SLO Pest & Termite
Serving San Luis Obispo and
Santa Barbara County
There are two species of widow spiders in California, the western black widow and the brown widow. Both are in the genus Latrodectus and are characterized by a similar body shape, reclusive habit, and haphazardly constructed cobwebs.
The western black widow spider—a native species—is widespread and is the spider posing the greatest threat to humans in the Western United States. It is well known in many localities, and nonprofessionals can identify it easily.
In the first decade of the 21st century, the non-native brown widow became established in Southern California, and although it isn’t nearly as dangerous as the black widow, it causes alarm because of its potentially deadly relative.
Several species of black widow spiders are common in North America, but in the Western United States, the only species is the western black widow, Latrodectus hesperus. Its habitat ranges from British Columbia to Mexico and throughout the Rocky Mountains to the western portions of the Great Plains.
In California, it is a common desert spider that is able to survive very hot, dry conditions. However, black widows also can be found in mountainous terrains above the 5,000-foot elevation in Southern California where snow covers the ground every winter. Outside of California, they are common in urban Colorado and in Central and Eastern Washington state.
Because the holes, cracks, crevices, trash, and clutter associated with human structures attract the western black widow, these spiders are often very common around homes, barns, outbuildings, and rock walls. In supportive habitats, a mature female can be found every few feet.
The mature female western black widow spider is about 1/2-inch long, not including the legs, and has a rounded abdomen and very characteristic coloration. She is shiny jet black all over her body and legs except for a red pattern on the underside of the abdomen, which looks, in perfect specimens, like an hourglass. Some specimens have a brownish or plum-colored tinge, but usually these are females that are so well fed the black pigment on the abdomen has expanded until it looks brown instead of black.
The red hourglass can vary from two perfect triangles whose points merge to make a perfect hourglass to two triangles separated by a space, a triangle and small bar, or just minimal almost imperceptible red coloration. The false black widow, which is discussed below, is chocolate brown and never has red coloration, although many people frequently mistake it for a black widow.
As easy as it is to identify an adult female black widow, the immatures look nothing like the mother. When baby black widow spiderlings emerge from their egg sac, they have tan legs and tan cephalothorax, the body part to which the legs attach, while the abdomen is mostly white with a few black spots.
As the spider grows, the background coloration of the abdomen becomes olive gray, and there is a longitudinal white stripe on the top of the abdomen and three diagonal stripes on the flanks with a small black dot at the uppermost portion of each diagonal stripe.
Like all spiders, as the spiderlings grow larger, they molt in order to shed their restrictive exoskeleton. With successive molts in females, the white stripes become thinner, the olive gray darkens toward black, and eventually the spider acquires its well-known black coloration. Some mature females retain one or two conspicuous, indented white lines on the front surface of the abdomen that look like a corporal’s chevrons.
In the youngest spiders, the space where the hourglass develops starts off being a whitish shield. As the spider grows and goes through several molts, the color of this shield turns from white to yellow to orange to red and changes from a shield with thick middle to a hourglass with a thin, tapered middle.
In contrast to the female, the male black widow retains the coloration of the juvenile. After it matures, it stops eating, and its abdomen shrinks, because its only task at that point is to mate. The male still retains its one longitudinal stripe and set of three diagonal flank stripes on each side the abdomen. The males are much smaller than the females in body length although sometimes their legs are almost as long as the adult female.
One more variation involves the longitudinal stripe that runs up the middle of the top surface of the abdomen in immature black widows. Sometimes it has a vivid red stripe within the confines of the white border. This coloration can cause anxiety for anyone who isn’t familiar with widow immatures, because they might incorrectly identify it as the Australian redback widow spider. This widow has red markings on the top but otherwise is uniformly black rather than mottled as an adult. Redback widow spiders aren’t found in North America.
The egg sac of the western black widow is a yellowish, tear-drop shape, tapered at the top and bulbous at the bottom. The margins of the sac are well defined, as opposed to some spider egg sacs, which look like fluffy cotton balls, making it difficult to determine exactly where the egg sac starts. The egg sac is very tough and difficult to rip apart.
A female western black widow typically lays about 300 eggs per sac. Because they can store sperm from their first mating, they can produce more than 10 egg sacs without subsequent matings without a decrease in the number of eggs or a reduction in the percentage of eggs that will hatch into spiderlings.
The western black widow spider is found almost everywhere in California where people live. Although they can be found inside homes, black widows typically are outside, around the home and in clutter. In the garage, they usually make webs by doors, near vents, and in other places where they are guaranteed lots of insect traffic for food. Because most people don’t tolerate large numbers of insects in their living spaces, widow spiders usually won’t find sufficient prey to survive inside homes.
Black widows are shy spiders that seek retreats such as a hole between two bricks or a pipe hole in a wall where they can hide during the day and then come out at night. In natural settings, you’ll often find them in rodent burrows and rock faces. The spider makes a web of tangled silk extending from this retreat hole.
The web doesn’t have a very recognizable pattern although it does have vertical support threads above and below the central areas where the spider sits while it waits for prey at night. The lower support threads also alert the widow to the presence of a prey item blundering into the web. In most cases the widow spider will seek a retreat near the ground as the home base for her web, which connects to the retreat, allowing the spider to emerge to catch both flying and crawling prey at night. However, some spiders will make a retreat well above ground level, such as in the eave of a house, and then drop down 10 or more feet before building their web.
Widow spiders come out at dusk. After making improvements to their existing web, they take up a position in the middle, their underside facing upward, to wait for prey. Any large disturbance of the web that indicates something larger than a prey item causes the spider to quickly move toward the safety of its retreat.
The silk of a mature black widow is very strong; running a finger through the web that a large spider has made results in an audible ripping sound. During World War II, black widow silk was used to make crosshairs for gun sights.
The black widow bite itself is painless or may feel like a little pinprick. Almost all medically important black widow bites are from the adult female, which is much larger than the male; the female also has stronger biting muscles and a larger venom reserve. At the site of the bite, you might see a little red mark or red streaking away from the bite. Within an hour, symptoms start to appear.
Bite victims might suffer from some but not all of the following symptoms: rigid stomach muscles, which some medical professionals have misdiagnosed as appendicitis; sweating, sometimes of just the bitten body part, such as a bite to the hand that results in only the arm sweating profusely; pain that can be local, radiating, or regional; urine retention; and—less commonly—numbness, agitation, fever, and patchy paralysis. Another symptom is bite victims will move or rock back and forth incessantly to try to lessen the pain from the venom injection process. However, these symptoms are the most severe manifestation; many black widow bite symptoms merely resemble the flu. Black widow bites don’t cause conspicuous swelling, necrosis, or deterioration of tissue around the bite.
As a neurotoxin, the venom of a black widow affects the nerve-muscle junction in the body. Normally the body’s neurons work like a light switch; they make the muscle, or “light,” turn on and then off again, so that the muscle can relax and be ready to contract again if needed. The venom causes the muscle to repeatedly contract. It would be like flipping on a light switch and not being able to turn it off again.
If bitten, seek medical attention immediately. You can place a cold pack on the bite to relieve the pain. An antivenom for black widow bites is available that works for all species that have been tested, worldwide. Response is fast, and bite victims can go from intense pain back to normal in 30 minutes. The antivenom is based on horse serum, so physicians need to monitor for anaphylactic shock. American physicians are somewhat reluctant to use antivenom for this reason and might prefer to have the bite victim simply endure the symptoms, which can be similar to a bad flu episode and that usually dissipate in a few days.