predator kills and eats a foul- tasting walkingstick, there is still a biological payoff. The predator will probably remember this unpleasant experience and avoid walkingsticks in the future. The sacrifice of one individual may spare that individual’s offspring and relatives from a similar fate.
Walkingstick eggs, like those of other large insects, may be consumed by the larvae of certain tiny wasps that deposit their eggs on or in insect eggs. Some stick insects, however, have evolved a cryptic countermeasure to this threat, too. Many species produce eggs that resemble seeds, and some walkingsticks that live on only one plant species deposit eggs that look like their host’s seeds. Presumably, seed mimicry makes it difficult for parasitic wasps to distinguish the eggs from the seeds.
Immature walkingsticks possess an extraordinary defensive adaptation called autotomy. If its leg is grabbed by a predator, a nymph can shed the leg from a joint near its body. Better to give up a leg and leave than to hang around and risk your life! This sacrifice is not as extreme as it may seem, for the nymph can regenerate its lost limb within two weeks.
No aspect of walkingstick life, not even mating behavior, has escaped investigation by evolutionary biologists eager to learn about every survival tactic. John Sivinski, a research entomologist in Florida, studied walkingsticks while a graduate student at the University of New Mexico. He had read that phasmatids mate for long periods of time. Diapheromera veliei, a species closely related to D. arizonensis, couples for 3 to 136 hours at one time, and in the extreme, a pair of Anisomorpha buprestoides may remain coupled for as long as 3 weeks.
Sivinski reasoned that because the transfer of sperm should require only a few minutes, protracted copulation must have a function other than fertilization. Following earlier work by Thomas Eisner, an eminent student of insect chemical defense mechanisms, Sivinski studied the predatory behavior of blue jays in the presence of both coupled and unmated walkingsticks to learn if the insects might pool their chemical defenses and hence survive longer together than apart. His research showed no survival advantage for males, but copulating females enjoyed significantly higher survival rates over non-copulating females.
Extended copulation does have a biological advantage for males: greater reproductive success. If a male fails to remain coupled after mating, his mate may immediately seek another consort. Sperm from this subsequent mate is stored along with that of the first, reducing the number of eggs that will be fertilized by the first male. Reduced fertilizations mean fewer offspring, so males that remain paired with a single female for a long time produce more individuals carrying their genes in the next generation than will males that copulate for short periods of time.
Remember the definition of biological success? Survivors must maximize their reproduction; otherwise, survival is biologically meaningless. The 2000 living species of stick insects attest to the biological success of their designs for survival.
Termites are morphologically uncomplicated insects, in contrast with their astonishingly complex social behavior. Superficially termites resemble and are sometimes mistaken for ants, which also exhibit social behavior. Sonoran Desert termites range in size from 4 to 11 mm (J - 7/16 inches) long, not including the wings of alates (the winged reproductive adult forms that appear occasionally, especially during and following rains). Workers are white with small head capsules. Soldiers have enlarged head capsules, and very formidable jaws, or in one of our species, a snout-like structure.
There are over 40 species of termites in 10 genera widely distributed in the Sonoran Desert.
|Families: Kalotermitidae, Hodotermitidae, Rhinotermitidae, Termitidae|
|Sonoran Desert genera:|
|Spanish name: termitos|
This highly successful group of social insects plays an essential ecological role in the decomposition and recycling of a nutritionally poor, highly resistant, but extremely abundant substance: cellulose. Cellulose is a poly-saccharide, that is, a large number of sugar molecules linked together by tight chemical bonds to form a very long, strong chain. Cellulose is the substance that gives plants their structure and is the most abundant organic compound in the world. Wood is mostly cellulose, and so are cotton and all paper products. In the Sonoran Desert, trees, shrubs, grasses, and cactus skeletons are the primary source of cellulose, which represents more than half of all the organic material produced by photosynthesis. Cellulose is durable because it is a physically strong material resistant to mechanical breakdown, but more important, very few organisms produce enzymes that can chemically break it down. Among those that do produce the cellulose break-down enzyme cellulase are fungi and tiny animals called protozoans. Termites do not produce cellulase, but all termites contain protozoans in their guts in a mutually beneficial relationship known as mutualism. Termites grind up the cellulose mechanically by biting off bits and chewing them up; then the protozoans in their guts break down the chewed mass into sugars, which are readily absorbed through the termites’ guts. Both the termites and their protozoans share in the nutritional benefit of these released sugars. Newly hatched termites are first inoculated with these indispensable protozoans by eating the feces of their older brothers and sisters.
The ecological importance of Sonoran Desert termites can best be understood by considering the following question: What would happen if we didn’t have termites in our desert? Well, because our aridity severely limits the abundance and distribution of wood decaying fungi, without termites, we would soon be neck deep in cellulose in the form of mesquite and palo verde wood, dead grasses, cactus skeletons and dung. Eventually, few living plants would be left to produce food for animals because there would be no space for new plant seedlings to establish and no nutrients to sustain their growth. All of the space would be taken up by dry, un-recycled cellulose litter, and all of the nutrients would be tied up in this material and thus unavailable for plants in the soil. Without plants fixing carbon-producing food, most animals would disappear. So, without termites, the whole desert ecosystem as we know it would simply collapse.
Our termites partition the desert’s cellulose into many ecological niches. For example, one drywood termite, Marginitermes hubbardi, feeds primarily on saguaro skeletons, and another very large primitive drywood termite, Pterotermes occidentis, is a specialist on palo verde wood. Gnathamitermes perplexus, the crust-building subterranean desert termite, feeds on grass, fine dry plant parts and the weathered outer surfaces of woody tissues of all kinds. Heterotermes aureus, the lowland subterranean termite, is an important consumer of native woods on the desert floor and also of pine.
In the Sonoran Desert large, well-established termite colonies of many species produce nymphs in the late spring. When these nymphs shed their external skeletons for the last time, fully-formed functional wings unfold from the wing pads, and the resultant individuals are called alates. Alates are reproductively mature males and females ready and eager to start new colonies. They stay in the parent colony until conditions are optimal (usually during or after rain); then they leave the galleries of the colony, surrounded by soldier termites ready to defend their brothers and sisters against ants and other enemies as they depart. The alates then take flight.
The season and time of termite flights depend on the species, but alates from all colonies of a given species in an area fly simultaneously. Just how far alates fly is not known for any species, but it is assumed that the flights of reproductives serve to assure new colonization some distance from the home colony. The simultaneous flights also promote outbreeding by increasing the probability that reproductives from one colony will mate with members of other colonies.
Soon after winged termites alight, they shed their wings by breaking them off at lines of weakness (like perforations in paper) near the point of their attachment to the body. Females may then use a chemical odor called a pheromone to “call ”males. Males attracted to this substance may be accepted or rejected by the female. A rejected male is forced to try his luck elsewhere. The fortunate male who is accepted by a female is permitted to follow her on the ground as she runs quickly about looking for the ideal place to start a new colony. During this “tandem running” phase of courtship, the male remains within touching distance of the female until she finds the “perfect” place (in the ground or in dead wood, again depending on the species of termite) to begin a new colony. The pair then settles into a monogamous relationship and cooperative family rearing.
After mating, the queen lays a few eggs that soon hatch into tiny termite larvae. These are fed and nurtured by both the mother and father until they are large enough to begin foraging for wood and other sources of cellulose, at which time the young termites take over the work of feeding the larvae that have hatched from a second set of eggs. When the parents feed their first batch of offspring, the protozoans required to produce the enzyme needed for cellulose digestion are transferred from the mother’s and father’s stomachs to the larvae, and this protozoan inoculum is all that is required to get a culture going in the offspring so that they too, with the aid of the microbes, can digest their own cellulose.
Termites can accurately be described as “tiny social cockroaches” because they evolved from a common ancestor with wood-dwelling cockroaches, to whom they are very closely related. They first appeared on earth during the age of the dinosaurs, about 100 million years ago. Termites are social in ways not unfamiliar to humans. We live together with others of our kind in complex societies, we divide the many tasks needed to support our communities and we care for our young long after they are born. Termites likewise live in complex societies, have division of labor, and care for their young.
A well-established termite society or colony minimally consists of a king and queen, which are responsible for producing offspring: soldiers, which defend the colony against its enemies; and workers, which collect and process wood or other sources of cellulose and feed the royal couple and the soldiers, which are unable to feed themselves. Workers also care for eggs produced by the queen, and they tend to the young termite larvae that hatch from these eggs. The categories of king, queen, soldiers, and workers in a termite colony are referred to as castes. All of these are wingless; however, after a termite colony reaches a certain size (a few dozen to several thousand individuals, depending on the species), the colony begins to produce nymphs. These nymphs have small pads on their backs that contain developing wings. Regulation of the development of different castes in a termite colony is controlled by chemicals in the colony that are transferred from individual to individual by social feeding called tropholaxis. Exactly how different developmental trajectories are regulated in termite colonies remains an entomological mystery.
Knee Deep in Dung
Termites eat dead plant material and herbivore dung, thereby removing this litter from the surface of the land, permitting sunlight and moisture to reach new growth. On its own, dry cow dung decomposes very slowly. Research conducted in southwestern deserts and desert grasslands by New Mexico State University’s Walt Whitford estimates that without the action of termites, cow pies would smother the land, covering 20 percent of the surface in 50 years.
