Previously, we stopped by the island of Borneo to visit gorgeous orchid mantises and strange spiny jungle nymphs. Our friend the jungle nymph taught us a few things about hiding in plain sight and how sharp her defensive spines were. Ouch! Well, this week we head about 3,500 miles further south to the eastern coast of Australia in New South Wales to meet the Macleay’s spectre, a.k.a. the giant prickly stick insect or Australian walking stick so named after the famed British naturalist William Sharp Macleay who described this magnificent insect. We met other charismatic members of the walking stick clan such as the twostriped walking stick, northern walking stick, and elegant phasmatid in previous episodes.
Females of this giant of the insect world measure more than five inches across from the tips of their outstretched forelegs to the tips of their hind tarsi (analogous to toes). And these are heavied body, massive insects with fully grown females weighing more than a thousand times that of a common housefly. To generate this much mass Australian walking sticks consume large amounts of foliage. In their native realm, the diet consists of leaves of eucalypts but our colony here at the University of Maryland Insect Zoo thrives on foliage of local plants including leaves of Photinia. With so much juicy biomass in play, one might think that predators like magpies or leaf-tailed geckos would find these to be excellent tucker. Ah, but Macleay’s spectre has several tricks to defeat the beaks and jaws of hungry predators.
Looking like a dead leaf gently swaying in a breeze is one way to fool the hungry eyes of a jungle predator. This form of mimicry called motion crypsis is even deployed while leisurely devouring a leaf by this magnificent mistress of disguise.
Trick number one, look like a dead leaf. With expanded plates on outstretched legs and an unusual posture of arched abdomen and thorax, a resting walking stick appears to be nothing more than a dead leaf waiting to drop from a plant. This camouflage could easily fool a visually astute predator searching for the symmetrical lines and characteristic shapes of other insects on the menu such as beetles or butterflies. Trick number two, sway in the breeze. Many walking sticks, including Macleay’s spectre, gently rock their bodies to and fro when a temperate breeze or vibration disturbs the substrate on which they rest. Scientists believe that predators learn to overlook irrelevant environmental cues like leaves swaying in a breeze as they hunt for tasty insect prey that often move in characteristically buggy ways. By swaying like a leaf or twig in the wind, walking sticks may send the would-be predator a “these aren’t the droids you’re looking for” message and be overlooked by their enemies. This feat of deception has been called motion crypsis.
A startling display of bristling spines warns predators to think twice before attacking.
Trick number three, show them your spines. Playing the part of a hungry predator, I reached out to grab a large female walking stick and was rewarded with a threatening display of outstretched forelegs, head arched backward, and abdomen arched forward, all body parts bristling with a phalanx of nasty looking spines. This posture clearly was a warning to attack only with the threat of imminent pain – a challenge I foolishly accepted. Trick number four, impale the enemy. As I grasped Macleay’s spectre, she forcefully embraced my fingers and hand, stabbing my skin with several spines that failed to draw blood but nonetheless left a lasting impression. I imagine a bird or lizard biting into a mouthful of said spines might forgo an attack on another walking stick at the next encounter.
Smaller male Australian walking sticks also use expanded body parts and unusual postures to deceive predators by resembling withered parts of plants.
Trick number five, like many other insects and some vertebrates, including critters such as crickets and leaf-footed bugs we met in a previous episode, Macleay’s spectres are able to shed a limb on demand. A special muscle allows a leg or antenna to snap off at the insect’s bidding under threatening circumstances. This phenomenon, known as autotomy, allows the insect to lose a leg and save its life by distracting the hungry predator. When the predator stops to examine or eat the severed limb, the bug makes its getaway. Clever morphological and behavioral adaptations allow this grotesquely beautiful giant to best its enemies and survive in a Land Down Under.
Acknowledgements
We thank Todd Waters for maintaining the Insect Petting Zoo at UMD and thereby providing the inspiration for this week’s episode. We thank Men at Work for “Do you come from a land down under?” but what does “where women glow and men plunder” mean? The interesting article “The swaying behavior of Extatosoma tiaratum: motion camouflage in a stick insect?“ by Xue Bian, Mark A. Elgar, and Richard A. Peters provided fascinating insights into the quirky behaviors of Macleay’s spectre.