Specimen Showcase | Sticks that Walk and Breath🌿

Have you ever seen a walking stick, the common name for the Phasmatodea insects? Chances are that you have encountered the 𝘚𝘪𝘱𝘺𝘭𝘰𝘪𝘥𝘦𝘢 𝘴𝘪𝘱𝘺𝘭𝘶𝘴 (Westwood, 1859), a very common stick insect species which can be found in Hong Kong on your way to school or work, but probably did not even noticed, as they are masters in camouflage.

They have a long and slender body, adults have an overall pale brown appearance while nymphs (young stick insects) have a green appearance. They own a pair of slightly opaque wings with pinkish veins, giving them their signature pink wings once deployed. Individuals spend most of their time in the trees, where they feast on leaves. The pink-winged stick insect is a widely distributed species within Asia, being found from Japanto India to Borneo, with populations introduced in Madagascar. This makes it one of the most broadly distributed species of stick insects in the world, even if it is likely that several cryptic undescribed species may hide under this species name. Just like its evasive taxonomy, this species is in nature a champion at hiding and evading.

For herbivorous species, escaping predators is a constant necessity. Fortunately, a plethora of defense mechanisms and strategies have evolved allowing animals to avoid becoming a meal to others. Here, let us introduce two of these life-saving mechanisms. The first is a collection of primary defenses that lowers the chance of predation, including through mimicry, that is perfectly illustrated by the pink-winged stick insect. But first, what is insect mimicry? It is an ecological strategy where the insect pretends to be another species or object. The goal is to deceive the predators so it does not notice or identify the potential prey, but instead perceives it as a background object or as another unpalatable or dangerous species. In the case of the pink stick insect, it is not perceived by potential predators (and other human observers as well) as an animal but instead as a piece of vegetation. In addition, scientists noticed that stick insects push the ruse further by keeping up-right positions to act as a better part of the plant. Even when the predator moves close to them, they remain completely still to not draw the predators’ attention with their movements. Very rarely, they blend in so well with the plant that predators, like frogs, will step on them because they perceive these insects are genuine sticks!

But even when detected, the pink-winged stick insect has one more trick in its sleeves… or rather on its back from which it unleashes its final secret weapon, a mixture of harmful and irritating chemicals. Researchers have uncovered the contents of their secret concoction. One of the chemicals detected is limonene, which can be found in the peels of citrus fruits, including oranges and lemons. These chemicals can cause all kinds of discomfort to the attackers, including irritation of the skin. When they are disturbed, for example being poked at, the bugs can either drench themselves with this distasteful substance or attack the predators by shooting the chemicals directly at them. Through the small ducts in the two glands situated on the sides of their thorax (chest), they would control the passage of secretions by muscles in the gland to decide whether to diffuse the chemicals into the air in the form of mist where they can get themselves covered in the chemicals, or eject a column of liquid at the attacker. Either way, most predators would be successfully repelled by now and these stick critters get to live another day.

Even when the above two measures fail and the stick insect is caught, they could still discard their leg as a last-ditch effort to free themselves. Living above the ground in the tree canopies, there is no doubt they are going to fall a considerable distance if they lose their grasp on the branches. So by sacrificing a leg, the insects are able to quickly distance themselves from the predator, reducing the chance of predators pursuing them. For them, the expression “break a leg”, takes another dimension. During falling, the stick insect spreads its pink wings to control the speed and direction of their fall, helping them to land squarely on their target. They might have lost a piece of themselves, at least it’s not the whole piece. But does that mean they have to make use of only one leg afterwards? Surprisingly, these insects are capable of regrowing their legs through shedding their old skin three times in a row. Although it can grow its leg back, we do not encourage any readers to experiment by attempting to yank their legs off, or else you might be attacked by their unpleasant secretions!