Hemiptera & Homoptera
Many people refer to anything small that crawls on the ground as a “bug,” and indeed many insects have the word “bug”in their name, such as ladybug and lightningbug (both are actually beetles). The hemiptera, however, are the “true” bugs of the insect world, having distinct features that set them apart from other insect orders. Hemiptera means “halfwing,” in reference to the unique front pair of wings, which are leathery near their base and membranous towards the tips. Most species hold their wings flat over their backs with the two membranous portions overlapping. This combined with a triangular structure called a scutellum (located between the attachment sites of the two front wings) creates an x-shaped pattern on the back of many species. True bugs have slender, beak-like mouthparts that arise from the front of the head, and are usually folded along the ventral surface of the insect except when feeding. While most hemipterans suck the sap from plants, some are predatory, sucking the body fluids of other arthropods or even the blood of vertebrates. All hemipterans undergo incomplete metamorphosis with egg, nymph, and adult stages. Nymphs look very much like diminutive wingless adults.
The Homoptera are close relatives of the Hemiptera and also have piercing-sucking mouthparts. In contrast to the Hemiptera, homopteran mouthparts arise further back on the underside of the head. Those forms that have wings have ones that are uniform in structure, hence their name, Homoptera, meaning “samewing.” Also unlike the Hemiptera, these insects hold their wings roof-like over their backs. All are plant feeders and most have incomplete metamorphosis. Many families within this order have very strange and complex life cycles with both sexual and asexual generations, winged and wingless generations, as well as individuals with much reduced, highly specialized structures.
The Hemiptera and Homoptera are large orders of insects with too many species to cover in this book. Below are examples of a few of the more commonly encountered species.
Probably everyone’s least favorite representatives from this order are the conenose or kissing bugs. These blood-feeding insects in the family Reduviidae are a nuisance during the spring and summer months. We have several species (Triatoma rubida being the most common) whose primary hosts are the wood rat (Neotoma ssp.). They are most active at night during the months of May and June (a dispersal period). They occasionally venture into peoples’ homes, attracted by porch lights and finding entry through holes in window and door screens. Humans are usually bitten while they are sleeping and often don’t feel the actual bites. The next morning, however, victims awake with large, hard, itchy welts. Some unlucky people who have been bitten several times over an extended period become “sensitized”to these bites and actually develop a dangerous allergic reaction. As the insect feeds, it injects a small amount of saliva that contains both an anesthetic and an anticoagulant. It is usually one of these two components to which the victim’s immune system reacts. In Central and South America kissing bugs carry Chagas’ disease (a trypanosome). The bugs defecate while feeding; the victim later scratches the bite and thereby inadvertently rubs the trypanosome (located in the feces) into the wound. Although the species in the southwestern U.S. might be able to carry the desease, their feces are not yet known to contain the parasite, but in rural Sonora below 4500 feet (1500 m) Triatoma dimidiata does carry the disease. One of the more spectacular hemipteran representatives of our area is the giant mesquite bug (Thasus gigas). These beautiful insects can be found feeding on the fresh green stems and pods of mesquite during the early summer months. They are the largest of the true bugs in our area. Adults are brownish-black with orange-red bands on their legs and along the veins of their wings. Each of the two antennae has a round medallion-like segment. Nymphs are a gorgeous red and white and are usually found in clusters feeding together on the mesquite pods. When handled, these insects produce a potent, stinky secretion, but are otherwise harmless. Eggs look like small brown pillows glued in rows along the stems of the mesquite. Nymphs hatch from eggs in the spring, grow to become adults, mate, lay eggs, and then die by the end of summer, leaving new eggs to overwinter until the following spring.
Among the larger and more familiar representatives of this order are the cicadas. These musical insects are prevalent during the summer months and are usually first heard during the hot dry days of June before the monsoons. Most of us are probably familiar with the song, a loud buzzing noise emanating from a tree or shrub. Males sing to attract mates and can be very hard to locate since they usually stop singing as you approach their perch. An observant person is probably more familiar with the brown husks of exoskeleton left by nymphs when they emerge as adults. There are several species of cicadas in the Sonoran Desert region, with one of the most common species at lower elevations being the Apache cicada (Diceroprocta apache). Adults emerge in June, feed on plant sap, mate, and insert their eggs just under the surface of plant stems. These eggs hatch, and the larvae drop to the ground and dig into the soil to feed on the roots of various plants. In their underground homes, nymphs live and feed for 3 or more years before emerging as adults.
The homopteran cochineal has a fascinating natural and cultural history. Cochineal belongs to the Coccoidea, or scale insects, a bizarre group of homopterans so specialized that they often do not look like insects at all. Cochineal, in the family Dactylopiidae, feed on cactus. You may have noticed a white moldy material growing on the prickly pear cacti in your yard or in the desert. Beneath the fluffy white mass reside the small cochineal insects. Females are dark red, wingless, legless, and sessile (attached to the plant and sedentary), and look like tiny grapes. The white material covering the insects is wax that females and nymphs secrete from special abdominal glands. Males are more insect-like and resemble small pink and white gnats, each with two long tails or caudal filaments. Males are most prevalent during the late summer when they emerge and mate. Once the females have mated, they lay eggs that hatch into tiny 6-legged nymphs called crawlers. These migrate to other pads on the cactus or crawl to a pad edge, secrete thin filaments of wax, and wait for the wind to pick them up and carry them to nearby cacti. Crawlers wander until they find suitable spots on cactus pads to settle, feed, and molt. As the tiny insects molt, they lose their legs and become sessile with only their mouthparts firmly tapped into the cactus.
Females, like most other homopterans, have incomplete metamorphosis, but males pupate within a tiny silken cocoon before emerging as adults.
Cochineals’ claim to fame is that their bodies contain a substance called carminic acid that produces a beautiful red dye. Native peoples from the Southwest, Mexico, and South America harvested these insects for the dye. Europeans discovered the dye when conquering the New World and the valuable product made many of them rich. Up to that time, most red dyes came from plant material. Cochineal, however, not only produced a superior red hue, but also withstood the effects of sun and washing much better than did the plant-derived dyes. Cochineal dye fell out of favor with the advent of synthetic dyes developed in the mid 1800s. However, it is making a comeback, as the more subtle hues of natural dyes are regaining popularity. It is also currently being used as a food and pharmaceutical dye, particularly since many of the synthetic food dyes have proven to be toxic. Look for cochineal or carmine on the labels of pink- and red-colored foods and medicines at the grocery store.
Other familiar homopterans are the aphids or plant lice. Most of us have encountered large groups of these little creatures feasting on the fleshy stems of herbaceous plants in the spring and summer. These tiny insects have very interesting and complex life cycles. Most species survive the winter as eggs and then hatch into females in the spring. These new females then begin to reproduce asexually on their host plants—which differ depending on the species of aphid—forming small colonies of clones. In many species, the colony eventually produces winged forms that fly off to a second host plant (sometimes a completely different plant species) and continue to reproduce. When fall approaches, winged forms migrate back to the original species of host plant and produce both males and females which mate and lay the overwintering eggs. An easy-to-recognize species that occurs in our area is the milkweed aphid, Aphis nerii. These beautiful aphids are bright yellow with black legs and antennae; they are found on various species of milkweed in the spring and summer months.
Beetles comprise the largest group of insects on Earth, representing one-quarter of all living organisms and one-third of all animals, with nearly 350,000 species grouped into more than 150 families. Beetles owe their success, in part, to an external skeleton, or exoskeleton, which functions as both skin and skeleton. The outer surface of the exoskeleton may be covered with spines, or hair-like structures, or coated with waxy secretions. These adornments may function as sensory transmitters of environmental information to the nervous system, or serve as additional protection from predators, abrasion, and desiccation.
The beetle head provides the housing for such delicate and primary sensory structures as the eyes and antennae. The antennae are equipped with sophisticated receptors used to detect food, locate egg-laying sites and assess temperature and humidity. Male beetles may have elaborate antennal structures to increase their powers of smell—especially those species that use specific scents, or pheromones, to locate mates. The mandibles are usually conspicuous and are variously modified to cut, grind, and strain foodstuffs, occasionally serving as a means of defense. The thorax is the powerhouse of the beetle body, enclosing an internal battery of muscles used to drive the legs and wings. The forewings of beetles are usually thick and leathery, protecting the delicate flight wings and the circulatory, respiratory, digestive, excretory, and reproductive systems housed within the abdomen.
|Families: Scarabaeidae, Tenebrionidae, Cerambycidae|
|Sonoran Desert species:|
fig beetle (Cotinis mutabilis), pinacate beetle (Eleodes spp.), palo verde root borer (Derobrachus geminatus), cactus longhorn beetle (Moneilema gigas)
escarabajo, mayate, coleóptero, cochinilla (cochineal), mayate verde (fig beetle), torito, longicornio (cactus longhorn beetle)
The stages of complete metamorphosis—egg, larva, pupa, and adult—serve to adapt each beetle species to a dynamic suite of seasonal and ecological conditions. Most beetles have fairly regular life cycles, with one or more generations per year. Beetles either produce eggs singly or by the hundreds, scattering them about haphazardly, or carefully depositing each one on or near suitable larval foodstuffs. Upon hatching, the larva begins its life with a single purpose: to eat. Beetle larvae may scavenge carrion, consume animal excrement, attack roots, mine through leaves or chew their way through wood. The pupa serves as the vessel in which dramatic transformations take place, reconstructing the tissues of a non-reproductive eating machine into a precision breeding instrument that may or may not require food.
The thickened wing covers of small, compact beetles protect them from abrasion and desiccation as they move about through soil, detritus and decomposing plant materials, allowing them to hide, feed, and reproduce among the myriad of niches found in the environment. The cavity beneath the forewings serves as a site for the storage of oxygen in aquatic species, while insulating desert-dwelling species from the heat and minimizing water loss through respiration. In fact, studies with the pinacate beetle, Eleodes armata, have shown that the cavities beneath their forewings may be warmer than the ambient temperature, indicating that they may function as both convective cooling and heat buffering systems.
Receiving little rain and exposed to often wildly oscillating fluctuations in temperature, deserts are home to a rich and dominant beetle fauna whose basic physical features are remarkably similar throughout the world. This parallel evolution is a result of the fact that beetles living in deserts have all adapted both behaviorally and morphologically to cope with the lack of water.
Many species of the beetle family Tenebrionidae are wonderfully adapted to desert life. These wingless and usually black species escape extreme temperatures by remaining buried in the sand during the heat of the day, where temperatures may be significantly lower. The pinacate bettles of the genus Eleodes are also called clown beetles because of their defensive stance—they stand on their heads. This action precedes the release of a foul, oily fluid from repugnatorial glands located at the tip of the abdomen, a defense that repels most predators. However, the grasshopper mouse is not deterred by the beetle’s odiferous offerings—it simply grabs the beetle with its paws, forcing the tip of the abdomen into the sand, and eagerly begins to consume the tastier head and thorax, stopping just short of the ill-tasting defensive glands. A relative of the pinacate beetle, the ironclad beetle (Asbolus verrucosus), however, is not endowed with such a chemical defense system. Rather, when disturbed, this beetle simply collapses to one side and pretends to be dead, an act that may last from several minutes to several hours. The thick, roughened exoskeleton of the ironclad beetle, reminiscent of the impenetrable metallic exterior of battle ships, serves to conserve moisture. Further protecting this beetle from desiccation is a bluish-gray waxy secretion exuded by glands whose ducts are located at the tip of knob-like projections on the forewings. The lighter color produced by the wax also reflects ultraviolet light, helping to keep the body cool. The larvae of both Asbolus and Eleodes resemble mealworms and are sometimes encountered burrowing through the sand where they feed on plant detritus.
Beetles employ a broad array of tactics to fool their enemies, using mimicry, camouflage, and warning coloration. The cactus longhorn beetle, Moneilema gigas, resembles the foul smelling and unrelated Eleodes, both in appearance and behavior. Tests have shown that species of Moneilema behave in a way similar to Eleodes when faced with predators such as lizards, wood rats, and skunks. Although these cactus longhorn beetles can be found on the ground wandering about, their preferred spiny host, the cholla, also affords them a considerable degree of protection. Adult Moneilema feed on the softer, more succulent portions of the cactus. The female lays her eggs at the base of the cactus inside an earthen case. Upon hatching, the larvae bore into the roots and stems of the cactus to feed. Their boring activities above ground are conspicuous, as they expel tar-like excrement and fluid from the wounds created by their feeding activities.
Beetles are a major component of the “F.B.I.”—fungi, bacteria, and insects— the primary agents of decomposition. Without them the planet would soon be covered with dead animals and plants. For example, a dead tree branch presents a number of feeding niches for long horned wood-boring beetles, in terms differently-sized stems and the different layers of tissue, from the bark inward. Furthermore, as a dead branch ages, there is a succession of different beetle species that work in concert to complete the recycling process. The adult palo verde root borer, Derobrachus geminatus, grows to more than 3 inches (76 mm) in length and may commonly be encountered during the early evening hours of summer, when it is often attracted to street lights and well-lit storefronts. Females lay their eggs just below the surface of the ground on the roots of a variety of native and non-native trees. The larvae, which may reach 5 inches (130 mm) in length, bore through the live roots of the host tree, consuming woody tissues. Microorganisms, such as bacteria, yeasts, and fungi located in the intestinal tract of these and other wood- boring beetles assist in the digestion of cellulose, whether the tissue is living or dead. These microorganisms are passed to the larvae from their mothers as they pass through a residue in the ovipositor as eggs. Upon hatching, the larvae immediately consume their own egg shells, which are laden with micro-organisms that are essential for their digestion.
Another conspicuous beetle that plays an active role in the decomposition of plant materials is the green fig beetle, Cotinis mutabilis. The buzzing flight of this scarab, a relative of the nocturnal June bug, fills the days of summer. It is easily distinguished by its matte green back and conspicuously shiny underside. Green fig beetles feed upon soft fruits, but are also encountered around mesquite and desert broom wounds that are oozing sap. Females search for piles of organic matter in which to lay their eggs. The c-shaped grubs, which prefer to crawl on their backs and may grow up to 2 inches (50 mm) in length, are commonly encountered in compost or dung heaps. The pupal cell consists of a hardened shell of soil and larval excrement.
Butterflies are one of the most popular and easily recognized groups of insects. Together with moths, they make up one of the major insect orders or groups—Lepidoptera —which number some 160,000 recognized species worldwide. There are over 250 species of butterflies in the Sonoran Desert.
Lepidoptera comes from Greek words meaning scaled and wing. Butterflies and moths can easily be distinguished from other insects by their wings, which are covered with thousands of tiny overlapping scales, much like tiles on a roof. Each scale is one color, but collectively a butterfly’s color pattern is produced by a complex mixture of differently colored scales. Butterflies are usually large, pretty, and diurnal. They are rarely pests, and consequently are well-liked by humans.
Dispersal and Species Richness
There are a number of factors to account for the rich butterfly diversity in the Sonoran Desert. In general, as one approaches the tropics, species richness increases. Also, a varied topography means a corresponding variety of microclimates, rainfall patterns, plant distributions, and therefore butterfly distributions.
The majority of butterfly species in the Sonoran Desert are rather sedentary, occurring in fairly close proximity to their larval foodplants. But at times, for reasons not fully understood, butterflies wander. Some species move at a particular season, some nearly any time. Some species are true migrants, in that individuals push northward early in the season and southward later. However, an interesting array of taxa are influx species, entering the Sonoran Desert yearly from other deserts, thornscrub habitats, and mountain ranges in northwestern Mexico. The strength, time of onset, and duration of the summer rainy season are thought to be responsible for the intensity of this influx phenomenon. Many of these visitors breed in the Sonoran Desert and comprise a significant or even dominant portion of the summer butterfly fauna. Several influx species, however, are on dead-end missions, there being no suitable plants to serve as larval hosts. The fact that there are both indigenous and influx species of butterflies in the Sonoran Desert accounts for the high number of species, and also delights the butterfly enthusiast.
The life cycle of butterflies is one of the true miracles in nature. Butterfly lives have four distinct stages: egg, caterpillar (or larva), chrysalis (or pupa), and adult. The term describing this series of distinct stages of development is complete metamorphosis, as distinguished from simple or incomplete metamorphosis, in that the animal progresses through life stages which are very similar to each other. (See the “grasshoppers” section for an example of incomplete metamorphosis.)
In most cases, butterflies produce one or more generations (broods) per year. The length of the complete life cycle varies greatly, ranging from weeks to a couple of years or more in desert adapted species. The lifespan of an adult butterfly varies as well, from merely a few days to as long as several months. After mating, butterflies oviposit (lay eggs), either singly or in clusters. Female butterflies typically oviposit on specific groups of related plants that will provide food for the caterpillars. The young caterpillars begin feeding and, because their skins do not expand to accommodate growth, must shed their skins several times. Each stage between molts is called an instar; each instar is larger than the previous one. The final molt produces a pupa, the resting stage during which the animal does not feed but undergoes the amazing transformation into a butterfly. The pupa of many butterflies hangs from a silk button called a cremaster. Other species’ pupae are held upright by a silken girdle. Some are disguised (cryptic), being generally green or brown and resembling leaves, stems, or wood. Others are covered in thornlike tubercles or bumps. Just prior to the emergence of the butterfly, the chrysalis usually changes color. Once free of the chrysalis, the butterfly pumps fluid from its swollen body to its shrunken wings. The newly emerged butterfly then lets its wings dry and harden before taking flight in search of food and a mate.
The Importance of Butterflies
Butterflies are important pollinators. They are also good indicators of the ecological quality of a habitat, as they are important components of the food chain, particularly as larvae. Few butterflies are a serious threat to economically important plants. In short, butterflies are benign, aesthetically pleasing, faunal members. In turn, the main threat to butterflies is the destruction and loss of their habitats. The channelization of riparian areas, draining of wetlands, lowering of water tables, growth of cities, and expansion of agriculture all contribute to this habitat loss. Widespread use of pesticides may also threaten healthy butterfly populations.
The Butterfly Families
Swallowtails are mostly tropical and include some of the largest butterflies in the world. Adults of both sexes have 6 walking legs, take nectar readily, and often flutter their wings even while perched. One of our region’s largest butterflies, with a 4 inch (10 cm) wingspread, is the giant swallowtail (Papilio cresphontes, also called Heraclides cresphantes), a brown and yellow species commonly encountered in urban areas. Its larvae, which feed mostly on the leaves of citrus, look much like fresh bird droppings. If the larvae are touched or disturbed, an unpleasant-smelling, y-shaped orange organ called an osmeterium, is everted from just behind the head. This device and the cryptic appearance are adaptations to avoid predators and perhaps parasites.
Another impressive swallowtail is the pipevine swallowtail (Battus philenor), which is slightly smaller than the giant swallowtail. Pipevine swallowtails are commonly encountered March through October. The butterfly’s upper surface is a dark, iridescent blue; the underside is blue with orange spots. The showy, red-orange caterpillars are poisonous due to the compounds taken from the leaves of pipevines (Aristolochia spp.), their larval food plant. The bright colors serve as warnings to would-be predators that the caterpillars are highly distasteful.
