Stick insects

Pol Turon Contreras

Stick insects are a big group of insects called phasmatodea that contain many species around the world in all continents except Antarctica. They are more abundant throughout the tropics and in areas of high vegetation and live on trees and shrubs where they can hide among branches. The name phasmatodea comes from the ancient Greek word φάσμα (phasma) meaning apparition or ghost because most of these insects look like plants. Stick insects are usually very long insects, especially the females that grow larger than the male. Most of them are around 2.5-30cm long although the biggest species, the chinese walking stick, can reach up to 64cm long, making them the longest insects in the world. 

The main characteristic that sets these creatures apart  from others is their camouflage. Stick insects have evolved to look like different parts of vegetation, most of them have evolved to look like sticks, doing this by having very elongated bodies mostly brown and green and some even having a few outgrowths that look like lichen helping them camouflage on bark. A few in the family phyliidae have evolved to be wider and greener to look like leaves and even look like they have bite marks making them even more real "stick”. This serves as excellent camouflage allowing them to hide in plain sight from predators. Stick insects are so good at this that when there is wind they start swaying from side to side so that they can look like a swinging branch which makes their act much more convincing to predators. Since moving too much can make them more distinguishable they are mostly nocturnal animals mostly moving around and eating at night where predators have a harder time seeing them.

Although they put on a very convincing display it does not fool all their potential predators, such as the bat. Since bats hunt using echolocation instead of their sense of sight they have no problem distinguishing a stick insect from an actual stick and very commonly prey on them. Some lizards and birds also can sometimes see through their camouflage. Due to their camouflage not being enough to deter all predators some species of walking stick have also evolved other defence mechanisms to deter predators such as spikes, chemical sprays that can temporarily blind predators or make them smell bad and when disturbed they start playing dead and fall to the ground and can stay like this for hours.

Due to the very inactive lifestyle of stick insects most of them have evolved to lose their wings. This adaptation might seem strange since the evolution of wings is the main characteristic that allowed insects to diversify and survive for almost 400 million years but since flying takes up so much energy losing this ability may have let the stick insects produce more eggs using that leftover energy. Even if that is the case flight is such a versatile trait that some stick insects still have it even if they are more clumsy flyers than most other insects. Researchers thought that these species simply didn’t evolve to lose their wings because many biologists thought that losing such a complex organ would be irreversible, this is known as Dollo’s law. However, newer research shows that stick insects re-evolved wings completely separately from their ancestors, doing what was thought to be impossible and potentially disproving a well known law of evolution.

In the species of stick insects that have re-evolved flight usually only the males have regained this mastery of the skies while the females are still stuck on the ground. This trait helps the males survive and find females while the females have more energy reserved to make eggs essentially getting the best of both worlds. 

Winged species of stick insects also have a special type of eyes called ocelli. There are three of them at the top of their head and while these eyes can only detect between light and dark they can process this information much faster than more developed eyes which helps them detect things in their environment and position themselves while flying. Since this trait is mostly found in flying species researchers believe it helps them have better reflexes and have a wider range of sight of their surroundings.

However, all stick insects have two highly developed compound eyes, like other insects like dragonflies, praying mantises and flies. These eyes are made out of thousands of tubes like structures each with its own lens and light receptors. They use these eyes to navigate and find predators, especially during the night where they're most active. But when stick insects are born they don’t have very good nightvision, but as they grow their eyes become better at detecting light and their vision increases by 8.6-fold by the time they mature into adults. Because of this young stick insects usually move and eat during the day and are quite inactive during the night, as they get older and their eye-sight improves they become more and more nocturnal until as fully grown adults they have a fully nocturnal lifestyle. 

Walking sticks have a very short lifespan of about one or two years so they mature quickly so they can lay eggs before dying. When a female is ready to mate, usually around autumn, she releases hormones to attract nearby males to her location. The mature males pick up those hormones using their antennae and follow them to the female and they mate. Then the female lays the eggs on the floor because stick insect eggs look like seeds so that gives them good camouflage, then the eggs stay there for about two months until they’re born. Walking stick eggs are also durable enough to survive the digestion from a bird, so if a bird mistakenly eats one they can travel in its stomach and get to disperse themselves. Females also have the ability to reproduce parthenogenetically, which means they can lay eggs that will later on hatch into females without having a male fertilize them.