Black Widow Spiders

Common Name: Southern black widow spider
Scientific Name: Latrodectus mactans Fabricius
Order: Araneae

Description: The body of the female, consisting of two regions (cephalothorax and abdomen), is typically shiny black with red markings. The red is usually an hourglass-shaped mark on the underside (venter) of the abdomen, but this may be reduced to remnants. Usually there is a single red spot just behind the spinnerets, sometimes a row along the back. There is much variation in body color with southern and western specimens being more strikingly marked than northern and eastern ones. The male has the abdomen narrower, with white lines along the sides which is shown by immatures as well. Young spiderlings are orange and white, and acquire more black in later developmental stages (instars) until some have little or no red except the hourglass markings. Immatures have the abdomen gray with white curved stripes. The body length of the female is about 3/8 inch and that of the male is about 3/16 inch, but sizes within different geographical populations can vary widely.

There are more than 25 Latrodectus species world wide. Contrary to common belief, the female does not consume the male in most situations, except when held together in cages from which the male cannot escape. Examination of the genitalia is the proper way to identify species in this group. The species in Texas can be separated to some degree by the shape of the red markings on the abdomen and the location.

The western black widow, Latrodectus hesperus Chamberlin & Ivie, usually has the hourglass marking connected or complete with the anterior triangle larger and wider than the posterior triangle. This species largely displaces the southern black widow in the western half of the state. In southwestern Texas through the Lower Rio Grande Valley and adjoining parts of Mexico, specimens of L. hesperus have been found in which the adults retain their brilliant immature colors. Further west, the coloration of the species appears to grade back to black.

The northern black widow, Latrodectus variolus Walckenaer, usually has the hourglass divided, typically with red spots on the dorsum and white lines on the sides. This species occurs throughout much of the eastern half of the U.S.

The brown widow, L. geometricus C. L. Koch, is dark to light brown with a pattern and an orange hourglass. They are sometimes called “gas station spiders” because of their habit of building webs in service stations. They are extending their range from the east but their presence in Texas is uncertain.

Life Cycle: Adults of both sexes have been found throughout the year in buildings. They can be very common at some locations and times. During the course of a summer a female may lay several egg sacs. The egg sacs are white to tan or gray, pear-shaped to almost globular of tough papery texture, about 1/4 to 1/2-inch in diameter. Each egg sacs contain from 25 to 400 or more eggs. Egg sacs are suspended in the web where the female stands guard nearby. The second stage (instar) spiderlings typically emerge about 4 weeks after egg sac production. Newly emerged spiderlings are not cannibalistic until 10 to 14 days after emergence, whereupon they may suddenly become highly cannibalistic. Males generally require fewer molts to mature than females.

Habitat, Food Source(s), Damage:  Widow spiders feed on a wide variety of arthropods. Red imported fire ants have been reported as their main food in cotton fields of East Texas. Boll weevils, grasshoppers, June beetles, and scorpions are also known prey. They are found in houses, outhouses, cotton fields, trash and dumps. Webs are commonly found in spaces under stones or logs, or holes in dirt embankments, and in barns, rural privies, and other outbuildings. The web is an irregular mesh usually built in a dark spot sheltered from the weather. The webs may also have a retreat, typically a 1/16 to 5/16 inch (2 to 8 cm) circular or semicircular silken tent. The spider spends most of the time in the retreat, venturing out onto the web for web maintenance or when attracted by prey vibrations. Webs are usually placed low to the ground.

Pest Status: Most notorious of all spiders in the United States; venom is highly virulent, but the spider is quite timid. Even when disturbed in its web it attempts to escape rather than to attack. Widow spiders are known from every state (except Maine where it undoubtedly also occurs) and several Canadian provinces. It is uncommon in the north but quite abundant in the south and west.