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A Vertebrate Looks at Arthropods

Barbara Terkanian

At close range even a familiar arthropod like a grasshopper may seem appallingly strange. The creature’s expressionless eyes give no indication that it can think or feel. Its colored armor plating seems more suited to a machine than to an animal. There are too many legs jointed in too many places. The abdomen pulses, the antennae twitch, and tiny oral appendages shift food to the mouth. Lean a little closer and the thing leaps, almost into one’s face, to veer off on crackling wings.

Such an experience may strengthen the conclusion that arthropods are so different from us that they are essentially alien life forms. But such an extreme first impression is not really justified. Arthropods are classified as animals, exactly as we are, and so they must have characteristics in common with us and with other members of the animal kingdom. Certainly they do. Arthropods share numerous physical, biochemical, and developmental characteristics with the other creatures classified into the 30-odd phyla composing the Kingdom Animalia.

We can concede that arthropods qualify as animals, but how does the grasshopper in the garden fare in comparison to vertebrate animals like ourselves? In fact, arthropods provide an interesting counterpoint to vertebrates. Curious parallels and strong contrasts exist between the two groups. Similarities in skeletal function are associated with historical parallels in the colonization of land and the evolution of flight. The giant size of vertebrates contrasts strongly with the much smaller size of arthropods, while the extraordinary diversity of arthropods eclipses the modest radiation of vertebrates.

Kingdom: Animalia
Phylum: Arthropoda

Animals with jointed external skeletons, such as lobsters, spiders, centipedes, and insects

Phylum: Chordata

Animals with dorsal nerve cords (vertebrates and others). The vertebrates—fishes, amphibians, reptiles, birds, and mammals—have bone or cartilage skeletons. Non-vertebrate members of this phylum lack skeletons.

The Design of Arthropod and Vertebrate Skeletons

Both arthropods and vertebrates have articulated skeletons. Components of the skeleton meet (articulate) at joints, which allows one part of the body to move in relation to another. Muscles spanning joints and anchored to different parts of the skeleton provide the power for movement. Articulated skeletons serve two functions. First, they allow their owners to retain a characteristic physical form. Second, they support an organism’s weight and resist the stresses of locomotion. Other kinds of animals, like snails, have hard shells into which they may retreat for protection, but these structures do not define the animal’s form or support its weight.

Arthropod and vertebrate skeletons are quite distinct from each other. Basically, the vertebrate skeleton is internal (an endoskeleton) while the arthropod skeleton is external (an exoskeleton). Here, both kinds will be referred to as skeletons. The vertebrate skeleton is buried under skin and muscle. Within the body, skull, and vertebral bones encase the brain and spinal cord, while ribs protect the heart and other organs. Long bones form the internal core of the legs. With arthropods, virtually all external structures as well as a few internal ones are covered by exoskeletal material, including eyes, mouthparts, antennae, body, legs, the fore and hind sections of the digestive tract, and some respiratory surfaces. Regions of flexible, unhardened exoskeleton serve as joints between neighboring segments.

That articulated skeletons arose only in arthropods and vertebrates is simply an evolutionary coincidence. The two groups are not closely related to each other. But functional advantages common to skeletons are related to two novel parallels in arthropod and vertebrate evolution: first, vertebrates and arthropods are perhaps the most successful of all terrestrial animals; and second, they are the only organisms ever to evolve genuine powers of flight.

Life on Land and in the Air

A way to appreciate the advantages of having a skeleton in a terrestrial environment is by comparing animals with skeletons to “soft-bodied” animals. The latter category includes snails, slugs, and a great variety of worms. Some of these animals are ancient residents of land environments. But compared to animals with skeletons, soft-bodied terrestrial animals display a limited variety of form and diversity of locomotion.

The external forms of soft-bodied terrestrial animals resemble each other despite substantial differences in each species’ internal anatomy. Nearly all have fleshy, cylindrical bodies without legs, and most lie full length upon the ground. These creatures are limited to slug-like forms because they depend on muscle to retain form and support their weight. Their aquatic relatives (octopus, squid, and various worms) can evolve complex forms because they live in water, which is denser than air and provides more support for attenuated appendages like arms. Even among vertebrates, purely muscular “limbs,” such as an elephant’s trunk or a chameleon’s tongue, are not employed to support their owner’s weight.

Animals with skeletons can use the passive strength of bone or hardened exoskeleton to support their bodies in air. Consequently, terrestrial vertebrates and arthropods have more diverse forms, unlike soft-bodied land animals. Turtles, flamingos, ballet dancers, and rhinos look nothing like slugs. Praying mantids, stalk-eyed flies, and scorpions cannot be said to resemble worms. Toes, legs, necks, antennae, tails, and wings are great departures from a basic cylindrical form. And all of these departures are of functional value to their owners. Long, jointed limbs would hardly seem to be an evolutionary innovation, but they and other jointed appendages account for the great freedom of movement enjoyed by terrestrial arthropods and vertebrates.

