All spiders in the United States are venomous predators. However, only a few spider species are venomous to people. A spider has two distinct main body parts: an abdomen and a cephalothorax (combined head and chest area). Daddy longlegs, or harvestmen, which have their head, thorax and abdomen fused into a single body part, are not spiders. A spider has eight legs, no antennae and spinnerets on the abdomen for spinning silk.
Of the approximately 3,500 species of spiders identified in the U.S. and Canada, no more than 21 species pose serious health risks to people. Spider species known to pose risks to people in the U.S. include five species of widow spiders and 13 species of recluse spiders. Three other species, the hobo spider and two species of yellow sac spiders, might pose some risks to people, but the scientific community does not agree whether they pose risks.
Widow and recluse species do not occur everywhere throughout the U.S. For example, only one recluse species (brown recluse) and three widow species (southern black widow, western black widow and introduced brown widow) occur in Oklahoma and north Texas. Each of these four species have distributions encompassing about one-quarter to half of the U.S. The two remaining widow species in the United States include the red widow, which occurs only in Florida, and the northern black widow, which occurs throughout much of the eastern U.S. and into southern Canada. Except for the brown recluse, most native recluse species occur only in southwestern states. Non-native recluse spiders in the U.S. include the Mediterranean recluse, which has been introduced into several states, and the Chilean recluse, which has been introduced into California.
Among widow spiders, only female widow spiders pose risks to people; males do not. When mature, southern and western black widow females have shiny black abdomens with a red or orange hourglass marking on the bottom side of their abdomens. When immature, female widow spiders frequently have other markings on the tops and sides of their abdomens; some western black widows maintain markings on the tops of their abdomens into adulthood. A brown widow typically has a brown, black and white patterned abdomen with a red, orange or yellow hourglass marking underneath. Widows normally are outdoor spiders but occasionally invade houses.
Both female and male recluse spiders pose risks to people. Other than a dark violin shape on top of the cephalothorax, a brown recluse does not have a pattern on the body or legs. A brown recluse can be brown, light tan or even blackish-gray. Often the abdomen is a lighter or darker color than the cephalothorax. Several harmless spider species, such as cellar spiders and pirate spiders, also have a dark violin shape on top of the cephalothorax but other characteristics vary from the brown recluse. Brown recluses commonly live in buildings in close proximity to people. Fortunately, widow and recluse species are normally nonaggressive and try to avoid confrontations with humans, which decreases the likelihood of bites.