Whites and Sulphurs (Pieridae):
Butterflies in this family are small- to medium-sized and most often white, yellow, or orange, with black margins. Some are among the first spring butterflies, even as early as January. Many emerge in a series of broods throughout the year. Still others are influx species during and after the summer monsoons. This family often exhibits mud puddling behavior wherein dozens of individuals group together on a patch of damp mud to drink and take in minerals and salts. One of the more showy species, the southern dogface (Colias cesonia, also called Zeren cesonia), has a distinctive outline of a poodle on each forewing. A common species, the cloudless sulphur (Phoebis sennae), exhibits strong sexual dimorphism in which the males and females have visibly different wing patterns or coloration. Males of this species are bright, clear, lemon yellow, while females are off-white with small dark markings. In addition to sexual dimorphism, some sulphur species, such as the tailed orange (Eurema proterpia), are seasonally dimorphic, there being short and long day-length color patterns evident. Early spring is a good time to search for orange-tips. The Sara orange-tip (Anthocharis sara) and desert orange-tip (Anthocharis cethura) both come to flowering wild mustards, plants that also serve as larval food plants for these butterflies.
Gossamer Wing Butterflies (Lycaenidae):
Lycaenidae is a large family of small butterflies. Despite their size, many are detailed with exquisite markings—truly jewels of the insect world. One of the world’s smallest butterflies, the pygmy blue (Brephidium exile), measures little more than ½ inch (12 mm) from wingtip to wingtip, and is fairly common in our region, especially in disturbed areas. The great blue hairstreak (Atlides halesus) is one of the largest and most spectacular of the gossamer wings, but it is solitary and uncommon. The caterpillars feed on desert mistletoes (Phoradendron spp.). While there are several broods during the year, the best places and times to look for great blue hairstreaks are wherever and whenever desert broom or seep-willow (both Baccharis spp.) are in bloom.
This largely tropical family is extremely diverse in appearance and in species content. Fewer than a dozen species occur in the Sonoran Desert; most of these having metallic spotting on the ventral (underside) surface of the wings. Metalmarks have long antennae for their size and usually perch with their wings opened flat. One species, the Mormon metalmark (Apodemia mormo), is quite variable from one location to the next, but all populations are associated with patches of various buckwheat (Eriogonum species).
Snout Butterflies (Libytheidae):
The single species of snout butterfly that occurs in the Sonoran Desert, the American snout butterfly (Libytheana bachmanii), can have huge population surges, particularly in the summer and fall, though in some years it is nearly absent. It has a wingspan of about 1¾ inch (44 mm) and is easily recognized by the long projecting sense organs called palpi (which resemble snouts) located between the antennae. The larvae feed on desert and netleaf hackberry.
Brush-footed Butterflies (Nymphalidae):
This is a very large and diverse family with most species medium-sized and generally orange and brown. Many well-known butterflies belong to this family, such as fritillaries, painted ladies, crescents, checker-spots, anglewings, admirals, longwings, and of course, monarchs. Nymphalid caterpillars feed on a wide variety of plant families. Most larvae are fiercely spined, and their pupae are usually sharply angled and adorned with silver or gold colors.
The gulf fritillary (Agraulis vanillae) is a large, vividly-colored, orange and black butterfly with brilliant silver spots on the underside. Gulf fritillary caterpillars feed exclusively on passion vines. Planting ornamental passion vines around the house can greatly increase the abundance of these butterflies.
One subgroup of the brushfoots, the milkweed butterflies (Danainae), is probably among the most noted and studied groups of butterflies, since it contains the monarch (Danaus plexippus). This well known species is a remarkably strong flier, migrating great distances every year to overwintering roosts in California and central Mexico. The monarch butterfly is not very common in the Sonoran Desert, but it can be seen regularly in late summer and early fall on its push southward. A much more common resident of the region, the queen (Danaus gilippus), is closely related to the monarch. The larvae of both species feed exclusively on various plants in the milkweed family (Asclepiadaceae). This family of plants is poisonous to most vertebrates, and the milkweed butterflies gain protection by ingesting its leaves.
This family is named for its rapid, skipping flight. Skippers are small- to medium-sized, differing from other butterflies in having larger bodies in proportion to the wings. They generally have broader heads and hooked antennae that continue past the clubs rather than ending at them. To some people, skippers resemble moths, being hairier and more robust and generally lacking the gaudy colors and patterns of the other butterfly families. But unlike most moths, most skippers are day-flying or crepuscular. Many skipper larvae feed on grasses. One common garden species, the fiery skipper (Hylephila phyleus), eats Bermuda grass and is common around desert lawns. Another common species, the funereal duskywing (Erynnis funeralis), is mostly dark, with white fringes on the hind wings’ edges. It is regular both in towns and in desert arroyos, often in close proximity to woody legumes, the larval hosts. Several skipper species have long tail-like projections on the hindwings. The most common of these, the dorantes long-tail (Urbanus dorantes), is an influx species and is at times abundant in late summer and fall.
Papilionidae, Pieridae, Lycaenidae, Riodinidae, Libytheidae, Nymphalidae, Hesperiidae
|Sonoran Desert genera: numerous|
|Spanish name: mariposa|
Butterfly or Moth?
Though butterflies and moths appear similar in many respects, there are some ways to distinguish between them. Generally when a butterfly lands and rests on a plant it holds its wings vertically, while moths tend to rest with their wings folded back almost horizontally. Moths have heavy, furred bodies, whereas the butterflies have more delicate, slender bodies with little hair. Butterfly antennae are thin and end with a knob at the tip. Moth antennae are often feathery and without a knob.
Color is not a reliable indicator, as some of the moths, especially the Saturnids, are beautifully colored and some butterflies, such as satyrs and mourning cloaks, have muted coloration. Also, not all moths are night fliers. Some species, such as the buck moths and the Calleta silk moth, fly by day. (You may notice the Calleta moth as it feeds on ocotillo leaves during the summer rainy season. Look for ocotillos stripped of leaves from the top down.)
Both butterflies and moths lay eggs which hatch into caterpillars. These caterpillars molt into a pupa, or resting stage. After a period of time—a few days to a season—the winged adult emerges from the pupal case. Moths tend to construct cocoons, protective silk coverings around themselves, before molting into pupas. Butterflies do not encase themselves in cocoons.
With over 142,000 described species worldwide, moths are a smashing evolutionary success, second among animals only to beetles in number of species. Over 12,000 species, grouped into 65 families, are found in North America alone. The moth fauna of the Southwest is particularly rich, as it includes the northern limit of distribution for many primarily Neotropical species. Within the order Lepidoptera, moth species outnumber butterflies and skippers nearly 15 to 1, with many species left to be described, especially among the numerous “microlepidopteran” families.
Why are moths so successful? All moths undergo complete metamorphosis; that is, their life cycles progress through egg, larval, pupal, and adult stages. Thus, the typical moth lives 2 ostensibly distinct lives, filling 2 distinct ecological niches; it is born as a terrestrial, vegetarian eating machine and is “reborn” as a winged creature of the night, hell-bent on completing its reproductive cycle. Yet this is not unusual for insects. Moths share a common body plan, including a head with large compound eyes and sensitive olfactory appendages (antennae). Also, in all but the “primitive” families (Micropterigidae, etc.), moths have long, tubular mouthparts fused into a proboscis; a powerfully muscled midsection (thorax) with two pairs of scale-covered wings and three pairs of legs; and a long, segmented abdomen that includes digestive, circulatory, respiratory, and reproductive structures. Male moths can often be distinguished from females by their broader, comb-like antennae, valve-like abdominal claspers, and smaller, more slender bodies. As in beetles, moths from different families vary widely in wing venation, shape and coloration, larval and adult feeding habits and behaviors, mating systems, population structures, thermal biology, and sizes, ranging from the minute clothes moth (Tineidae) with its ¼ to 3/8 inch (7-10 mm) wingspread, to the bat-sized hawkmoths (Sphingidae) and giant silkmoths (Saturniidae). Unlike beetles, the overwhelming majority of moth species are herbivorous as larvae and adults; there are far fewer examples of carnivores, fungivores, and detritivores among moth lineages. The complex relationships between moths and their host plants may hold keys to understanding why there are so many moths.
Given the stupendous diversity of moths, and our incomplete knowledge of moth distribution and abundance, especially in the Southwest borderlands, our purpose in this section is to outline a few of the salient characteristics of moth biology and suggest a few activities by which the reader might gain an appreciation for moths as dynamic, complex organisms.
Arctiidae (Tiger Moths), Geometridae (Inchworms), Lasiocampidae (tent caterpillars), Lymantriidae (tussock moths), Micropterigidae (mandibulate moths), Noctuidae (owlet moths), Prodoxidae (yucca moths), Saturniidae (giant silkmoths), Sphingidae (hawkmoths), Tineidae (clothes moths)
|Spanish names: palomilla, mariposa de noche, polilla|
Five Features of Moth Biology in the Southwest
1. Plants, Caterpillars, and the Arms Race
The caterpillars of most moths are highly specialized and eat only one or a few plant species. Unfortunately, moth caterpillars are infamous for the exceptional cases; the decimation of crop plants by extreme generalists such as the cabbage looper (Trichoplusia ni; Noctuidae), and the destruction of wool clothing and stored grains by moths in the family Tineidae. The repeated association of certain moth and butterfly lineages with specific families of host plants worldwide suggests that these relationships are ancient. A closer examination reveals complex suites of plant defenses, both chemical (terpenoids, alkaloids, phenolics, cyanide-generating compounds) and physical (hairs, spines, tough leaves, oozing resins, and latex), designed to keep caterpillars at bay. Humans owe a debt of gratitude to moths and other insects for such biochemical plant wealth, which, quite coincidentally, provides us with a pharmacopoeia of natural drugs, insecticides, flavors, and fragrances.
