Arachnophobia: Should You Be Afraid of Spiders?
Fear of spiders, arachnophobia, is a widespread problem in western societies. This fear is not only of spiders but can extend to all land arthropods with more than six legs—the arachnids. It can range from simply avoiding arachnids to panic attacks, high heart rates, and flight behavior. It's not clear if arachnophobia is a learned response or something instinctive. However, it is much commoner in western societies than elsewhere. In other parts of the world, spiders may even be part of the diet. Fear of spiders can result in stress, wasted time, and environmental costs through overuse of insecticides. Insecticides are often used to kill spiders, but unless you directly spray the spider or its web, insecticides have little effect.
What are arachnids?
Arachnids include spiders, mites, ticks, scorpions, harvestmen, whip scorpions, camel spiders, and vinegaroons. Of these, only spiders, mites and ticks, and scorpions have venom. Arachnids differ from insects in several ways. They have eight or more legs and two major body parts, the cephalothorax (a fused head and thorax) and abdomen. They don't have wings or antennae. Spiders are different from other arachnids because they can spin silk from structures called spinnerets at the end of the abdomen. They are much more commonly found in homes and gardens. Spiders do not have chewing mouthparts or legs modified to capture prey, so they rely on venom to both incapacitate their prey and begin digestion..
In California, spiders commonly encountered around buildings include jumping spiders, tarantulas, orb weavers, sheet web spiders, cellar spiders, wolf spiders and widow spiders.
Which spiders are dangerous?
Culturally, we're taught to fear certain spiders more than other arachnids, except for scorpions. Among these are black widows, brown recluse, and hobo spiders. Many issues are attributed to spider bites. However, in most cases of “spider bites” spiders were not actually involved. A sharp pain followed by development of a lesion or sore is not a sign of a spider bite but could instead be due to a staph or strep bacterial infection.
Humans are largely unaffected by the venom of most spider species. During the entire 20th century, only about 100 deaths from spider bites occurred worldwide.
Only a few spiders have venom that is dangerous to humans, including the Sydney funnel web spider of Australia, widow spiders (Latrodectus), Brazilian wandering spider, and the African sand spider.
The only potentially dangerous spiders in California are widow spiders. Both the native western black widow spider and the invasive brown widow spider occur in California. These are smooth bodied, shiny spiders, with long slender legs, and a bulbous abdomen in the females. They appear to be hairless. Females have a bright hourglass shaped mark on the underside of the abdomen.
Widow spiders make messy webs in sheltered sites. The silk is several times as strong as silk produced by other spiders and has a different feel when touched. These tough silk threads are also smooth and homogeneous. During WWII widow silk was used to make the crosshairs in gunsights for the U.S. Army.
Widow spiders are abundant in urban and suburban habitats. In some regions of California there might be as many as 20 to 30 black widows per property. They are shy, reclusive spiders and live in quiet dark places. They generally retreat into hiding places when confronted by any animal larger than they are. Actual widow bites are rare and rarer still do they inject venom. Instead many widow bites are what are called dry bites. Black widow venom is neurotoxic and can cause fever, muscle and joint aches and muscle cramping. It does not cause sores, and it is rarely fatal. There is an effective antivenom available for treating black widow bites.
Recluse spiders also have bad reputations. These are ground-dwelling spiders in the genus Loxosceles that somewhat resemble wolf spiders. There are several species across the U.S., but only two occur in California, L. deserta and the introduced L. laeta, but most people focus on the brown recluse, L. reclusa. California has the largest number of diagnosed brown recluse spider bites in the U.S., yet the brown recluse does not occur within 1,000 miles of California. Essentially, all recluse bites in California are probably bacterial infections.
True to their name, recluse spiders are very shy and occur in dark quiet places. Recluse spiders appear to be even more reluctant to bite than widow spiders. In an example of this, in the early 2000's a Kansas family began collecting spiders in their century old farmhouse. After one year they had collected more than 2,000 brown recluse spiders. No one had ever been bitten.
Hobo spiders, Eratigena agrestis, are found in Washington and Oregon, and perhaps far northern California. This spider is also a ground dweller and resembles a small wolf spider. Online, its bite is said to be as dangerous as recluse spiders, causing necrotic sores. This is also an urban myth. Sores attributed to hobo spiders are generally caused by bacterial infections.
For more information on spiders, see Dr. Kimsey's webinar on the UC IPM YouTube channel.