Of the many spider species found in California, only a few present any danger to humans, and even those species are unlikely to cause serious harm.
Western Black Widow Spider
The western black widow spider (Latrodectus hesperus) is the most common California spider with potentially dangerous venom. The female black widow has a glossy black body about 1/4 to 1/3 inch in length and a red or orange hourglass-shaped marking or similar marking on the underside of the abdomen. Males of the species are about one-half the size of females and brown with lighter-colored markings on their abdomens.
Female black widows build tangled webs in secluded locations, sometimes inside buildings, and they spend almost all of their time in the webs, waiting for prey and defending their eggs.
The venom of the female black widow contains a neurotoxin that can cause breathing difficulties, severe pain, nausea, high blood pressure and muscle weakness. The bite is rarely fatal for healthy adults, but vulnerable individuals, such as children and elderly individuals, may be at risk.
Although sightings of brown recluse spiders (Loxosceles reclusa) are often reported in California, no known native populations of them are in the state, and the reports are likely to be misidentifications. Other recluse species, however, are known to live in the state, and some are potentially dangerous.
The desert recluse (Loxosceles deserta) is most common in the Sonoran and Mojave deserts and in the lower San Joaquin Valley. The Chilean recluse (Loxosceles laeta) is sometimes found in Los Angeles County. Both species can be identified by their six eyes, which are arranged in pairs; recluse spiders are usually brown, with fine hairs covering their abdomens and a body length of 3/8 inch or less. These species are night-hunting spiders, and when they live inside structures, they spend the day hiding in out-of-the-way cracks and crevices.
The venom of all recluse spiders contains a toxin that sometimes causes the death of tissue around the site of a bite. The resulting wound is slow to heal, and in rare cases, the bite may cause severe tissue damage or even death. Most bites, however, do not produce a serious reaction.
Sac spiders are typically found outdoors, but some species, including the yellow sac spider (Cheiracanthium mildei), often wander into homes when the weather is cool or when a plentiful supply of small insects is indoors. Sac spiders are hunting spiders that don't build webs but instead roam at night in search of prey; during the day, they stay away from activity and may lurk high on walls or ceilings. They are small, usually about 1/4 inch in body length, and are pale tan, yellow or green.
Sac spider venom includes a cytotoxin that can cause a reaction similar to, but typically less severe than, that of a recluse spider bite. The sac spider bite is usually immediately painful, and sometimes a blister and slow-healing sore will develop at the site of the bite. Reactions to sac spider bites are generally not significant except in individuals who are especially sensitive to the venom.
If you suspect you've been bitten by a recluse spider or a sac spider, then use over-the-counter pain medication to relieve pain, apply ice to the site of the bite to reduce swelling and elevate the body part that was bitten. Medical attention is generally not necessary unless the bite site shows signs of infection or you begin to experience any whole-body symptoms.