Caterpillars, in turn, have evolved numerous physiological and behavioral strategies to counteract these defenses, from detoxification or rapid excretion of plant toxins to avoidance of older, better defended leaves. Tobacco hornworm (Manduca sexta) larvae will avoid snipping the veins of tobacco leaves, thus reducing the amounts of nicotine marshalled by the plant in its defense. Some specialized caterpillars co-opt the toxins from their host plants for their own defenses, and advertise their acquired distastefulness with bright, vivid colors.
There are additional, more subtle levels to the wars between caterpillars and their host plants. When caterpillars remain undaunted by chemical or physical deterrents, plants may use extrafloral nectaries or other foodstuffs to purchase the services of ants and wasps as caterpillar exterminators. These security guards can be bribed, however, and certain caterpillars do so with glandular secretions and resume eating plant tissues with impunity. And so on. Caterpillars on plants are vulnerable to many other hazards. The scents of wounded leaves and grass, the by- products of caterpillar foraging, are attractive to the parasitic wasps and flies that appropriate caterpillar tissues for the nutriment of their own young. In addition, caterpillars are preyed upon by birds, wasps, and other visually foraging predators. In order to survive, they defend themselves by being distasteful or covering themselves with stinging spines, or through bluff and deceit: they mimic leaves, twigs, galls, flower buds, bird droppings, and even snakes.
2. Moths as Pollinators
Few people realize that the voracious hornworm, looper and armyworm caterpillars that defoliate desert wildflowers, crop plants and garden vegetables, eventually become nectar-feeding adult moths that render important pollination services to many of the same plants. Moth pollination is more prevalent in the Southwest than in other regions of North America, largely due to warm evenings, favorable climate, and proximity to the moth-rich canyons and thorn- scrub of northern Mexico. Moths visit flowers in search of nutritious rewards, usually nectar, and transfer pollen as a consequence of their contact with floral structures and forging movements between flowers. Many night-blooming plant species, especially in grasslands and dune areas, appear to be specialized for moth pollination, but since most moths feed opportunistically from a variety of flowers, disperse widely during their lifetimes, and neither defend territories (like hummingbirds) nor provision young with floral rewards in local nests (as do many bees), most moth-pollinated plants employ alternative reproductive strategies. These include self-pollination, recruiting other (diurnal, or day-active) pollinators, or simply waiting for the next flowering season. Thus, moth pollination is a risky proposition, and moth-flower mutualisms are not very exclusive.
One noteworthy exception to this pattern is the relationship between yucca flowers and the small, white moths (of the genera Tegiticula and Parategiticula in the family Prodoxidae) that spend most of their lives associated with yucca plants. Yucca moths are among the few examples of “active” pollinators, animals that intentionally collect pollen from anthers and apply it to stigmatic surfaces. A female yucca moth uses her unique mouthparts (tentacles) to gather a pollen ball from yucca anthers, then walks or flies to another flower, deposits a number of eggs within the flower’s ovaries, and slam-dunks the pollen ball into its stigmatic cavity. Like wasps that bury the bodies of paralyzed spiders with their eggs, the mother moth’s pollination services ensure that her young will have food (developing seeds) when they emerge as hungry caterpillars.
The yucca plant and moth are absolutely dependent upon one another for reproductive success, yet the terms of their contract are usually complex. First, the yucca plant must sacrifice a significant percentage of its seeds as food for the moth larvae, although limited feeding damage enhances seed germination. Second, if yucca moth females deposit too many eggs within a single flower, the plant can selectively abort that flower, effectively killing all larvae within it. Finally the yucca-moth mutualism (living together in such a way as to increase each other’s reproductive success) is vulnerable to exploitation by cheaters: other moth species lay eggs within fertilized flowers but do not pollinate the flower.
3. Migration and Dispersal
The Southwest is an unusually good place to witness impressive directional movements of insects, especially of conspicuous desert butterflies like the snout butterflies and painted ladies. Such movements are noteworthy for the animals’ prolonged flight in the same direction, at a fixed speed, just a few feet above the ground. True round-trip migration, such as that performed annually by monarch butterflies and many birds, is a relativity rare phenomenon. Most cases of mass movements by moths probably are examples of one-way dispersal, such as the northward flush of black witches (Ascalapha odorata) into our region from northern Mexico toward the end of the summer. Dispersal need not be limited to adults; the movements of thousands of green- and black-striped hornworms (larvae of the white-lined sphinx moth Hyles lineata), across the desert floor provides one of the most memorable images of the summer monsoon. There are many potential causes of mass dispersal, such as periodic population explosions, seasonal changes in day length or humidity, and the availability of food or hostplant resources, but the actual causes are unknown in many cases. The movement of adult moths over great distances, often across political boundaries, has important implications for moths as biotic resources for plants (through pollination), predators, and parasites. With increasing fragmentation and conversion of wild habitats to agricultural lands and subdivisions, these movements also affect populations of moths and their biological interactions with plants and other animals.
4. Moths and Bats: Eaves- dropping That Really Pays Off
While the age of the earliest fossil moths suggests that they shared the world with dinosaurs and flying reptiles, we probably can never know if or when moths or their ancestors abandoned daylight for a relatively predator-free night. However, with the fall of the dinosaurs and the rise of the mammals, new and deadly predators of the night skies arrived: the bats. Fast, maneuverable fliers equipped with sensitive sonar guidance systems, bats are the number one threat for night-flying moths. But moths have developed an array of sensory and behavioral strategies that enable them to avoid becoming evening snacks for a bat.
Many night-flying moths have pairs of ears positioned on both sides of their abdomens that are tuned to exactly the sound frequencies emitted by hunting bats. These sensitive ears allow the moths to eavesdrop on the hunting cries of bats and to attempt to avoid them. Moths have two levels of escape behavior at their disposal when they hear a bat using sonar to search for food. If their bat-detecting ears inform them that a bat is on the way, but still distant, the moth turns away from the direction that the cries are coming from and leaves the area. However, if the bat gets very close before it is detected, the moth suddenly executes a series of high-speed acrobatic maneuvers, usually ending in a dive for the ground or the shelter of nearby bushes. Some moths confuse bats by emitting sounds similar to those emitted by a bat closing in on prey. Sometimes the moths can evade the hunting bats; sometimes they become dinner.
Very small moths and very large moths usually do not have bat-detecting ears. The smaller moths are too small a morsel for the bats to chase after, and many of the larger moths, such as hawkmoths and giant silkmoths, may be too large for bats to catch and eat. There is another small group of species in the tiger moth family (Arctiidae) that actually advertise their poisonous nature to hunting bats. Moths in this group are poisonous due to toxins in the plant species that their larvae eat. Diurnally-active, poisonous insects typically bear bright and conspicuous color pattern so that visually hunting predators learn to associate their horrible flavor or poison-induced sickness with the bright colors. But what do you do if you are poisonous, active at night, and your major predator uses sound and not vision to identify its prey? The small group of tiger moths, which share these characteristics, make unique sounds that their potential bat predators can detect and associate with their poisonous nature. When the sensitive ears of one of these moths detect a hunting bat in the area, at first it attempts to avoid the bat. If the bat gets too close, just as the moth initiates its evasive maneuvers, it also emits a burst of high-frequency clicks that the bat detects with its very sensitive ears. Bats that have learned to associate these clicks with an unpleasant meal initiate their own evasive maneuvers and leave the moth alone.
5. Mating Systems: The Scent of a Female
It is easy to understand why the colorful and conspicuous day-active butterflies typically have mating systems that rely heavily on visual communication of species and sexual identity. In contrast, even though moths possess visual systems especially adapted for their active night life, most species identification and sexual information in moths is communicated via air-borne chemical signals known as pheromones. In some insects, such as the highly social bees, ants, and termites, a complex chemical “language” exists that coordinates activities in the colony and allows for group defense and the dominance of the queen. Moths and many other insects appear to have only a very imited chemical vocabulary, usually amounting to “Hey baby, I’m a fantastic guy,” and “OK, I’m ready to mate.”
In a large majority of the moth species so far studied, the female moth determines when mating will occur by releasing her sex-attractant pheromones. These pheromones typically are a blend of closely-related chemical compounds, which she synthesizes in a special gland near the tip of her abdomen. The sex-attractant is unique to each particular species, and males are rarely confused into following the scent trail of the wrong species. Male moths often trace a side-to-side zigzagging flight track as they follow the wind-borne pheromone trail to its source. In most moth species studied, once the male arrives at the female’s location and physical contact is made, mating proceeds almost immediately. However, in some moths, upon his arrival a male releases his own unique courtship pheromone and fans it over the female with his wings. It has been demonstrated in a few species, and suspected for others, that the female moth uses the quantity or quality of the male’s pheromone to assess his “quality” as a potential mate. It is interesting to note that many of the chemical compounds identified from male pheromones are also common components of the scents of flowers. At least some male moths remember to bring a bouquet!
