Black Widow Spiders

The Black Widow spider (Latrodectus) is represented by five species in North America north of Mexico. Two of these species, the brown widow (L. geostricus) and the red-legged or red widow (L. bishopi), are restricted to a southern distribution. The red widow appears to be found only in southern Florida. The remaining black widow species were historically considered to be only one species but are now recognized as three distinct species. Identification is based upon the pattern of red coloration on the underside of their abdomen and by their geographical distribution, although the northern and southern black widows may coexist in some states.

The Venom
Although almost all spider species possess poison only a few species produce a venom virulent to be harmful to humans. Black widows are very timid spiders and are not known to agressively bite humans. However, the venom of black widows is a neurotoxin and it may interfere with the nerve impulses to muscle tissue. After being biten from a black widow one may feel painful rigidity in the muscles of the abdomen and a feeling of tightness in the chest. Other symptoms include an increase in blood pressure, a rise in body temperature, nausea, and sweating. Death is uncommon (less than 1% of the reported cases), but in the elderly or very young death may occur from asphyxia 14-32 hours after being bitten. Seek medical attention if you suspect you have been bitten. Without medical attention the symptoms can last 5 days and a complete recovery may take weeks.

Natural History
Black widows belong to the Family Theridiidae, a group of spiders known as the comb-footed spiders because of the specialized comb structure on tarsus IV. This comb is used to create the characteristic tangled web as exemplified by the domestic cob-web spider (Achaearanea tepidariorum) found in many houses throughout the US. The Black Widow spider rarely, if ever, builds their cob-webs inside a house. Black widow webs are an irregular tangled mesh built in dark spots sheltered from the weather. Typical web sites include spaces under large rocks or logs, in holes in dirt embankments and occasionally in barns, outhouses (always check under the seat), and other out-buildings. Females may occassionally kill and eat a male after mating but this is more the exception than the rule. After mating, females will lay several egg massess (gray or tan/yellow depending upon the species) in the web. Females can produce egg sacs the following year with some females having the good fortune to live and reproduce up to three years.

Southern Black Widow
Latrodectus mactans

Northern Black Widow
Latrodectus variolus

Western Black Widow
Latrodectus hesperus

Southern New England to Florida west to eastern Oklahoma, Texas and Kansas. More common in the southern part of the range.

New England and adjacent Canada south to Florida, and west to eastern Texas, Oklahoma and Kansas. More common in the northern part of the range. Western portions of Texas, Oklahoma, and Kansas north to the adjacent Canadian provinces, west to the Pacific Coast states.
Anterior

Posterior 		Anterior

Posterior 		Anterior

Posterior
Ventral side of the abdomen (underbelly) showing the typical variation in the red hourglass markings. Note that anterior half of the marking is wider and the posterior ends are more generally rounded in their shape. The hourglass is usually broken down the middle into two parts.



The hourglass mark is typically complete with both halves triangular with the anterior triangle longer and broader than the posterior region.
Egg sacs nearly spherical with a grayish tinge, even when freshly made. Egg sacs typically yellow or tan and more pear shaped. Egg sacs typically creamy yellow to tan, never gray and more pear shaped.
















   Specific components of the venom (alpha-Latrotoxin) can bind to presynaptic cholinergic receptors causing permanent blockage of the nerve synapse. In some cases the venom can create extreme facial contortions (known as facies latrodectismi) where the red-flushed face becomes sweat-soaked with swollen eyelids, inflammed lips, and contracted masseter muscles.

   The lethal dose of the black widow venom is reported as an LD50 of 0.0009 mg venom / gram body weight (mouse). LD50 is a standardized amount representing the dose required to kill 50% of the experimental animals,
Click here to return to the black widow page.