I’m on a bench facing an ivy-covered fence, half-watching the black-and-yellow-banded wasps patrolling near ground level, ready to move if they get too chummy. Suddenly one wasp tumbles out of the ivy, grappling with a twig longer than its body. The wasp is having trouble getting airborne, and it finally drops the twig and moves on. And then, before I happen to look away, the twig starts to crawl off.
That gets my full attention. I pick it up to see it’s not a twig but a twig-mimicking stick insect, a walkingstick, a phasmid. It’s a couple of inches long, drab brown, with an elongated wingless body and skinny legs. I’ve always thought of phasmids as creatures of the South, at least, and primarily the tropics; I didn’t even know they occurred in California. (Jerry Powell and Charles Hogue’s California Insects illustrates 3 species, none quite matching this one.)
The walkingstick is sitting in my palm. Suddenly the first pair of legs stretches forward, concealing the head to produce an even more twiglike effect. It continues to hold that posture. I notice a drop of bright green fluid on its thorax and figure the wasp has injured it. Is insect blood, if blood is the applicable word, supposed to be green?
Eventually I let the stick insect go on about its business. But the encounter—a perfect PBS or Discovery Channel moment—stays in my mind. It looks like an unsuccessful predation attempt. But why did the wasp relinquish its victim?
Stick insects—2,500 species worldwide, most, in fact, tropical—are placid plant eaters, specialized relatives of the grasshoppers, mantids, and roaches. They’re masters of procrypsis: visual disguise. Most have either attenuated sticklike forms or flattened leaflike shapes, and come in appropriate shades of brown or green. Their slow movements and tendency to freeze when threatened add to the illusion.
Protective coloration is not that unusual: The world is full of moths that look like tree bark, mantids the color of orchids, lizards disguised as rocks. The 19th-century naturalist-artist Abbott Thayer was convinced that all colors and patterns in nature were cryptic. Flamingos, he claimed, had evolved their coloration to blend into tropical sunrises and sunsets. Thayer’s ideas were not widely accepted.
Mimicry is not foolproof. The stick insect’s disguise might protect it against visual predators, like birds. But the wasp had known it wasn’t just a twig. Wasps, it turns out, have an extraordinarily acute sense of smell. Tarantula wasps can sniff out spiders in their underground lairs. The Department of Defense’s Advanced Research Projects Agency is investigating the use of parasitic wasps to detect explosives and nerve gas toxins.
So what if a predator figures out that the twig is edible? Some phasmids have a second line of defense, the ability to secrete chemicals that repel attackers. I’ve been unable to find any reference to chemical defense in California’s stick insect species; the technical literature is sparse. But the trait is widespread among phasmids. It’s possible, then, that the green liquid wasn’t blood after all.
The handful of stick insects whose defensive behavior has been studied produce the stuff from glands on the thorax. From the structure of the glands, it’s been speculated that some manufacture two kinds of chemicals that combine as they are released, as in the notorious bombardier beetle. Some produce an invisible spray, others viscous drops. One Australian species emits a substance described as colorless, volatile, and smelling strongly of toffee. The secretion of a Javanese form makes dogs sneeze.
Thomas Eisner, an entomologist at Cornell who discovered astonishing things about chemical warfare in nature, did the first substantive work on stick insects’ defensive secretions in the 1960s. His subject was an American species, the large Southern walkingstick (Anisomorpha buprestoides). This insect, he found, could direct its spray with impressive accuracy. Eisner ran trials with a variety of predators, including ants, beetles, mice, blue jays, and a South American opossum; all except the opossum were deterred. In most cases the stick insect fired on physical contact by a predator. The jays, though, were blasted on first approach.
The active ingredient of the Southern walkingstick’s spray, dubbed anisomorphol by Eisner, is chemically similar to nepetalactone, the main component of catnip. (Eisner apparently did not use cats in the predator trials.) Other scientists found the same chemical in the secretion of the coconut stick insect, a major pest of coconut plantations in the South Pacific. A species from Taiwan, however, uses actinidine, which it appears to obtain from its diet of pandanus leaves.
The most startling variant was described by Eisner and other researchers a few years ago. The Peruvian firestick, a large and relatively gaudy tropical phasmid, bases its defense on quinoline, whose closest chemical relative is naphthalene, a coal tar derivative. This phasmid, as Eisner wrote, “appears to have hit upon the expedient of producing an insect analogue of ‘moth balls.’”
There are other odd things about this group of insects. Earlier this year a Brigham Young University evolutionary biologist named Michael Whiting published the results of his study using DNA to reconstruct the phasmid family tree. Some stick insects have wings; others, like the one I found, do not. It was always assumed that the winged forms were ancestral to the wingless forms. However, Whiting discovered that in one lineage, phasmids had lost their wings, then re-evolved them. This is not supposed to happen. It would be like snakes re-evolving legs, or blind fish re-evolving eyes. It looks as if these insects had retained the genetic instructions for making wings and were able to turn them back on when circumstances again favored flight.
At least one stick insect, a South African species, has co-opted ants into safeguarding its eggs. A number of plants—some in California, many more in the chaparral-like fynbos of South Africa—produce seeds with attached fat-and-oil-rich bodies called elaiosomes. Ants carry elaiosome-bearing seeds back to their nests, eat the food body, and eject or ignore the seeds. Away from their parent plants, where they would likely have been consumed by rodents or other seed predators, the seeds have a better chance of germinating.
The eggs of phasmids are often seedlike, and in some species have an external bit called the capitulum whose function was unclear. Two entomologists at Rhodes University in South Africa documented ants carrying the eggs of a local phasmid off (using the capitulum as a handle), then eating the capitulum, which did not prevent the eggs from hatching. The capitulum seems to be a fake elaiosome. They speculated that by dispersing the stick insect’s eggs, the ants inadvertently reduced the eggs’ vulnerability to predators and parasites, and gave the larval phasmids more elbow room.
There’s a nice symmetry here—insects that imitate plants hatching from eggs that mimic seeds. It’s tantalizing how little we know about these bizarre creatures, living their obscure lives right under our noses. I may never find out whether stick insects in California defend themselves with pseudo-catnip or bio-naphthalene, or lay eggs packaged with ant-snacks. In fact, if it hadn’t been for that wasp, I might never have known there were stick insects in California at all.
