Insects & Spiders: Stick Insect

Range: all continents except Antarctica Habitat: tropical forests and woodlands

Masters of disguise

Stick insects, as their name implies, are insects that have taken camouflage and imitation to the extreme by developing the appearance of a stick or twig. Typically these insects are shades of brown, although some may be green, black, gray, or blue. You might think that stick insects hide among sticks on the ground, hoping to blend in, but most stick insect species are usually found sitting right out in the open within the leaves of a tropical tree. They usually stay perfectly still, but when they need to move, they are even able to camouflage their motion. It is common to see them walk in a swaying motion, pretending to be a twig caught by the wind. Other stick insect species have lichen-like outgrowths on their bodies that help camouflage them on tree bark.

Other forms of protection

When camouflage is not enough, some stick insects use active forms of defense to handle predators. For example, the species Eurycantha horrida can release an awful-smelling brown liquid. Other species have brightly colored wings that are invisible when folded against their body. However, when they feel threatened, they will flash open their wings, then immediately drop to the ground and again hide their wings. The predator is often confused as it searches for a brightly colored insect but only sees a pile of drab, brown sticks on the ground!

The incredible, concealable egg

You've probably realized by now that stick insects have developed adaptations to fool predators. Clever ways to lay eggs are also included in their bag of tricks. Female stick insects use two main methods of laying eggs: dropping eggs on the ground or placing them in a hard-to-reach place. Some stick insect species will drop one egg per day somewhere on the ground during their day's travels. These eggs are commonly small and resemble seeds. By dispersing her eggs far and wide, the female prevents a predator from lunching on a cluster of her eggs.

Other females lay their eggs in places that are hard for predators to find. For example, some stick insects will lay eggs in the soil, in hollow parts of plants, or glue them to bark or the underside of leaves. Species that lay their eggs underneath leaves tend to hatch faster than species that lay eggs elsewhere, as the eggs need to hatch before the leaf falls off the tree and exposes the eggs!

Most stick insect eggs are covered by a hardened shell or capsule with a node called a “capitulum” on one end. The capsule of some species contains fats and other goodies that lure ants. The ants will bring the capsule underground into their nest, remove the capitulum, and feed on the nutrients it contains. After they are done eating, the ants toss what's left, which includes the stick insect egg, in their nest garbage dump area. The egg incubates in the safety of the ant nest, out of sight of predators. A few months later the all-but-forgotten hatchling makes its way out of the ant nest!

A nymph's life

No matter how their egg is laid, stick insect hatchlings, called nymphs, hatch from the egg as miniature versions of adults. They then go through successive molts to eventually reach adult size. This process is called incomplete metamorphosis: egg, nymph, and adult. Molting happens when the old exoskeleton is shed and the larger body that had grown inside it expands and hardens into a new one. The time between molts is called an instar.

Who needs males?

One of the most interesting things about stick insects is their ability to reproduce parthenogenetically. This is a form of asexual reproduction where the unfertilized females produce eggs that hatch into females. If a male fertilizes the egg, it has a fifty-fifty chance of turning out male. If no males are around, the line continues with females only. At the San Diego Zoo, we have had a group of female-only leaf insects for several years now!

Providing food for others

Because stick insects are a very nutritious and filling meal for many birds, reptiles, spiders, and primates, they are mostly nocturnal. They are herbivores that munch on leaves with their powerful jaws, called mandibles. Their droppings contain broken-down plant material that becomes food for other insects.

Even though stick insects can sometimes avoid diurnal predators, they are not safe from bats. Bats that use echolocation can hone in on the tiny noises made by stick insects and turn the insect into a tasty meal. The stick insect's elaborate camouflage doesn't help it in the dark. It's a good thing bats are not fooled by stick insect camouflage; without bats to eat them, we could find ourselves living a little too close for comfort with millions of stick insects!