Spiders are arachnids, a group of arthropods that also includes scorpions, harvestmen, mites, and ticks. Approximately 3,000 species occur in North America.

Spiders, like insects (another group of arthropods), have jointed legs and a hard external or outer skeleton. Spiders have four pair of legs, with a body divided into two regions (cephalothorax and abdomen), while insects have three pair of legs and the body divided into three regions: head, thorax, and abdomen. Spiders have no wings or antennae, but have enlarged, sharply pointed jaws called fangs (chelicerae).

All spiders are predators. They feed on a wide variety of insects and other soft-bodied invertebrate animals. Spiders attack and subdue their prey by biting with their fangs to inject a poison. As predators spiders are beneficial.

All spiders spin silk, but the silk is used in a wide variety of ways. Most spiders construct a silken case to protect their eggs, but not all spiders make a web. A few use silk threads much like a parachute to aid in dispersal on wind currents, e.g., balloon spiders.

All spiders are poisonous, but fear of all spiders is unwarranted because most are either too small or possess poison that is too weak to harm humans. Only a few have bites that are dangerous to humans. However, the bite of these few species can cause serious medical problems and possible death under certain circumstances.

The most dangerous spiders to humans in North America are the widow spiders (usually known as black widows) (Fig. 1), the brown spiders (also known as the fiddlebacked spiders), the tarantulas, an innocuous-looking sac spider, and a funnel-web spider.

 Fig. 1.   Black widow feeding on a fly.

Five species of widow spiders occur in North America. However, the single species occurring in Washington is the western black widow Latrodoctus hesperus. The adult female is a velvety jet black, but males and immatures are striped with white or yellow. The underside of the abdomen of the adult female usually shows two reddish markings, often joined to resemble the shape of an hourglass. The back of the abdomen is usually entirely black, but may be marked with a broken stripe of white, red, or yellow spots.

An adult female, including legs, is 3—4 centimeters (about 1¹/₂ inches) in diameter. This species is usually associated with dry, undisturbed piles of firewood, old lumber, dry crawl spaces, outbuildings, rock piles, or bales of hay. Poorly constructed wooden pit privies are favored haunts for these spiders. Widow webs are poorly defined, amorphous sheetings of very strong, fine silk.

The bite of the adult female is more toxic than that of juveniles or males. However, widows are shy, retiring spiders and bite only reluctantly, usually only when molested.

Widows are more aggressive when they are protecting an egg sac. The bite of the widow spider causes little immediate pain and may go unnoticed. Slight local swelling and reddening at the bite site are early signs, followed by intense muscular pain, rigidity of the abdomen and legs, difficulty in breathing, and nausea. There is little first aid advised other than cleaning the bite and calming the victim. Consult a physician as soon as possible. Pain can be relieved with injections of calcium gluconate. In untreated cases symptoms generally fade in 2—3 days. Widow bites are more dangerous if the victim is a small child or an elderly person.

Fig. 2.. A female brown recluse or violin spider, so called because of the dark brown violin-shaped pattern on the cephalothorax.

Fig. 3. The shaded pattern shows areas where the brown recluse spider is well established. Reports of spiders outside these areas from as far north as Maine and Minnesota, and as far west as southern California are based on specimens known or presumed to be carried there by commerce.

These spiders also are known as fiddlebacked or violin spiders, as they have a dark violin pattern on the front portion of the body. They have only three pair of eyes instead of four like most spiders. Their overall size is 2—3 centimeters (³/₄—1¹/₄ inches) in diameter. Brown recluse spiders vary in color from tan to dark brown. They readily enter human dwellings and hide during the daytime in baseboard or ceiling cracks, behind or in furniture, or in undisturbed piles of clothing.

The bite of the brown recluse spider either may go unnoticed with no aftereffects or may be followed by a severe localized reaction characterized by scabbing, sloughing off of affected tissue (tissue necrosis), and very slow healing. Again, as with other suspected spider bites, consult a physician if pain and other discomfort follow the bite.

Fig. 4. Tarantula crawling in the desert in Central America.

These spiders are normally nonaggressive, and they rarely bite. Even if they do, their bite is not usually considered to be dangerous and causes little lasting pain or few serious health problems.

The hazard of these spiders is a physiological fear because of their large size, and the risk of developing a hypersensitive or allergic response to their body hairs. When agitated or uneasy, tarantulas rake their abdomen with their hind legs to throw very fine abdominal hairs in the direction of danger. These hollow, needlelike, barbed hairs readily penetrate human skin to introduce a toxic material that can cause a serious skin rash, an allergic response, and possible anaphylactic shock. Anyone handling a pet tarantula should recognize these potential hazard problems.

Fig. 5. Aggressive house spider in a well-defined funnel web.