Five Active Ways to Learn About Moths
1. Light Traps
The inexorable attraction of moths to light is axiomatic in Western culture, yet no truly satisfactory explanations for this behavior have come to light, so to speak. Nevertheless, there is no better way to gain an initial appreciation for the diversity of moths in your area than by setting out a strong ultraviolet or mercury vapor light, hung over a white sheet in an otherwise dark, wild place. Insect lights that run off automobile cigarette lighters or portable generators are available from biological supply houses; alternatively, you could run an extension cord from your porch outlet. Pull up a chair and watch the guests arrive; take notes on which species arrive at different hours of the night and at different times of year. Examine the degree of wear and tear on the wings: are the moths freshly emerged and naive, or are they time-worn and battle weary? Look at the moths’ antennae and genitalia: are they all of one sex, or are equal sex ratios attracted to the lights? What do you observe when you take your light to a different habitat or elevation? Who shows up to eat the moths that are attracted to your light? Which moths never seem to get eaten? If you are interested in collecting moths, this is a good way to start.
Relatively little is known about the adult feeding behaviors of most moth species. There are entire families of moths, such as the giant silkmoths (Saturniidae), tussock moths (Lymantriidae) and the familiar tent caterpillars (Lasiocampidae), that don’t feed at all as adults. Others, including many hawkmoths (Sphingidae) and owlet moths (Noctuidae) are avid nectar feeders and can be important pollinators of night-blooming plants. Some of these nectar-drinkers also tipple at flowing sap or rotting fruit, resources that are exploited to a much greater extent in tropical than in temperate regions. Setting out fermented bait is an exciting way to observe these moths in action; you may attract a different fauna than you will find at light traps, and you’ll have a creative way to dispose of stale beer and overripe bananas! Simply mix old beer, rotting fruit and sugar or molasses and let it brew in a warm, dark place. When the concoction becomes pungent, paint or splash it onto tree trunks along a trail at dusk, then return after dark with a lantern and a field guide. You may see large underwing moths (Catocala spp.)—whose camouflaged, bark-like forewings conceal colorfully banded hindwings—as well as other owlet moths, inchworm moths (Geometridae), ants, crickets, and millipedes. When you learn the species by sight, keep notes on which ones are attracted to bait at different times of year. Experiment with the kinds of bait you use, adding the proverbial “eye of newt” to the cauldron; remember that most insects are less squeamish about road kill and excrement than we are—the funkier it smells, the better!
3. Night-bloomers and oth Gardens
One of the greatest thrills for a moth enthusiast is watching a large hawkmoth unfurl its 4 inch (10 cm) long proboscis to drink from a trumpet-shaped flower while hovering in place at its threshold. Hawkmoths are effective pollinators of a guild of specialized night-blooming plants throughout the Southwest, including sacred datura (Datura wrightii), sweet four o’clocks (Mirabilis longiflora), and tufted evening primrose (Oenothera caespitosa), which produce pale, fragrant flowers with nectar tubes as long as the moths’ extended tongues. In exchange for reproductive services, these plants provide copious sucrose-rich nectar, the high-octane fuel required by hawkmoths to maintain hovering flight. The strong floral perfumes are thought to attract moths from a distance, after which the moths appear to be guided to these pale trumpets by visual cues. Plants with bunches of small tubular flowers, such as scarlet gaura (Gaura coccinea) and fairy duster (Calliandra eriophylla), attract many owlet and inchworm moths, which may spend up to 20 minutes perched on a single inflorescence, drinking leisurely from each flower. As mentioned above, yuccas are pollinated exclusively by small, satin-white yucca moths, whose frenetic mating, pollinating, and egg-laying activities are endlessly entertaining. Find populations of these and other night-blooming plants and wait until dusk, then watch as their flowers open and the moths arrive. Do flowers and their moth visitors segregate by size? Do moths stick to one type of flower, or do they sample from a buffet? Dab a few flowers of one plant with day-glo paint powder, wait until a few moths pass through, then scan all the nearby flowers with a portable ultraviolet lamp (the kind used to illuminate scorpions) to follow the moths’ trails. Better still, plant your own moth garden with a variety of night- blooming, fragrant plants, and provide a nectar filling station for wayfaring moths.
4. Raising Caterpillars
In the Southwest, as in most places, chances are good that if you find an unfamiliar caterpillar, it will grow up to be a moth. This is especially true for caterpillars that mine within leaves, bear stinging spines, and spin plush silken cocoons. Modern insect science has benefited greatly from the contributions of dedicated amateur entomologists who have reared adult moths from caterpillars, and described their larval anatomies and behaviors, food plant preferences, and the timing of their life cycles. Despite the efforts of amateurs and scientists, little is known about the immature stages of most adult moths, and many await discovery.
Caterpillars can be raised in jars, terraria, or even plastic bags, as long as old leaves and grass are removed regularly and growth of mildew is prevented. When you find a new caterpillar, provide it with plenty of freshly cut leaves from the plant it was eating or walking on, along with twigs, dry leaves and soil on which to pupate. If it refuses to eat, you may have to offer it a living plant, or a buffet of different plant species from the area in which you found it. As the caterpillar grows, make drawings or photographic records of its different larval stages (instars), the pupa (cocoon) and the adult that emerges from it. Once you have reared a given species successfully, you can experiment with alternative food plants. Will your caterpillars accept leaves of another plant related to its host plant? Would they rather die (some do)? Never let caterpillars run out of food, as even temporary starvation enhances the probablility of death due to viral infection. Be persistent, as many moth life cycles involve dormancy (diapause) during one or more of the stages of development, and some caterpillars are very difficult to rear in captivity. Also, don’t be discouraged if a parasitic wasp or fly emerges from your pupa; instead, photograph it or collect the specimen. Records of this kind are extremely important, as we know even less about moths’ parasites then we do about moths!
5. Captive Moth Sirens
Many moth species, especially the giant silk moths and hawkmoths, feature mating systems in which newly emerged females emit volatile sex pheromones that attract males from a considerable distance. Rearing moths from caterpillars may provide you with many virgin females, and the means to conduct an interesting census of male moths in your area. (You can also obtain female moths at light traps and bait, but these may already have mated.) Place female moths into small mesh or wire cages; position the cages along a trail, near a forest clearing, or on your porch; then observe. If males of this species are about, they should find their way to the cages in short order. At what time do they arrive? How abundant are they in suburban areas, as compared with rural habitats? Do males from more than one species arrive at the cages? If you are interested in moth breeding, this is an excellent way to attract wild mates and begin the life cycle anew.
Bees comprise a highly diverse group of hymenopterous insects in the Sonoran Desert region. Superficially, bees (especially the parasitic cuckoo bees) resemble some wasps, except that bees are usually hairier and more robust, and they possess specialized structures for carrying pollen back to their nests. Together with ants, bees and wasps form a natural group referred to by taxonomists as the aculeate, or “stinging,” hymenoptera; the stinger is called an aculeus. Only females sting, since the aculeus evolved from the ovipositor or egg-laying tube. Sonoran Desert bees range in size from the world’s smallest bee, Perdita minima, which is less than .08 inches (2 mm) to carpenter bees (the genus Xylocopa), gentle giants that may have body lengths of almost 1½ inches (40 mm) and weigh over a gram. Our native bees burrow into the ground or create nests inside hollow, pithy, dried stems or abandoned tunnels left by wood-boring beetles. All bees are herbivorous except for parasitic forms that prey on other bees.
Herbivorous bees feed on pollen, nectar, and oils offered as floral rewards by flowering plants. Most bees have solitary lifestyles in which females act alone to construct and provision nests, but there are also social forms, such as the familiar black and yellow bumblebees.
Andrenidae, Apidae (includes former Anthophoridae), Colletidae, Halictidae, Megachiliade, Melittidae, Oxaeidae
|Sonoran Desert genera:|
Agapostemon, Andrena, Anthidium, Anthophora, Ashmeadiella, Bombus, Centris, Coelioxys, Colletes, Diadasia, Epeolus, Exomalopsis, Halictus, Heteranthidium, Hylaeus, Megachile, Melecta, Melissodes, Nomadopsis, Nomia, Osmia, Panurginus, Peponapis, Perdita, Psithyrus, Sphecodes, Stelis, Svastra, Tetralonia, Triepeolus, Xenoglossa, Xenoglossodes, Xeromelecta, Xylocopa
|Spanish names: abeja (bee), jicote, abejorro (bumblebee, carpenter bee)|
There are at least 45 genera in 7 families, and perhaps as many as 1000 species of bees distributed within the Sonoran Desert bioregion. Unlike most other groups of organisms, bees are most abundant in numbers of both species and individuals in deserts and savannahs, rather than in lowland rainforests. The region around Tucson, Arizona is thought to host more kinds of bees than anywhere else in the world, with the possible exception of some deserts in Israel. In the United States, there are about 5000 species of bees. On a global scale, there are approximately 25,000 named species, but it is likely that as many as 40,000 different species exist.
Bees, a highly successful group derived from wasps, live in almost all terrestrial habitats within our region. Except for the parasitic cuckoo bees, all female bees make their living by foraging in search of protein-rich pollen and sugary nectar from flowering plants. By moving pollen around from flower to flower and plant to plant, bees perform vital and often unappreciated roles as the most important group of pollinating animals on earth. Yet bees are not out to “help” flowers; they collect pollen and nectar in order to feed themselves and their larvae.
Of the approximately 640 flowering plant taxa growing in the Tucson Mountains near the Desert Museum, approximately 80 percent of these species have flowers adapted for and pollinated by bees. Similarly, at least 30 percent of our agricultural crops require bees to move pollen between flowers. Not only are we dependent upon these “forgotten pollinators” for over a third of our food, but for other products as well. Cotton cloth is a product that eventually results from bee pollination, and so are many beverages and medicines made from other fruits and seeds.
