The hobo spider is now widely recognized as dangerous. The Centers for Disease Control lists it as such, as do medical textbooks and publications like the The Journal of the American Medical Association
. Doctors know the signs of hobo venom—a blistering wound ringed with yellow, like the moon in a halo of smog, often accompanied by headaches and, in rare cases, disturbed thinking.
But skeptics remain. In 1998 evolutionary biologist Greta Binford of Lewis and Clark College and some of her colleagues at the University of Michigan tried to replicate Vest’s experiment. When they injected hobo spider venom into rabbits, however, the rabbits developed nothing worse than a red bump. Like several other prominent skeptics, Binford notes that the hobo spider is rarely caught in the act of biting and then taken to a competent specialist for identification. Its appearance is unremarkable, so its supposed victims can’t be expected to distinguish it from dozens of other spiders. In Europe the hobo has never been implicated in human injuries, although its venom is nearly identical to that of North American hoboes.
In four of the cases that Darwin Vest investigated, a hobo spider was captured or crushed near the victim. But Vest noted that one of these victims—the 42-year-old woman mentioned at the beginning of this story—had a history of phlebitis, a circulatory problem. According to Rick Vetter, an arachnologist at the University of California at Riverside, phlebitis sometimes causes necrotic lesions. 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 confirmed white-tailed spider bites and found not a single necrosis. Vetter would like to see hobo bites subjected to a similarly rigorous study. He points out that a mistaken diagnosis can have serious consequences: Certain skin cancers, for instance, look like necrotic arachnidism and can be fatal if left untreated.
Even if hobo spiders are responsible for the lesions, their bites may not always be 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. Vest’s sister, Rebecca, who worked with him in his investigations, reports that hoboes often give dry bites. 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 hoboes pack a more potent venom than females. It is typically the male hobo, wandering away from its web in search of a mate at the end of summer, that bites people.
People vary considerably in their reactions to venom. I have been bitten by brown recluses a number of times. Though the stinging sensation that developed after a short delay made it clear that I’d received venom, I never developed a sore or any systemic symptoms, and the same is true of most bite victims. 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. It would be comforting to think that a few hundred million years of evolution have put considerable distance between us and our insect kin, but only some of us are immune to insect-killing venoms.
Although hundreds of medically significant cases are diagnosed as spider bites in the Pacific Northwest each year, hard evidence is elusive. Rod Crawford, curator of arachnids at the Burke Museum of the University of Washington, notes that a handful of human deaths have been attributed to the hobo spider but that even a physician’s diagnosis is shaky evidence in the absence of the culprit. Like the recluse before it, the hobo has become what Binford calls “a medical dumping ground”—a default diagnosis when a better one can’t be found.
Agelenids are remarkably tolerant of one another, as spiders go. I have seen a spindly male living on the fringes of a female’s web, suffering no abuse from its larger mate. Perhaps he was helping to guard the eggs. I have seen, too, a bed of wandering Jew covered with 20 or so 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, which 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 agelenids 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 false black widow (Steatoda grossa) and the American house spider (Achaearanea tepidariorum). Most effective of all was the giant house spider, an agelenid with a leg span as 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. In the past 25 years, the giant house spider has 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.