Latrodectus mactans; she doesn't always live up to her name

Description

The largest member of the comb-footed spider family, a female black widow averages about 1/2 inch in length; the male is considerably smaller. It is shiny, coal black in color, and has long slender legs and a round abdomen. The female is almost always marked with a red hourglass-shaped spot on her abdomen.

The male's legs are larger than the female's, and each joint is orange brown in the middle and black on the ends. The sides of the male's abdomen is marked by four pairs of red and white stripes.

Young spiderlings are orange, brown, and white. They acquire their black coloring with age.

Distribution and Habitat

Black widows are found throughout much of the United States, from Massachusetts to Florida, west to California, and throughout all four deserts of the American Southwest. They are also found in Canada, Mexico, the West Indies, and South America.

The black widow spins a coarse irregularly designed web which often has a short funnel of silk. The male spins a similarly textured web but much smaller. Cool dark places are chosen, in cellars, outbuildings, ruined or abandoned houses, under doorsteps and porches, beneath floorboards or in piles of rubbish. In heavily populated areas they are often found within city dumps and littered areas. In desert regions they can be found in almost any crevice in the soil, as well as in cacti and agave plants.

Reproduction

When adult, the male spins a very tiny web, upon which he deposits seminal fluid. He then charges his pedipalps with the sperm, abandons his home, and goes off in search of a mate. Once he has located a female black widow the male vibrates the threads of the female's web to make sure she is of the right species, and for her to recognize him as a mate. In mating the male simply transfers the sperm from the reservoir in his pedipalps to the female's body. One mating is sufficient for several bouts of egg-laying, since the female stores the sperm and uses it over a period, often of months.

The eggs are laid in silken cocoons, which are guarded by the female guards until the spiderlings hatch. Once hatched, however, the spiderlings leave the web and fend for themselves.

Diet

Black widows feed primarily on insects, but will also take other arachnids. Most prey is encased in silk upon being trapped in the web. Once secured, the prey is paralyzed by the black widow's venom. Saliva left behind by the spider's bite starts digesting the prey almost immediately, and within an hour or two the spider is able to feed.

Habits and Behaviors

Black widows are timid, sedentary, solitary, cannibalistic, and nocturnal.

The only time more than one black widow will be found together is during mating and just prior to newly-hatched spiderlings leaving their mother's web.

The black widow spins a web which acts as a defense mechanism against predators. When a potential predator comes in contact with the web, it becomes entangled in the threads allowing the spider to wrap silk around it and then inject it with poison.

A female black widow usually hangs upside down in her web so that her red hourglass mark serves as a warning signal to a predator.

This spider will usually drop out of its web at the slightest disturbance and feign death.

Other Information

The bite of a black widow is distinguished by a double puncture wound. Black widow venom is a neurotoxin, meaning that it blocks the transmission of nerve impulses. A bitten human usually suffers from a painful rigidity in the area around the bite, but if treated properly and promptly the bite is rarely fatal.

Despite its common name, a female black widow does not invariably eat the male after mating. In fact, it is actually fairly common for the male to live long enough to mate with several females. It is possible, however, that after several matings the male may become enfeebled enough to become entangled in a female's web, upon which time the female would act in the same way she would if any other type of prey found itself caught in her web.

Scientific Classification

phylum Arthropoda
class Arachnida
order Araneae
suborder Neocribellatae
family Theridiidae (cobweb spiders)
genus & species Latrodectus mactans

Source

Animal Diversity Web animaldiversity.org

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This page was last updated on September 29, 2018.