Without the pollination services bees provide, many plants would not produce seed-laden fruits from which the next generation of plants would grow. Without bees, there would be few or no fleshy berries or fruits to sustain birds, mammals, and other wildlife. The tunneling activity of bees aerates the soil and allows water from infrequent rains to quickly penetrate and reach plant roots; and bees’ nitrogen-rich feces fertilize the soil. The bees themselves often provide food for lizards, mammals, birds, insects, spiders, and other arachnids.
In their daily quests bees harvest foodstuffs from flowers for themselves and their larvae. Pollen is a rich food source of amino acids, proteins, fatty acids, vitamins, minerals, and carbohydrates. Nectar provides the energy boost from sugars that bees need to fly. Some desert bees (Centris) have specialized scrapers on their legs for harvesting oils from glands on the undersides of specialized flowers in the ratany and malpighia families. These energy-rich oils are are mixed with pollen as larval food and are also used to help construct brood cells. Other bees collect small pebbles, plant hairs, or floral resins that they use as building materials. Some bees, such as mason bees in the genus Osmia, also require water and mud with which to construct their adobe-like nests. Leafcutter bees (Megachile) remove circular pieces of leaves to fashion into cell walls.
The vast majority of desert bees dig burrows in the ground for their brood cells. Cells can be just a few inches deep or six feet or more down in sandy soils. Many species line their burrows or cells with waxy secretions produced by glands within their abdomens. This lining waterproofs the cells, maintains humidity, and keeps organisms like fungi from destroying the food and the developing larvae. Most bees are solitary, that is each female selects a site for her nest, excavates the tunnels, forms and provisions the rounded cells, and lays an egg within each one. This behavior is called “mass provisioning,” since the mother bee collects and prepares at one time all the pollen and nectar food each developing larva will need to complete its life cycle from larval stages to pupa and, finally, through complete metamorphosis into a newly emerged adult. After laying an egg in each cell, the solitary female has no further contact with her progeny. Although solitary, many of our Sonoran Desert bees routinely nest with other females in very large aggregated nest sites. Among them is is our common cactus bee (Diadasia rinconis) which pollinates prickly pear, cholla, and saguaro cacti. Its aggregations during the spring cactus bloom may number in the hundreds of thousands of individual nests over an area the size of 2 to 3 tennis courts.
Other native desert bees don’t go to the bother of excavating their own nests. Instead, they actively search out the abandoned exit holes and tunnels of wood-boring beetles (usually buprestids and cerambycids) in dead limbs or standing dead trees. These bees are known by their common names of leafcutter (the genera Chalicodoma and Megachile) and mason bees (the genus Osmia). Once a beetle burrow is located, these females bring back cut pieces of leaves, resins and pebbles or mud balls with which to fashion cells and their thick, protective capping plugs.
Most of our Sonoran Desert bees have but one generation per year, with adults usually emerging with the spring or summer wildflower blooms. Some species, however, have 2 or even 3 generations per year. In a typical life history, adult males and females mate soon after emergence from their natal cells. Females construct and provision nests and lay eggs, and the larvae develop rapidly underground. During cooler months the larvae usually stay in a resting condition, or diapause, at either the prepupal or the pharate adult stage (the pupal stage just prior to the final molt to the adult proper) until the following spring or summer when they complete their metamorphoses and emerge as adults.
A relatively small number of our desert bees are truly social. Bee sociality indicates that these species’ evolutionary paths have diverged from those of their solitary ancestors. In the case of some sweat bees (the family Halictidae), a queen looks like other females in the colony, though she differs in having highly developed ovaries. She may or may not secrete pheromones that elicit feeding and grooming behavior in her daughters. The queen lays all the eggs within the colony. Sweat bee colonies are usually small, consisting of a few dozen or at most a few hundred individuals.
Highly social bees in our region include the introduced honey bee and the native black and yellow bumblebees in the genus Bombus. Bumblebee colonies are annual in nature, established by an already-inseminated queen who emerges from her winter retreat in spring and finds a suitable mouse nest or other underground cavity in which to nest. The queen is larger than her daughters, and lays all the eggs after she has produced, initially, a small brood of workers. Males and queens are produced late in the season and in the colony cycle. They mate, the males die, and the inseminated queens spend the fall and winter “hibernating” below ground until the next spring.
Farther south in Sonora, Mexico (near Alamos), extremely social bees live in colonies with many thousands of individuals—the so-called “stingless bees,” in the genera Melipona and Trigona. These queens are physogastric, with abdomens swollen full of eggs. They are not able to fly once they begin laying eggs, and never leave the colony again. Thus, these social bees represent a still greater caste differentiation between queens and workers. Stingless bees store as much as several quarts (liters) of honey in waxen storage pots that look like clusters of grapes. Indigenous peoples who find these nests within hollow trees sometimes transport the hives back to their villages, where they tend the bees and routinely harvest their honey and beeswax.
The infamous Africanized honey bee closely resembles other North American honey bees—even experts have to examine them closely to tell them apart. Africanized or not, a few foraging honey bees are usually no cause for alarm. The only true danger is encountering an Africanized colony. In such cases stay clear of the colony and contact a beekeeper or pest control company.
Wasps comprise an enormous and diverse assemblage of insects ranging from the smallest known insects—tiny parasites of insect eggs—to immense cicada killers and tarantula hawks. Most wasps are predators whose young feed on other insects or arthropods, but a few groups have become vegetarians, similar to bees, and collect pollen to be fed to their larvae. Both sexes of wasps are typically strong fliers, but some species are flightless, and in others, one sex, usually the male, is an excellent flier while the other sex is flightless. Velvet “ants”—actually female wasps that superficially resemble ants—are an example of this.
Perhaps the most conspicuous and commonly seen wasps in the Sonoran Desert are the paper wasps (Polistes). Paper wasps are large, (about 1 inch, or 20 to 25 mm) social wasps that build paper honeycomb nests. Paper wasps are longer, thinner, and more smooth and shiny than honey bees and have longer, narrower waists (called petioles) than do bees. Common paper wasps include the yellow paper wasp, whose color is true to the name; the Navajo paper wasp, which is deep chocolate brown with the end of the abdomen yellowish; and the Arizona paper wasp, which is slightly smaller and more spindle-shaped than the other 2 and is brownish-red with thin yellow cross bands on the abdomen.
|Division: Aculeata||Families: Pompilidae, Mutillidae, Sphecidae, Vespidae|
|Sonoran Desert genera:|
Pepsis, Dasymutilla, Sphecius, Sceliphron, Polistes, Vespula
|Spanish name: avispa|
Some of the most impressive insects in the Sonoran Desert are the enormous tarantula hawks (Pepsis). These 1 to 1¾ inch (25 to 45 mm) wasps sport brilliant gun metal, blue-black bodies carried on fiery orange wings. The desert contains more than a dozen species, some of which have jet-black wings instead of orange wings.
Velvet ants (Dasymutilla) are among the more colorful of all organisms in the desert. Females of these large ant-like wasps can be seen scurrying over sandy or bare soil surfaces during warmer seasons of the year. More than 3 dozen species live in the Sonoran Desert of Arizona. They range in size from tiny J inch (4 mm) species to huge 1 inch (25 mm) giants. Most are clothed in red, orange, yellow, or silver coats of hair-like setae (bristles) and look like moving fuzzy cotton balls. Particularly large velvet ants include the black and red D. klugii and Satan’s velvet ant, which is black with a yellowish-white furry abdomen. The glorious velvet ant (D. gloriosa) is a long-haired, totally white velvet ant that looks like a creosote bush seed on legs.
Cicada killers (Sphecius grandis) superficially resemble huge yellowjackets or hornets. Yellow with tan patches, they are 1 to 1½ inches (25 to 40 mm) long. These powerful fliers have large compound eyes.
Mud daubers (Sceliphron caementarium), sometimes called dirt daubers or mud wasps, are thin, 1-inch (25 mm) long black wasps with yellow legs, and long, yellow-thread waists. They are named for their habit of building mud nests under bridges or eves of houses, or in other protected areas.
The paper wasp is social insect whose life cycle begins as a solitary mated queen. The queen overwinters deep in rock cracks, behind peeling tar paper, or inside enclosures. In spring the queen builds a paper nest suspended from a thin stalk in a protected rock crevice, among thick vegetation such as dead fan palm leaves, or under the overhang of a man-made structure. She constructs a small cluster of paper hexagonal cells and lays an egg in each. The queen then feeds the larvae that hatch from these eggs a diet of caterpillar “meat balls.” When the first young worker wasps emerge from their pupal cells, they assume the tasks of hunting caterpillars, collecting material for making papier mâché for nest expansion, and collecting water for cooling. The queen then ceases all work except egg laying. By late spring, the colonies have grown to contain 20 to 50 wasps; by late summer as many as 200 wasps may be present. At this time new queens and males are reared. After mating, the new queens imbibe nectar to fatten for the winter. By late fall, the queen mother and workers die, the nest is abandoned, and the next generation of queens goes into hibernation.
