Stick insects, members of the class Arthropoda and the order
Phasmatodea, are herbivores, spending their lives on foliage, moving slowly, depending mostly on camouflage to save them in a world of hungry predators. They are prey for birds, reptiles, spiders and primates. Although they resemble twigs very closely indeed, they do not live in piles of fallen sticks, but up among the greenery that is their food.
They stay relatively still, but when they move they can resemble twigs trembling in the wind. Some look like curled dried leaves hanging on a branch. It takes a sharp eye to appreciate their intricate beauty even in an insectarium. Their size ranges from one to twelve inches, and their colors vary, although they are usually shades of brown. There are about three thousand species, many still unclassified. They live on every continent but Antarctica, mostly in tropical regions, but in some of the temperate zone as well.
Some of their adaptations are fascinating. To improve the chances of the next generation, some females drop the eggs one by one throughout the habitat, not grouping them in a vulnerable mass as some insects do. Stick insect females hide their eggs on the underside of leaves, or under soil, or in plant structure. The eggs often resemble seeds, with a bulb at the end, the
capitulum, which contains fats.
Some stick insects leave these eggs where ants will carry them to their nest. There the ants eat off the fatty capitulum, and toss the egg on their garbage dump. It hatches there, safe from birds, mice, and weather. Some nymphs, young, are even said to resemble ants, lending them added camouflage as they make their way out of the anthill and into the larger world.
There are two genders of stick insects, but the females do not need to breed in order to produce eggs. This adaptation is called parthenogenesis. If a female does not mate, all of her offspring will be female, but if she does, half the eggs will be male. Females are generally larger than the males, although not in every species. The males however, tend to have wings, often brilliant ones, and the females in general do not. Stick insects with wings may suddenly flash them at an attacking predator, and then fold them and drop down among the forest litter, invisible again.
The life cycle of the stick insect is an incomplete metamorphosis. Once the nymphs hatch they look like miniature stick insects (though often without wings). Each nymph will go through several molts before it reaches maturity, shedding its exoskeleton, eating it, and growing a new one. Eating the exoskeleton, the outer shell, hides the stick insect’s traces from predators, and reuses protein. Each period between molts is called an instar, and males have an average of five while females average six. While juvenile, at least some of the species can drop legs if they are seized by a predator, and grow them back at the next molt. Stick insects may live for one to two years. The eggs incubate for a period between three months and one and one half years, depending on species.
Stick insects are nocturnal as a rule. This way, they avoid most of the local predators when they are active and gathering food. However, this leaves them prey to bats, which find them by echolocation, sonar, and are not fooled by their camouflage.
So in general, the stick insect survival strategy relies on passive defense that uses camouflage and inactivity when most predators are active. Not always. Both the vivid orange and black Peruvian fire stick,
oreophoetes peruana, and the American walking stick, Anisomorpha bupestroides, can spray a defensive chemical when molested. This painful chemical temporarily blinds such predators as birds and mice, and encourages them to find other food.