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.