Spiders as a whole are seldom aggressive, generally biting only when threatened or injured. Nearly all spiders have venom glands, but the toxicity of the venom to humans is so low as to be insignificant. The severity of the reaction to a spider bite is influenced by the species of spiders, the area of the body where the bite occurs, the amount of venom injected, and depth of the bite. Only two spiders occurring in this area can cause poisoning in humans: the Northern Black Widow Spider (Latrodectus variolus) and the Brown Recluse Spider (Loxosceles reclusa).

If bitten by any spider suspected of being poisonous you should do the following: (1) remain calm,

(2) collect the specimen, if possible, for a positive identification (specimens may be sent to Insect Diagnostic Lab, Department of Entomology, Michigan State University, East Lansing, MI. 48824),

(3) seek medical attention and contact your local poison control center immediately (consult the inside cover of your phone directory, or dial for operator assistance), and

(4) if unable to get professional assistance, follow the first aid instructions given in this section.

The Northern Black Widow is found throughout Michigan's lower peninsula. Its preferred habitat is marginal land with sparse vegetation: in stumps, hollow logs, and piles of debris. Occasionally it may be found indoors. Its relative, the Southern Black Widow (L. mactans), is considered much more dangerous because of its larger range and willingness to invade man's habitat.

The poisonous female Northern Black Widow is approximately 1/2", excluding the legs. The overall length may be in excess of 1 1/2" at maturity. It is shiny black or brown-black with a spherical abdomen having two red triangular shaped marks (a "split- hourglass") on the underside. Its southern relative has the true hourglass marking. The males are harmless, and only approximately 1/2 the females' size. They have light streaks on the sides of the abdomen, with no red markings. Juveniles of both sexes resemble the male and are harmless.

The female spins a tangled coarse web in dark undisturbed places, and encloses her eggs in a whitish egg sac which she guards until the spiderlings hatch. Being shy and nocturnal, she will not leave her web voluntarily and will attack only if her web is disturbed. Her bite is slight, and may not be felt. It is followed by burning, local swelling, and redness. Two puncture points may be visible. The pain intensifies after 1-3 hours, and may last up to 48 hours. The venom is neurotoxic, effecting the human nervous system, and cramps in the legs, arms and chest occur concurrently with the local swelling. Additional symptoms may include headache, nausea, tremors, stuttering, and a slight rise in body temperature. The severity of the symptoms depends on the age and sensitivity of the victim, with small children most severely affected.

First Aid for Northern Black Widow Bite: If bitten by a Northern Widow, contact your physician immediately. The physician may give a specific antivenin or calcium gluconate.

The Brown Recluse, is found east of the Rockies and as far north as Ohio, Indiana, and Illinois. No specimen has been collected in Michigan. The males and females are similar in appearance, being 1/4-1/3" in length and are equally toxic. With legs extended, they may be as large as a half dollar. Their color varies from orangish-yellow to dark brown, and the body is covered with short hairs. The legs are long and slightly darker than the body. Distinguishing characteristic include six eyes (most spiders have eight) arranged in 3 pair, called diads, and a violin shaped marking on the cephalothorax (the head and thorax, which bears the legs). Development from egg to mature adult requires about 1 year, with total lifespan years.

Though its natural home is in sheltered places outdoors, it is commonly attracted to buildings of all types, especially those that are dry, littered, and contain many insects for prey. Their web is irregular, and it is used as a resting place rather than a snare. The Brown Recluse is a hunter, leaving the web and actively searching for prey. Egg sacs are hung in the web and are guarded by the female. Bites usually occur when a person puts on shoes or clothing in which the spider is hiding. Because of its habits, it is often inadvertently transported by man.

The amount of venom injected and the sensitivity of the victim are both factors determining the seriousness of the bits. The venom is cytotoxic, damaging the flesh of the victim. There may be no initial pain felt, or a stinging sensation may appear immediately. Intense pain follows, lasting 2-8 hours. A small blister forms at the bite, and a large area surrounding the bite swells and reddens, becoming firm to the touch. Victims sometimes experience nausea, abdominal cramps, stiffness of joints, and fever. The tissue in the infected area becomes ulcerous, sloughs away, and results in a pitted area that eventually fills with scar tissue. The wound heals slowly, usually in 6 to 8 weeks, and leaves a permanent scar.

First Aid for Brown Recluse Bite: If bitten, call your local poison center immediately or see a doctor. Apply antiseptic solution to prevent infection, and ice packs to the surrounding area. Corticosteroid injections may be given by a doctor to curtail reactions to the bite. No specific antivenom is available. The Brown Recluse bite is especially dangerous to children, the elderly, and those in poor physical condition.

Another spider worth mentioning is the tarantula. There are approximately 30 species of tarantulas found in the southern U.S. Because of their popularity as pets there is a chance of humans being bitten. Though the venom is rarely more poisonous than that of bees, the local poison center should be contacted immediately should anyone experience a bite.

General sanitation aids in preventing a spider infestation of any type. Caulking cracks and crevices, tight fitting screens and doors all prevent entrance into buildings. Since many spiders can live several months without food, simply eliminating the food source (insects) does not bring immediate results. Should an infestation warrant chemical control, follow all instructions on the label and observe any stated precautions.

For a complete listing of suggested control options for all home, yard and garden insect pests contact your local Extension Service, found under local government in the phone book.

Read and follow instructions on the pesticide label. Heed all warnings. Check with your physician if you have any concerns regarding your personal health risk. Revised by Tom Ellis, M.S., Department of Entomology



MSU 2003