|Giant House Spider--cousin, competitor, and occasional predator of the hobo|
(The Book of Deadly Animals is now available in the US. I’m going to celebrate by running here an expanded version of a story from the book.)
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The hobo spider's danger to people is now widely recognized. The Centers for Disease Control list it (along with the widow and recluse spiders) as dangerous, medical textbooks concur, and publications like the Journal of the American Medical Association have published case studies. Doctors know the signs of hobo venom—a blistering wound ringed with yellow like the moon in a halo of smog, devastating headache in many cases, disturbed thinking in an occasional one.
But skeptics remain. One is Greta Binford of Lewis and Clark College. In an unpublished study, Binford and some colleagues at the University of Michigan attempted to replicate Vest's experiment. They were unable to produce necrotic lesions by injecting hobo spider venom into rabbits. The Michigan rabbits developed nothing worse than a red bump. Binford points out that the hobo has never been implicated in human injuries in Europe, where it has been known for centuries. She analyzed venom obtained from European and North American hobos and found no chemical differences.
Like several other prominent skeptics, Binford notes that the hobo spider is rarely caught in the act of biting and brought in for identification by a competent specialist. Because the hobo's appearance is not especially distinctive, the average bite victim can't be expected to sort it out from dozens of other spiders. Rod Crawford of the Burke Museum at the University of Washington deals with hundreds of spider identifications each year. He points out that a species ID by a layman is virtually useless. Even doctors get very little training in such identification and are likely to be mistaken. Still, Crawford is satisfied with Vest's evidence, which linked a captured or crushed hobo with human symptoms in four cases.
But even here, it's possible to contest the evidence. Rick Vetter of the University of California, Riverside points out that one of these victims—the 42-year-old woman I mentioned earlier, who found a hobo spider in her clothing—had a history of phlebitis, a circulatory problem that sometimes causes necrotic lesions. The phlebitis, says Vetter, could have caused the symptoms Vest blamed on the hobo. Vetter also notes that the Australian white-tailed spider, once widely accepted by doctors as a source of necrotic arachnidism, has recently been exonerated. Researchers studied 130 cases of definite white-tailed spider bites, finding not a single necrosis. Vetter would like to see hobo bites subjected to a similarly rigorous study. He points out that mistakes have serious consequences. For example, misdiagnosis of ailments like basal cell carcinoma, which can look like necrotic arachnidism, could be fatal.
There's another complication: Venomous animals aren't always venomous. It has long been known that black widow spiders, like some venomous snakes, can deliver "dry bites" to warn off larger animals without wasting venom on them. Typically, these are followed by a dose of venom if the harassment persists. Rebecca Vest, who worked with her brother Darwin in his investigations (and who first proposed the name hobo spider to replace the inaccurate "aggressive house spider"), reports that dry bites are common for hobos. Widows vary in their toxicity with age, health, and gender, and these factors seem to come into play with hobo spiders as well. For example, male hobos pack a more potent venom than females. It is typically the male hobo, wandering away from its funnel-shaped web in search of a mate at the end of summer, that bites people.
People vary considerably in their reactions to venom. Only a minority of people, for example, show any lingering symptoms after a dose of brown recluse venom. I myself have been bitten by recluses a number of times. Though the stinging sensation I felt after a short delay made it clear that I'd been envenomed, I never developed a sore or any systemic symptoms. The whole experience was less painful than a mosquito bite—and, taking into account the possibility of mosquito-borne disease, less dangerous. It may be that hobo venom is similarly selective. After all, its function is to subdue insects, and any effect it has on us comes about because we're related to insects. It would be comforting to think that a few billion years of evolution have put considerable distance between us and our insect kin, but chemically, that's not the case. We are only sometimes different enough to be immune to insect-killing venoms.
Medical journals have attributed a handful of human deaths to the hobo spider. Crawford says perhaps 100 cases of medically significant bites are reported in Washington every year, but he adds that a physician's diagnosis is shaky evidence in the absence of the culprit. Like “recluse bite” before it, “hobo spider bite” has become what Binford calls "a medical dumping ground"—a default diagnosis when a better one can't be found.
Meanwhile, isolated cases have suggested that a few of the hobo's fellow agelinid spiders are occasionally dangerous. The Western grass spider (Agelenopsis aperta) has been implicated in a single human fatality. And a giant house spider (Tegenaria duellica) produced a serious injury in a human being—not because of venomous effects, but because of an allergic reaction.
The agelinidae are, as spiders go, remarkably tolerant of each other. I have seen a spindly male living on the fringes of a female's web, suffering no abuse from his larger mate. Perhaps he was helping to guard the eggs. I have seen, too, an entire bed of wandering jew covered with twenty or so discrete funnel webs, the inhabitants apparently unconcerned about the proximity of neighbors. But I've also seen what happens when two come into conflict. A flurry of legs, then the sudden collapse of one spider. It folds up in the grasp of its enemy. The effect is something like a child's hand crushed in an adult's.
As it happens, this tendency for some agelinids to eat others may help explain why the hobo has apparently harmed people in North America but not in Europe. Darwin Vest, who considered pesticides an irresponsible way to control spiders, examined the question of what predators might naturally control hobo populations. The most effective predators proved to be other spider species, like the cellar spider (Steatoda grossa) and the American house spider (Achaearanea tepidariorum). Most effective of all was the giant house spider, an agelinid with a leg span broad as a human palm. The giant is so closely related to the hobo that the two may interbreed, and it not only preys on the smaller species, but also competes with it for food. Vest suspected it was the giant that kept the hobo out of European houses all along. The giant has, in the last 25 years, established itself in the Pacific Northwest. Rebecca Vest reports that hobo populations in southern Idaho have shrunk noticeably in that same period. It may be that the hobo, though equally venomous wherever it turns up, simply has fewer chances to bite in Europe. And perhaps the same situation will eventually prevail here as the giant house spider, an unrecognized ally long ago suspected of spreading the Black Death, expands its range across America.