Without skeletons, most soft-bodied animals creep or writhe over surfaces by contracting muscles of the body wall. Terrestrial animals with skeletons use their limbs to walk, trot, run, leap, brachiate, and fly. They can do so because they are constructed differently from soft-bodied animals. Typically, arthropods and vertebrates stand with their bellies clear of the ground. This means that as one of these animals moves, its body passes through air, instead of over the ground, where friction has a greater effect. The foot of a running arthropod or vertebrate touches the ground just long enough to propel the animal forward with each stride. This enables it to run swiftly, compared to ground-bound, soft-bodied animals. Some arthropods and vertebrates are swifter yet, because they have evolved the ability to fly.

Flight is exclusive to animals with skeletons. Birds, bats, and a few species of dinosaurs have evolved flight. Among arthropods, only insects can fly, but since insects amount to 70 percent of all animal species, arthropods account for the vast majority of flying animals. All flying animals must be able to satisfy stringent flight requirements. They must have extensive respiratory systems to supply quantities of oxygen to flight muscles, sophisticated nervous systems to coordinate rapid flight responses, and of course, they must have functional pairs of wings.

Wings have arisen by different pathways in arthropods and vertebrates. In vertebrates the foreleg is modified into a wing; as a consequence the limb becomes useless or compromised for quadrupedal locomotion. Birds walk only on their hind legs. Bats use their wings as legs, while keeping their extraordinarily long fingers and extensive wing membranes folded out of the way. Arthropods have sacrificed nothing for flight; their wings evolved as completely new structures rather than as modifications of existing limbs. If one counts, insects retain three pairs of walking legs and two pairs of wings. All three pairs of legs may be used for locomotion or adapted for specialized tasks such as prey capture. Sometimes one pair of wings is modified into yet another arthropod tool while the remaining pair is used for flying. For example, beetle forewings have changed into sturdy protective coverings for the membranous hind wings. The hind wings of flies have become balancing devices necessary for their dizzying aerial maneuvers.

Interestingly enough, there appear to be limits to the sizes flying animals can attain. The largest flying birds weigh about 30 pounds (13.5 kg). For tiny arthropods, the very density of air becomes an important factor. Insects weighing less than 3/10,000 of an ounce (a milligram) can drift wingless through air just as plankton does through water. Size is an important matter to a flying animal. It is also an important distinction between vertebrates and arthropods.

Vertebrates Are Giants; Arthropods Are Small

Vertebrates are the biggest animals ever to evolve on Earth. We know of giant dinosaurs, huge amphibians, massive sloths, enormous moas, and the colossal blue whale. In comparison, the very biggest terrestrial arthropods can be held in the palm of the hand. Pterygotus, a long-extinct aquatic arthropod, did qualify as a giant—it approached 10 feet (three meters) in length. Ancient exceptions aside, there is little overlap in the size ranges for species belonging to each of the two groups. Small vertebrates are still larger than the great majority of arthropods, the tiniest of which are best viewed under a microscope. So why does gigantism arise frequently in vertebrates but not in arthropods?

An increase in size has physical consequences for any creature. As an animal doubles in size, its weight increases eight-fold, but the weight bearing capacity of its skeleton is only quadrupled and the strength of its muscles is merely doubled. Because of their great weight, large vertebrates have skeletons which are disproportionately heavy and robust compared to those of small vertebrates. Presumably, terrestrial arthropods could reach horror-movie size simply by developing big, sturdy skeletons. But they have not done so during hundreds of millions of years on Earth. It seems that the costs associated with large size affect arthropods more strongly than vertebrates.

A heavy, cumbersome skeleton, risk of injury, and complications during molting all become more serious problems at large size. The arthropod skeleton accounts for a greater proportion of its owner’s total weight than does the skeleton of a vertebrate. As an arthropod gets larger, the proportion of weight attributed to the skeleton will increase faster than it does for a vertebrate. At some point, the advantages of increased size will not compensate for the difficulties associated with a heavy skeleton. When that happens, natural selection will favor the smaller individuals in a population.

One important difficulty for large arthropods is the risk of injury. As the outermost part of an arthropod’s body, it is the rigid skeleton that comes in contact with the environment. Without the cushioning effect of soft tissues, it is more vulnerable to abrasion and impact damage than the internal skeleton of vertebrates. Running becomes hazardous because all of the weight of a heavy arthropod would come down on the relatively small area of the foot. Without the shock absorption provided by the hooves, paw pads, cartilage, and ligaments found in vertebrate extrem-ities, an external skeleton might be expected to fracture under the force of impact. A simple fall might be even more damaging.

Finally, molting, necessary for growth, causes other problems at large sizes. Just after molting an arthropod is essentially a soft-bodied invertebrate. The skeleton is still soft, and does not provide good support. Worse, an arthropod cannot rely on muscles to define form the way soft-bodied animals do, because arthropod muscles are designed to exert force against a rigid skeleton, and until the skeleton hardens, many muscles are useless. Instead, an arthropod gulps air or water in order to hold its form until the skeleton hardens. The larger an arthropod’s size, the more difficult this process becomes. Each time an arthropod molts it must undergo this risk. Vertebrates do not molt their skeletons as part of growth so they escape these risks completely.