In Europe, members of the genus Tegenaria are often called house spiders since they are common inhabitants of houses and adjacent vegetation. Only a single species of Tegenaria, T. chiricahuae, is native to the United States. It occurs in caves and other dark places in Arizona and New Mexico. The other six species of Tegenaria found in the United States are of European origin. They were probably introduced to this country very early through commerce. Three species occur in the Pacific Northwest.

Tegenaria domestica, the domestic house spider, is common in both the Northeast and the Northwest, while Tegenaria gigantea, the giant house spider, has been collected to date in the Pacific Northwest and in Winnipeg, Canada. Neither of these two species are known to cause serious bite reactions in humans (Fig. 6). T. gigantea has been encountered more frequently than T. agrestis in the last few years.

Fig. 6. Domestic house spider, Tegenaria domestica, in the funnel web.

Fig. 7. A sexually mature male Tegenaria agrestis.

Sexually mature males and females are abundant from mid-summer (July) through fall.

During this period males tend to wander relatively long distances in search of females. Eggs are laid into a spherical silken sac spun by the female, usually in September or October in the Pullman area. The sac is then placed within or adjacent to the funnel, usually on the underside of a rock or other object. This sac is usually covered with a thin layer of soil, wood chips, or other debris, including prey. The debris coated sac then is often covered with another layer of silk. Eggs hatch the following spring. Most Tegenaria molt about 10 times over a span of 2 years before reaching sexual maturity. Immatures are commonly found wandering in the spring searching for web sites.

Investigations of aggressive house spider bites show the venom produces skin injuries, or lesions, similar to those produced by the brown recluse. Therefore, ulcerating lesions of this type occurring on humans in Washington, Oregon, and Idaho are probably due to bites by the aggressive house spider. Surprisingly, males are somewhat more venomous than females.

Bites commonly occur as a spider is squeezed against the body when a homeowner picks up a firewood log with a spider on it or when a spider is located in an article of clothing and is squeezed when the clothing is put on.

The initial bite is not painful. It has been described as producing a very slight prickling sensation. However, a small, insensitive, hard area appears within 30 minutes or less, and is surrounded by an expanding reddened area of 5—15 cm in diameter (2—6 inches) (Fig. 8).

Fig. 8. Typical bite sequence pathology for the aggressive house spider.

Fig. 9. A Tegenaria agrestis bite on finger.

Fig. 10. Sac spider, Chiracanthium inclusum.

Fig. 11. Hololena s p., a funnel-web spider common in eastern Washington.

Fig. 12. A comb-footed spider.

Fig. 13. A trapdoor spider.

Fig. 14. A wolf spider with young on her back.

Fig. 15. A jumping spider.

Fig. 16. Garden spider in her web.

Fig. 17. A crab spider that matches the color of the flowers on which she rests.

Fig. 18. A sheetweb spider.

Fig. 19. A hackled band weaver.

Spiders are far more beneficial than they are dangerous. The benefits we realize from spiders preying on insects, mites, and other spiders far outweigh the low potential health hazard to humans.

Most spider problems can be solved without the use of chemicals. The first, and perhaps best, suggested controls are mechanical. Inspect door and window casings in the house for closeness of fit. Repair holes large enough to admit spiders. Other areas to inspect include entry points of water pipes and electrical lines. Caulk any cracks and other small openings.

Before bringing firewood into your home, inspect it for spiders or their egg sacs. In addition, keep woodpiles and other debris away from the house. Use a good vacuum that will readily remove all spiders and webs from corners and nooks. Crush spiders by stepping on them as they run across floors, or capture them with your vacuum; they will die rapidly in the dry, enclosed bag. Depending on the type of vacuum used, it might be best to dispose of the vacuum bag immediately after capture so the spider will not escape.

If spiders are numerous and constantly entering the house, you may want to use an effective insecticide to kill the unwanted intruders. Chemical controls for these pests are not included here because products in the marketplace and registration status thereof change too frequently. Chemical recommendations can be obtained from the Pacific Northwest Insect Control Handbook which is revised annually. Information from this book is available from field representatives, County Extension Agents, and your regional Plant Diagnostic clinics (located in Prosser and Puyallup). Do not exceed the recommended dosage given on the label. Materials are probably best applied only along baseboards, door casements, and corners, and only where spiders are present.

For more detailed information on the aggressive house spider please consult the following paper available at the Washington State University Library. Akre, R.D. and E.A. Myhre. 1991. Biology and medical importance of the aggressive house spider, Tegenaria agrestis, in the Pacific Northwest (Arachnida: Araneae: Agelenidae). Melanderia 47: 1-30. Copies can be purchased from Dr. Richard Zack, Department of Entomology, WSU Pullman, WA 99164-6382.