Month: August 2006

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Micrathena gracilis: not, perhaps, the specific name that most of us who have tried to walk through an August woods sticky with her webs would have chosen. But is she not slender and graceful, this spider, apart from the spiked club of her abdomen?

The ventral side of her abdomen appears to bear the spiral template for the web, whose silk emerges from its central point. Every morning at dawn she spins her web anew, spanning some path or opening in the forest where the light pools. If a human or other animal should blunder into her web, she’ll have a new one completed in a couple of hours. If it lasts until sunset, she devours the silk, leaving only the three foundation threads. The forest of the night throbs with the call-and-response of katydids — a noise like the surf, or a bank of industrial looms — while hundreds of spiders quietly erase their work.

She too can stridulate, though I’ve never heard it: a low-pitched buzz or hiss, they say, designed to frighten off predators.

The male is a fraction of her size, and has only two, flattish spines where she has ten. His spinning is limited to a single, non-sticky strand that he lays across her web: a path bisecting the net that spans the path. He hides under a nearby leaf, waiting for the right moment to place himself at her service but not at her disposal.

They each have two copulatory organs, and each pair must make contact, necessitating some complicated gymnastics. To ensure success of this risky business, he dances vigorously at the end of his thread, making the entire web vibrate as he bobs and waggles his skinny rear end. I would have to suppose the female finds this spectacle deeply entrancing. During copulation, if all goes well, the male flips over onto the female’s abdomen, and she can return to hunting flies with her new appendage sprawled on his back among the graceful peaks.
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See here (PDF) for a complete description of the courtship and mating practices of M. gracilis.

Yesterday afternoon, I stopped by the milkweed patch on the way back from a walk to the Far Field. A few monarchs still sailed back and forth, no doubt looking for places to lay their eggs, and a tiger swallowtail and great-spangled fritillary patronized the few remaining blossoms. The clusters of gray-green pods poking out from under the big, flat milkweed leaves almost suggested a miniature banana plantation.

I was pleased to see that the juvenile assassin bug I’d photographed a couple of times for my milkweed patch photo set was now an adult, and readily identifiable. It turned out to be one of the most bizarre and distinctive of the 160 species of assassin bugs found in North America: the wheel bug, Arilus cristatus. It’s a very large, eye-catching insect with a shiny dark patch at the end of its wings and a semicircular crest — the eponymous wheel — crowning its thorax like the blade of a circular saw, or a misplaced mohawk. I spotted it in the top leaves of a milkweed plant not three feet from where I first photographed it as a juvenile on July 17, and just as before, it reacted to me and my camera with the wary poise of a street fighter, sidestepping away in a manner that clearly said don’t fuck with me.

And I wouldn’t want to: web sources describe its bite as much more painful than the sting of any hornet. In fact, assassin bugs used to be used as instruments of torture in Central Asia (and might be still, under dictators like Karimov and Turkmenbashi). Their mouthparts are essentially weaponized straws, first injecting a poison that paralyzes their insect prey and turns their insides to soup, then sucking them dry.

The function of the “wheel” is completely unknown. That’s not too surprising, really, because like many common insects, wheel bugs have rarely been studied. The best thing on the web appears to be an article at the Hilton Pond nature center website, which includes general background on bugs (i.e. hemiptera) for the entomologically challenged, as well as some excellent close-up photos — much better than mine. See also Bugguide, the insect wiki, for more photos of wheel bugs in larval and adult forms.

If you like trees as I do, you should be grateful for the presence of wheel bugs. They’re one of the few predatory insects capable of feasting on hairy caterpillars such as fall webworms; tent caterpillars, which were in outbreak mode in many parts of the northeastern U.S. this spring; and gypsy moth caterpillars, which about a month ago defoliated hundreds of acres at higher elevations around Central Pennsylvania. With all the gypsy moth egg cases and empty cocoons on our ridgetop oaks right now, I fear we may be headed for a defoliation here next year.

Predators are different. The quality of attention that they bring to bear on their surroundings can seem both charismatic and a little intimidating, as anyone who as ever gazed into the eyes of a hawk or a captive leopard can attest. Praying mantises, with their large, space-alien eyes and their unique ability to swivel their heads, strike us as highly intelligent, but it’s not the kind of intelligence that, in humans, would lend itself to multi-tasking or associative thinking. I doubt that the mind of a predator can easily accommodate the kind of broadly focused awareness most useful to omnivorous habitat generalists such as crows, raccoons, bears, or humans.

By “mind,” of course, I mean the whole of the nervous system and its responses, whether learned or instinctual. For such a finely tuned instrument as an assassin bug, the brain itself may be somewhat superfluous. In a series of famous experiments on a related species from the American tropics — Rhodnius prolixus, the carrier of Chagas’ disease — the British entomologist Sir Vincent Wigglesworth surgically removed the brains of a number of captive assassin bugs and found that they not only survived, but far exceeded their normal lifespan.

Though some human males do identify strongly with the single-minded hunting prowess of natural predators, they should remember how most male praying mantises meet their end: as food for their mates. Apparently female wheel bugs, too, are in the habit of eating their partners as soon as they have completed their single-minded task. Cannibalism is common among wheel bugs; they are not given to displays of sociability. Neither the wheel nor a second set of tiny eyes on top of the head are present in the larval form, when mating is not a concern, so perhaps they evolved to help the male fend off attacks from the female prior to mating. If anyone has a better theory, I’d love to hear it.