Arthropods may be unable to attain the impressive sizes of vertebrates, but their small size is related to a big distinction—their extraordinary diversification.

More than a Million Arthropod Species

Arthropods are the most diverse of all animals, comprising over 85 percent of all living animal species. Estimates for the number of species in one class of arthropods, the insects, range from 1 to 10 million. In contrast, the entire Phylum Chordata—vertebrates and their close relatives—amounts to fewer than 5 percent of all living animals. The remaining 10 percent are accounted for by other invertebrate phyla, such as molluscs. Why is there such an overwhelming number of arthropod species compared to all other kinds of animals? Why are there relatively few vertebrate species, despite their sophisticated internal skeletons and access to terrestrial environments? The answers have to do with arthropods’ relatively small size, their capacity for rapid change, and their long tenure on Earth.

Small animals can exploit habitats more fully than large ones. A single plant may be a meal to a vertebrate, but to arthropods it can be a universe. One species might complete larval development in a flower bud, while another species spends its entire life feeding on the woody stems. A large plant like the saguaro can support an entire community of arthropods throughout its life and after its death. Other habitats, such as the surface of water and the bodies of other animals, are used by arthropods, but are inaccessible even to the smallest vertebrates. Winged insects or ballooning spiders can travel great distances, colonizing new habitats quickly. As they invade new habitats arthropods undergo selection which favors individuals best equipped to survive in the new conditions. Over time, the better-equipped individuals may come to differ so much from their ancestors that they become distinct species.

Arthropod populations can undergo rapid change. Agricultural pests are well-known for swiftly evolving tolerance to previously devastating pesticides. Short generations, multiple generations per year, and large populations are conducive to the prompt emergence of new forms, and under the right conditions, new species. Vertebrates are also capable of change and speciation, but because of their longer generation intervals these processes tend to require more time. While some arthropod species—fruit flies for instance—change and diversify rapidly, others, such as scorpions and horeshoe crabs, settled into a useful design early on and have remained unchanged for millions of years.

Finally, arthropods have been around for a long time. Trilobites, an early and now extinct group of marine arthropods, lived 500 million years ago. Early terrestrial forms, like scorpions, are known from 400-million- year-old fossils. Reptiles, the first entirely terrestrial vertebrates, did not arise until the Carboniferous period, approximately 100 million years later. Insects evolved flight 150 million years before birds, dinosaurs, and mammals. There has been plenty of time for diversification and evolution of many exquisite, bizarre, and intriguing arthropod species.

Similar, Different, and Interesting

Remarkable parallels and contrasts can be developed when arthropods and vertebrates are compared. But there are reasons to focus on arthropods alone, without considering them in relation to other animals. In the name of survival, arthropods have evolved forms ranging from familiar to outrageous to beautiful. They evade enemies, feed, and reproduce by methods that are sometimes ruthless, sometimes subtle, and frequently ingenious. As is their habit, arthropods in the Sonoran Desert have diversified, giving this region a rich inventory of fascinating species.

You are invited to form a closer acquaintance with native Sonoran Desert arthropods by reading about them in the following chapters. The chapters are organized on the basis of arthropod phylogeny, which reflects the evolutionary relationships between species. As you proceed through this section, you can use the phylogenetic listing following this introduction to keep track of arthropod groups.

These intriguing and largely harmless creatures are among the most visible and approachable of all Sonoran Desert animals. Keep an eye out on your next desert walk, and you may see an arthropod or two that you have previously encountered in the pages of this book.

Additional Readings

Alcock, John. In a Desert Garden: Love and Death Among the Insects. NY: W.W. Norton, 1997.

Conniff, Richard. Spineless Wonders: Strange Tales from the Invertebrate World. NY: Henry Holt, 1997.

Evans, Arthur V. An Inordinate Fondness for Beetles. New York: Henry Holt & Co., 1996.

Friederici, Peter. Strangers in Our Midst: The Startling World of Sonoran Desert Arthropods. Tucson: Arizona-Sonora Desert Museum Press, 1997.

Imes, Rick. The Practical Entomologist. New York: Fireside, 1992.

Smith, Robert L. Venomous Animals of Arizona. Tucson: University of Arizona, 1982.

Werner, Floyd G. and Carl Olson. Learning about & Living with Insects of the Southwest: How to Identify Helpful, Harmful and Venomous Insects. Tucson: Fisher Books, 1994.

A Phylogeny for Arthropods Mentioned in this Book

In essence, phylogenies are family trees. Researchers group living things into increasingly specific categories based on reconstructions of the organisms’ evolutionary histories. Since most of the branching in the arthropod family tree took place before humans came into existence, the tree structure is deduced from the study of fossils, and the examination of molecular, developmental, anatomical, and behavioral characteristics of contemporary species.

Many characters important in determining relationships between species are not observable in the field. The informal notes in this phylogeny are intended to help readers develop a feel for arthropod classification by using visual characteristics alone.

Kingdom Animalia