Southern Black Widow (Latrodectus mactans): Facts and Venom

What does a southern black widow look like?

The adult female is shiny, jet black, with a large, almost spherical abdomen and long, slender legs. On the underside of the abdomen she carries a red hourglass, usually complete in this species, with the two triangles joined in the middle. Many females also have a red spot on top of the abdomen, just above the spinnerets, the silk-spinning organs at the tip of the body.

Males look very different. They are tiny, with a slimmer abdomen and long legs, and are brownish or grayish with red and white markings along the sides of the abdomen. Young spiders start out pale, patterned in orange, brown and white, and darken with each molt.

Widows belong to the family Theridiidae, the cobweb or comb-footed spiders. The name comes from a row of stiff, curved bristles on the last segment of each hind leg, which the spider uses like a comb to fling sticky silk over prey.

Until 1970, all three North American black widows were treated as a single species, Latrodectus mactans. That year arachnologist B. J. Kaston separated the northern and western black widows. As a result, many older books use “mactans” for any black widow anywhere in the US, which is why range maps and facts from different sources sometimes disagree.

How big is a southern black widow?

The female’s body, not counting legs, is about 8–13 mm (0.3–0.5 in) long. A female heavy with eggs can have an abdomen more than 12 mm (0.5 in) across. With legs spread, a large female measures about 25–38 mm (1–1.5 in). The male is far smaller, with a body of about 3–6 mm (0.1–0.25 in).

Spider weights are rarely measured. Even a large, egg-laden female weighs only around a gram (0.04 oz) or less.

Habitat and range

The southern black widow lives across the southeastern United States, from Florida north to about Ohio and west to Texas. It also occurs in parts of Mexico and the Caribbean. In the East its range overlaps with that of the northern black widow, and in Texas and Oklahoma it meets the western black widow.

It favors dry, dark, sheltered places close to the ground: under stones and logs, in woodpiles, hollow stumps, rodent burrows, rock walls and crevices. Around people it settles in sheds, garages, barns, crawl spaces, water meter boxes, stacked bricks and the undersides of outdoor furniture. It seldom lives in heated living areas but may move indoors for shelter in cold weather. In the days of outdoor toilets, privies were a classic place for bites.

For a map of all three US black widows and their favorite hiding places, see where do black widow spiders live?

Diet and hunting

The southern black widow is a carnivore that eats mostly insects, such as beetles, crickets, cockroaches, flies and grasshoppers, along with woodlice, millipedes, centipedes and other spiders.

Its web looks like a messy tangle, but it has a clear design. Above is a framework of supporting threads, in the middle a dense tangle where the spider rests, and below a set of vertical “gumfoot” lines anchored to the ground with sticky droplets. When a beetle or ant walks into one of these lines, the thread snaps free and springs upward, leaving the prey dangling.

The widow then rushes out, combs silk over the struggling insect with her hind legs, wraps it and bites it. Like other spiders, she cannot chew solid food. She injects digestive enzymes, then sucks up the liquefied tissue, often after carrying the wrapped prey back to her retreat.

Widow venom contains a family of toxins called latrotoxins. Some act mainly on insects, one on crustaceans, and one, alpha-latrotoxin, on the nerves of vertebrates, which is why the bite affects people.

Behavior

Southern black widows are solitary, sedentary and mostly active at night. A female rarely leaves her web. She usually hangs belly-up in her web after dark, which displays the hourglass, and hides in a retreat at the edge of the web by day.

Despite their reputation, widows are timid. Web-building spiders have poor eyesight and sense the world through vibrations. When disturbed, a widow usually retreats, drops to the ground on a silk line or curls up and plays dead. Experiments with the closely related western black widow found that the spiders seldom bit when they were simply poked and bit mainly when squeezed, which matches how people get bitten.

The red hourglass is a warning signal. A 2016 study found that it is more conspicuous to birds, which are potential predators, than to the insects the spider eats, so it can warn off attackers without scaring away prey.

Venom and bites

Only adult females have fangs large enough, about 1 mm (0.04 in) long, and venom glands big enough to affect people. The venom triggers a flood of chemical messengers from nerve endings, a condition called latrodectism. Typical effects are pain spreading from the bite, painful muscle cramps that often affect the abdomen, back or chest, sweating, nausea and a raised heart rate and blood pressure. Symptoms usually ease within a few days.

