Spiders are in the same group as ticks and mites: the Arachnids. 45,700 different species exist in this group and are found in nearly every habitat worldwide. Spiders play a significant beneficial role in our environment by consuming nuisance insects and other arthropods that often annoy humans. There are about 2,000 identified species of spiders in the U.S.

All spiders use venom to kill and digest prey. The specific components of spider venom varies significantly between species, but most spiders are of little public health significance since few possess venom that will affect humans. Still, spider bites can result in uncomfortable pain and swelling. Spiders usually only bite people in self-defense, or because someone has accidentally disturbed a spider in an unexpected area.

The Black Widow and Yellow Sac Spiders are the only two species of public health importance in Alameda County. The Black Widow Spider has a powerful neurotoxic venom that may require medical intervention if bitten. The Yellow Sac Spider is responsible for inflicting many harmless but painful bites to humans, compared to most other species. Bites result when this spider drops on a person and uses its fangs for stability.

Brown Recluse Spiders (Loxosceles reclusa), a well-known notorious species, does not occur in California or Alameda County. However, the Chilean Brown Recluse Spider (L. laeta) and the native Desert Recluse Spiders (L. deserta) are closely related species that are found in southern California and can inflict similar venomous bites.

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A Guide to Spiders

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