Toxicology: Beware the Brown Recluse

Menace though it may be, the black widow spider is something of a has-been. For potency of venom, a little-known spider called the brown recluse is now No. 1. Also called the fiddler be cause of a violin-shaped mark on its head, this species is spreading across the Central and Southern states.

A closely related species in South America has been known for 80 years to cause severe and sometimes gangrenous bites, but the brown recluse (Loxosceles reclusd) was not believed to have such a potential until the late 1950s, when doctors at the University of Missouri identified it as...