While most spider species possess venom, few possess the capability to substantially harm human beings. This article focuses on the most common of these in Colorado, the western black widow spider---the only well-established species with an undeniably dangerous bite (despite a timid demeanor). The brown recluse spider, also infamous for its potent venom, is not native to Colorado, although isolated specimens are occasionally identified as a result of introductions. Other types like sac spiders, wood louse hunters and so-called "hobo" spiders (introduced from Europe) can bite painfully, although the necrotic effects of the latter species' venom may be doubtful.
Western Black Widow
This species, found statewide, belongs to the genus Latrodectus, which includes widow spiders on all continents except Antarctica. The western black widow, like most of its relatives, is small and retiring. Females, who are most likely to bite humans, are usually 1/4 to 1/3 inch in length and characterized by the roundness and shininess of their black bodies and the famous red "hourglass" on the underside of their abdomens. Males, unlikely to ever be seen by people, are far smaller than females, who will occasionally eat them during or right after mating, hence the species name.
Avoiding Widow Bites
Be on the lookout for widow spiders in their preferred habitat: dark, rarely disturbed nooks and crannies in both human (garages, basements, closets, crawlspaces, stacked wood) and nonhuman (rocks, holes) environments. Females rarely bite outside of their nests, which are small, sticky and irregular in shape; even then, they are likely only to be aggressive when actively defending their egg sacs.
Colorado State University Extension reports a range of symptoms following a bite, sometimes initially undetectable but typically announced by an acute pain and eventually redness. People afflicted by the widow's neurotoxin may experience severe cramping, sweating, labored breathing and other conditions. While the bites are rarely fatal, severe cases may warrant medical attention.