Hobo Spiders, Part 3 of 4

Black Widow Spider (Photo by Hodari Nundu)



(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.)


It used to be said that no US spider was really dangerous, and this view held sway well into the 1920s, despite well-attested reports of deaths from the bites of widow spiders. It was only after intrepid biologists like William Baerg and Allan Blair subjected themselves to widow bites in the lab, and suffered horribly, that the prevailing opinion changed. It took thirty more years for scientists to sort out the two syndromes apparently caused by widow bites—one featuring extravagant pain spreading rapidly throughout the body, the other the slow death of the flesh around the bite. Eventually it was demonstrated that the second syndrome should have been blamed all along on the unobtrusive recluse spiders.

That ought to clear everything up, but it hasn't. Specialists are routinely annoyed by pseudofacts claiming that the average person inhales four spiders per year in his sleep or that recluse bite symptoms can be cured with tazers. Many myths mix in a pinch of reality. The blush spider, for example, must have been inspired by the widow, which used to infest outdoor toilets and bite people in the genitals. And the false reports of camel spider venom read like an exaggerated account of the true effects of recluse venom.

It's taken a long time to sort out the truth behind hobo spider bites. They produce symptoms similar to those caused by recluse venom, but they occur in areas outside the recluse's range—the Northwest quadrant of the US and adjacent parts of Canada. After the brown recluse's danger came to public attention beginning in the 1950s, doctors in the Pacific Northwest began to diagnose certain lesions as recluse bites. But these diagnoses didn't fit the known range of the recluse. No member of its genus is regularly found in the northern half of the US.

In the early 1970s, this mystery came to the attention of toxinologist Darwin K. Vest, an autodydact whose work on cobras, rattlesnakes, and other venomous creatures had won him respect in scientific circles. While working at Washington State University's Pullman campus, Vest learned that the zoology department there often received queries about "necrotic arachnidism"—flesh-killing lesions apparently caused by spider bites. Vest tackled the problem by looking into the cases of 75 patients in the Pacific Northwest diagnosed with this affliction. He exonerated spiders in most of these injuries, blaming them on insect bites, cigarette burns, and other causes. Vest surveyed the homes of 22 remaining patients. Collecting by hand and with sticky traps, Vest and his team collected thousands of specimens. None of the homes yielded recluses, but sixteen of them revealed healthy populations of the hobo spider. Sometimes a single sticky trap measuring about 15 by 30 cm would fill with hobos overnight.

The presence of hobos in such numbers was suggestive, but it proved nothing. The average home in any temperate region is likely to host several dozen species of spiders. Most people don't realize that they spend every day of their lives close to spiders, so that seeing one the same day you get a bite or scratch proves nothing.

Vest decided to bring hobo spiders, and several other suspect species, into the lab for tests. He and his team milked live spiders with mild anesthesia and micro-pipettes under a dissecting microscope, working carefully so that the spiders could be released unharmed. The spiders were so small that the capillary action of the pipettes was often enough to draw venom from the fangs. When that technique didn't work, the researchers sometimes resorted to mild electric shock, using a nine-volt battery to make the venom glands contract and prompt the release of a droplet or two. Since each spider produced only a minuscule amount, the researchers had to milk a great many to obtain a workable sample. Their result: The hobo spider venom produced necrotic lesions in rabbits. To confirm this result, Vest shaved the backs of rabbits and held a hobo spider down on each bald patch, forcing a bite. The lesions that formed were similar to those found in human victims.

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