Serious cases are uncommon. A review of US poison center records from 2000 to 2008 found 23,409 reported widow spider exposures. Of nearly 9,900 people with symptoms, about 65% had minor effects, 33.5% moderate and 1.4% major effects, and no one died. Reported bites peaked in September.

Anyone with severe pain, cramps or difficulty breathing after a suspected widow bite, or any young child who may have been bitten, should seek emergency medical help immediately: call 911 in the US. For a closer look at the risk, see how dangerous is a black widow?

Reproduction and life cycle

When a male matures, he spins a tiny sperm web, deposits sperm on it and draws it into his palps, the leg-like feelers beside the jaws, which work as mating organs. He then leaves home to search for a female, guided by scent chemicals on her silk. At her web he courts carefully, plucking and vibrating the threads so that she recognizes him as a mate rather than prey.

The female’s appetite for her partners has been greatly exaggerated. In North American black widows, many males survive mating and leave, and cannibalism is much more common in laboratory containers than in the wild. The full story, including the Australian redback that sacrifices himself, is in do black widows eat their mates?

A female stores sperm and can fertilize several batches of eggs from one mating. Over a summer she may produce four to nine round or pear-shaped, cream to tan, papery egg sacs, each typically holding a few hundred eggs, and guards them in her web. The eggs hatch in about 20–30 days. Many spiderlings die, some eaten by their siblings, and the survivors drift away on silk threads, a habit called ballooning. They mature in a few months.

Lifespan, predators and conservation status

Females usually live 1–3 years. Males live only a few months and stop growing once mature.

Widows have plenty of enemies. The blue mud dauber (Chalybion californicum), a metallic blue wasp, is a major predator, stinging widows and packing them into its mud nest as food for its larvae. Spider wasps, centipedes, praying mantises and some birds also take them. Egg sacs are attacked by a tiny wingless wasp, Baeus latrodecti, and by the larvae of small flies. Where the introduced brown widow has spread, it competes with black widows and sometimes kills them.

The southern black widow has not been assessed by the IUCN, so its official status is Not Evaluated. It is common and widespread, and no threats to the species as a whole are known.

Related widow spiders in North America and Europe

There are about 35 widow species worldwide, found on every continent except Antarctica. In the US, the northern black widow (Latrodectus variolus) of the Northeast, Midwest and southern Canada usually has a split hourglass and red spots along its back. The western black widow (Latrodectus hesperus) ranges from Texas to the Pacific Coast and north into Canada. Two other widows live in the South: the introduced brown widow and the red widow of Florida’s scrublands. See venomous spiders in the US for how they compare.

Europe has its own widow, the Mediterranean black widow (Latrodectus tredecimguttatus). It is usually black with about 13 red spots on its back, which gives it its scientific name, and lives in grasslands and fields from southern Europe to Central Asia, where it is known as the karakurt. Because it lives in fields, bites in Europe were traditionally linked to the grain harvest. No widows are native to Britain or northern Europe.

Myths and facts

“Black widows always eat their mates.” They do not. Males of North American black widows often survive, and males of the western black widow commonly mate with more than one female.

“Black widows are aggressive.” They are shy, poorly sighted spiders that rarely leave their webs. Nearly all bites happen when a spider is pressed against skin, for example in a glove, a shoe or a stack of firewood.

“A black widow bite is usually deadly.” It can be very painful, but deaths have become exceedingly rare. No deaths were recorded among the 23,409 widow exposures reported to US poison centers from 2000 to 2008.

Relationship with people

The scientific name roughly means “murderous secret biter”, and the spider’s fearsome image is old. In 1933 Allan Blair, a professor at the University of Alabama’s medical school, let a female black widow bite him to prove to doubters that its venom affects people. The pain was so severe that he was taken to a hospital, where he stayed about two days before making a full recovery.

Today, alpha-latrotoxin is an important research tool. Neuroscientists have used it to study how nerve cells release their chemical messengers, and it helped reveal some of the proteins involved in that process. In gardens, sheds and woodpiles, black widows also help keep down insects.

Living near black widows is mostly a matter of respect: wear gloves when moving wood, stones or stored items, and shake out shoes and gardening gloves left outside. Like the brown recluse, the southern black widow is dangerous only when it is handled or trapped.