By remaining motionless for hours, this stick insect’s masquerade has evolved to deceive visually astute predators.
Last week we visited the Giant Malaysian Leaf insect, a master of disguise. This week we take a short hop roughly 600 miles north to Vietnam to visit another member of the phasmid clan called the Vietnamese Walking Stick. Unlike its island relative to the south, this walking stick bears a closer resemblance to the North American Northern Walking Stick and the Twostriped Walking Stick we met in previous episodes. Although lacking the tough armor of beetles and evasive flight of butterflies, they have perfected the art of crypsis, that is, they have an uncanny resemblance to features of their environment. This ruse of looking like branches or twigs of the trees on which they feed helps them avoid detection by hungry predators. In addition to having greatly elongated body regions and appendages matching the colors and textures of twigs, walking sticks move and pose in ways designed to fool sharp-eyed predators. As walking sticks search for leaves, they sway slowly back and forth mimicking the movement of a branch in the breeze. When not feeding or actively moving about, they assume a branch-like position with the front pair of legs extended directly forward. Their ability to hold an unflinching pose for hours is the envy of every mime.
Please don’t expect a lot of action in a video of a walking stick, but they certainly are interesting to watch. A tiny young nymph goes stick on stick with an adult female and another lady takes a leisurely nocturnal stroll.
Rather large eggs of the Vietnamese Walking Stick resemble seeds that are collected by ants. Eggs are taken underground where parasitic wasps are unlikely to find them, thereby increasing the stick insect’s prospects for survival.
The colony of Vietnamese Walking sticks at Maryland’s Insect zoo is composed entirely of females. Like the Giant Leaf Insects we met last week, Vietnamese Walking Sticks are able to reproduce asexually. Visually astute predators like birds are not the only threats to survival of walking sticks. Tiny parasitic wasps attack their eggs. Stick insects have evolved yet another clever ruse to avoid attack by parasitic wasps. Eggs of walking sticks have a remarkable resemblance to seeds of many plants species that depend on ants for dispersal. These seeds have a small nutrient-rich structure on one end called an elaiosome that entices an ant to carry the seed back to its subterranean colony, thereby assisting in the dispersal of a plant’s progeny. The eggs of many walking sticks have an appendage on one end called the capitula that is the spitting image of an elaiosome. The egg of the walking stick so closely resembles seeds of ant dispersed plants that ants mistakenly gather these eggs and bring them back to their nests in the soil. Underground the eggs of walking sticks are no longer exposed to the peril of attack and death by parasitic wasps. What a clever evolutionary strategy! Upon hatching, the tiny walking sticks escape from the earth, climb nearby vegetation, and assume the slow motion role of a twig.
Acknowledgements
Thanks to Chris Sargent and Todd Waters for providing the inspiration for this episode and for keeping our arthropods happy and thriving in the University of Maryland’s Insect Zoo. The wonderful article “Capitula on Stick Insect Eggs and Elaiosomes on Seeds: Convergent Adaptations for Burial by Ants” by L. Hughes and M. Westoby was used as a reference for this episode.