Tarantula hawks, so named for their huge size and hawk-like hunting strength, hunt tarantula spiders. During the warm months female wasps search the ground for tarantulas. Once prey is located, the tarantula hawk bites onto a leg of the tarantula and with its long, strong, sharp stinger, pierces the spider near a leg base, and injects paralyzing venom. The limp, but living, spider is dragged into an appropriate hole, sometimes the spider’s own burrow, where a single egg is laid on the spider. The wasp then seals the burrow to complete her work. When the egg hatches, the larva consumes the spider and then pupates; the next spring the adult emerges to complete the life cycle. Males do not hunt, but are frequently seen visiting flowers of milkweeds, western soapberry trees, or mesquites.
Velvet ants, cicada killers, and mud daubers have life cycles similar to tarantula hawks. Wingless female velvet ants search the soil surface for burrows of wasps or bees that can serve as hosts. Male velvet ants fly above the surface looking for females. Once a female velvet ant locates a nest of a suitable host, she enters, opens a cell of a host larva that has completed feeding or has already pupated, lays an egg on or near the host, and closes the cell. She does not sting or paralyze the host. Cicada killers search for cicadas, which they sting to paralyze, and then transport back to their nests dug in sand or soft soil. One to 3 cicadas are provisioned per cell for each young. Mud daubers provision their mud cells with spiders they have captured and paralyzed by stinging. The larvae of all of these wasps feed on the provisioned prey, consuming it entirely, and pupate to emerge the next year as adults.
Ecology and Biology
Most wasps are specialized hunters that track down their prey using smell and sight combined with knowledge of the habitat, activity periods, and behavior of the prey. A solitary wasp usually subdues its prey with a sting that either kills the prey or paralyzes it briefly or permanently. (Tarantulas stung by tarantula hawks can live completely paralyzed for months.) Social wasps, including paper wasps, never sting their prey. Instead, they use their powerful cutting mandibles to chew the prey into pieces to feed directly to their larvae. The venom of social wasps is used only for defense. The most serious predators of social wasps are birds, mammals, and reptiles. The sting and venom have evolved to be effective weapons against these large animals. Venoms are ideal defenses because they can be injected via the stinger directly into the assailant’s body where they cause pain, toxicity, or both. The stingers of wasps, and their relatives the ants and bees, are modified ovipositors used to deliver venom rather than to lay eggs. Thus, only females can sting; males are completely harmless. Unlike honey bees, which can only sting once because the barbed sting remains in the victim’s skin after stinging, thus evisceratating the bee, wasps have smaller barbs on their stings and can withdraw the stinger to sting several times. The effectiveness of wasp stings and venoms as defenses has allowed wasps to evolve bright warning colors of red, yellow, orange, or white on a black or dark background. These conspicuous warning color patterns are called aposematic, and their brightness usually correlates with the degree of painfulness of the sting. Potential predators and humans alike learn to avoid beautiful aposematic wasps. Failure to heed the warning often results in an excruciatingly painful, and possibly toxic, sting. Indeed, tarantula hawks deliver the most painful sting of any United States or Mexican insect, a sting that is many times more painful than that of a honey bee. Velvet ants are also true to their bright colors. They not only possess the longest stingers of stinging wasps, but deliver a powerful sting that is not soon forgotten. Although wasps can sting people, they rarely do so, and then only when they are captured, or in the case of paper wasps, when their nests are threatened or disturbed.
Ants are familiar creatures. Although they are small as indi- viduals, they are social, living in cooperative colonies, and these colonies are huge. Ant colonies are made up entirely of females, and include one or more queens and many workers. All of the ants streaming in and out of a nest entrance are workers, who protect the colony, collect food, and care for the larvae. Even though the workers are females, they lack the reproductive abilities of the queen, who lives deep within the nest and does little besides produce eggs.
All ants’ bodies are divided into three parts. The head includes the antennae that detect smells, compound eyes, and capable jaws. All legs are attached to the mid-section, called the thorax. The gaster is the last segment and contains most of the internal organs, including defensive organs, including the glands that produce formic acid. When working with ants, beware the gaster!
Ants are most closely related to wasps. Imagine a wasp without its wings—it looks like an ant. Flying ants (the new queens and males) are often mistaken for wasps.
|Sonoran Desert genera:|
Pogonomyrmex, Messor, Pheidole, Solenopsis, Myrmecocystus, desert Neivamyrmex
|Spanish name: hormegas|
Ants hatch from eggs laid by the queen and go through a series of larval stages before becoming adults, just as do butterflies. Ant larvae cannot move about on their own, however, and are completely dependent on workers for their care.
Just as individual ants go through stages of development, so do ant colonies. An adult ant colony raises reproductive (sexual) male and female forms, which can be recognized (in the desert) by the presence of wings. Reproductive females (who will become queens) look like workers but are 2 to 3 times as big; males are smaller than workers with even smaller heads and gasters. Mating occurs at species-specific times of the year. Most species fly immediately after summer rains; a few (Messor and some Myrmecocystus) fly in the winter. Reproductive individuals released within one area congregate at a mating swarm where one female may mate several times; sperm are stored for the lifetime of her colony, which may be more than 20 years!
Many desert ants (especially some Messor, Acromyrmex, Myrmecocystus, Solenopsis, Pogonomyrmex, and Pheidole), cooperate in founding colonies. Multiple new queens work together to start a small colony and raise their first workers. By working together, these queens can get underground faster (avoiding heat and predators) and produce more workers to more rapidly establish the territorial limits for their colony.
The period of colony founding— when the new queens must fly, mate, locate a good nest site, and avoid predators—is when most colonies fail. Only a few will survive. Once established, a desert ant colony may live for a decade or more. This means that the queens that establish the colonies are among the longest lived insects we know.
Long-lived, underground nests protected by thousands of ants devoted to bringing in food, offer attractive environments to other insects besides the ant-architects themselves. A variety of beetles, roaches, crickets and silverfish have evolved the ability to live in ant homes. Should we be surprised that they live in our homes as well? Perhaps the most interesting of these guests are other ant species. Some ants are “kitchen thieves”that are small and secretive. They live in the cracks and other hide-aways of regular ant nests and eat the crumbs of food that are left out. Other ants are more insidious. They sneak into a host nest of a closely related species and lay eggs destined to become reproductive males and females. Workers of the host colony raise these eggs as if they were their own. When it comes time to fly and reproduce, these social parasites propagate their own genes, not those of the colony that reared them.
The Sonoran Desert is a great place to watch ants. They are the most abundant animal in this habitat and lack of ground cover makes them easy to see. In the desert most species of ants build underground nests that protect them from the harsh conditions. Here they can store or even grow food, find ample water, and avoid the environmental extremes of the soil’s surface.
One way to look at ant diversity is to classify them by the foods they eat:
Many desert ants (especially Messor, Pheidole, Pogonomyrmex, and Solenopsis) harvest seeds that they use as food for their larvae. Seeds of several grasses and annual plant species are preferred; seeds of perennial plants—especially cacti—seem not to be preferred. Seeds are stored in chambers toward the top of nests where dry conditions discourage germination. Ants have interesting behaviors when learning the different types of seeds that are available to them.
Pogonomyrmex workers have large squarish heads that contain powerful muscles for crushing seeds. The workers are ½ inch (13 mm) in length and are brick red to black. They have unforgettable stings. The typical nest of many species has a prominent cleared area, with a central opening and several permanent trails radiating from it. Another ant, Messor pergandei, is the only species of this worldwide genus that extends into the Sonoran Desert. Workers are ¼ to ½ inches (6 to 13 mm) long and are a shiny, jet black; they do not sting.
Over a dozen species of Pheidole live in the Sonoran Desert. Their workers come in two distinct forms: a small “minor worker” class and a much larger “soldier” class that crushes seeds, and sometimes enemies too.
Our desert Solenopsis are related to the infamous “imported fire ant” that is the scourge of the southeastern United States. The desert fire ant is a natural part of the Sonoran Desert community; unfortunately, it does resemble its eastern relative in its aggressive behavior and annoying sting. Most fire ants are J inch (3mm) long with some as large as N inch (8 mm); they are shiny brick red to black.
Another common group of ants in the Sonoran Desert are the leaf-cutting or fungus-growing ants. Acromyrmex ants are related to the larger leaf-cutting ants of the tropical Americas. Acromyrmex versicolor is common in the Sonoran Desert. Its workers collect leaves and other plant parts to insert into fungus masses, which they grow in chambers deep within their underground nest. The fungus is completely dependent upon the ants for its care and propagation; the ants, in turn, eat a portion of the fungus as their sole source of solid food. Long columns of leaf-cutter ants search across the desert for plant matter for their fungus gardens when conditions permit in the fall and spring and on cool summer mornings; at other times, they remain underground. The fungus garden is started from a small “plug”of fungus brought by the queen from her home colony.
Honey pot ants
Another common food source in the desert is the liquid nectar of plants and the “juice” of other insects; both of these, however, are available only seasonally. Honey pot ants (Myrmecocystus) have solved this seasonal problem with specialized members of the colony that store liquid food in their engorged gasters. When other members of the colony need food, these living storage vessels share their stored reserves.
The ants described above eat seeds, fungus or nectar. Some ants prefer meat; these are the desert army ants (genus Neivamyrmex). These ants raid the nests of other desert ants and occasionally take other prey as well. Because they are predatory and deplete the prey in any one area, they are nomadic and move from place to place. They have no permanent nest structures, and instead tend to live in temporary quarters such as hollows under trees or kangaroo rats’ nests.
Ants’ ability to live in colonies and excavate deep nests where the seasonally abundant food of the desert can be stored has made them remarkably successful in the Sonoran Desert. Further, in this environment, ants are easily collected and observed; this has made them model organisms for studies of development, behavior, and ecology.