A female twostriped walkingstick dwarfs the male on her back.
Last week we headed to Australia to meet the giant prickly stick insect known as Macleay’s spectre. With snow still on the ground from a surprise autumnal storm, let’s continue our southern expedition and stop at Hutchison Island on the east coast of Florida to meet the twostriped walkingstick. These giants of the insect world are among the largest insects in the continental United States. One gorgeous female I captured was almost four inches long. Twostriped walkingsticks enjoy a vegan lifestyle dining on leaves of crepe myrtle, roses, oaks, rosemary, and several other trees and shrubs. While the details of the life cycle of this curious insect are not fully known, autumn is the season when mating pairs are most abundant.
In addition to their notable markings, one striking feature of these beauties is a curious and obvious discrepancy between the size of females and the size of their mates. In the world of mammals and birds, the general rule is that males of the species are larger than the gals. However, in the realm of insects and spiders, when it comes to size, females are often larger. Does size really matter and, if so, what forces shape larger females and smaller males? Several explanations have been proposed to help explain these size differences in insects. For many species of insects, there is often a direct and increasing relationship between the size of the body and the number of eggs a female can produce. Big mammas simply leave behind more offspring than small ones and thereby they may gain an advantage in the evolutionary game. This may be accomplished by molting an extra time relative to the male and by spending slightly more time growing and becoming larger than their puny mates.
The males of many insects have a slightly different situation. Their contribution to the mating game is rather small, just a deposit of sperm to fertilize the female’s eggs. While larger size can be important when battling other males for access to mates, being at the right place at the right time may play just as important a role in the walkingstick’s mating game. Rapid development may allow a small male walkingstick to gain an advantage over larger ones. By being the first of his cohort to reach maturity, find a mate, and win her affections, a small male may father more offspring than a larger one.
Just behind its head and above the front leg you can see the small opening from which the twostriped walkingstick discharges its noxious secretion.
With their remarkable size, one wonders how an insect so large goes unnoticed by scads of hungry predators that might like to make it a tasty meal. Like many other bugs we have visited, such as stink bugs, elegant phasmatids, Eleodes beetles and Nasutitermes termites, twostriped walkingsticks pack a potent chemical wallop. Just behind the creature’s head are small openings leading to underlying glands. When under attack the walkingstick can discharge a highly irritating secretion from these ports. Upon capturing the large gal featured in this episode, I was treated to a discharge of an unctuous liquid across the surface of my hand. While this caused me no real discomfort other than a smelly hand, one unfortunate human who was apparently eyeballing a walkingstick experienced a rather nasty surprise. A quote from a scholarly account of this incident includes the following: “The pain in his left eye was immediately excruciating; being reported to be as severe as if it had been caused by molten lead.” Although I suffered no misfortune by these beautiful creatures, caution should be observed when handling them - and please don’t look them in the eye!
Acknowledgements
Special thanks to brave Dr. Shrewsbury for spotting and capturing walking sticks featured in this Bug of the Week. Resources used for this episode include the articles “Behavioral causes and consequences of sexual size dimorphism” by Wolf U. Blanckenhorn; “Achieving high sexual size dimorphism in insects: females add instars” by Toomas Esperk, Toomas Tammaru, Soren Nylin, and Tiit Teder; and “Defensive spray of a phasmid insect” by Thomas Eisner. To learn more about twostriped walking sticks please visit the following website: