Southern Black Widow (Latrodectus mactans)

Physical description and identification

An adult female southern black widow is one of the easiest spiders in North America to name with confidence. The abdomen is a shiny black sphere, so smooth it looks wet, carried on long, fine black legs. Turn her over — safely, with the web between you and the spider — and there is a red hourglass on the underside, usually complete, with the two triangles touching.

Adults

• Size: the female's body is roughly 8–13 mm (0.3–0.5 in) with a leg span of 25–38 mm (1–1.5 in). The familiar half-inch figure is body length, not span. Males are tiny at 3–6 mm.
• Colour: mature females are uniformly glossy black, often with a small red spot above the spinnerets as well as the ventral hourglass. Males and immatures look like another species: grey-brown, with cream side stripes and small red dots down the back.
• Eyes: eight small eyes of roughly equal size in two rows of four, the side pairs set close together. No widow has an enlarged forward-facing pair.
• Distinctive features: like all cobweb spiders, the widow has a curved comb of serrated bristles on the fourth leg, used to fling gluey silk over prey from a safe distance instead of biting first. The female's fangs are long enough to break human skin; the male's are not.

Eggs and egg sacs

The female wraps her eggs in a smooth, papery, cream to tan, pear-shaped sac about 8–12 mm across, with no spines or tufts — a useful field mark, because the brown widow makes a spiky one. Each sac holds roughly 200 to 400 eggs, and spiderlings hatch inside within two to four weeks, moult once, then chew their way out.

Spiderlings

Emerging spiderlings are pale translucent cream with dark legs and develop the striped, spotted juvenile pattern over the following months. Only females turn fully black, at the last moult or the one before. Most of the brood disperses within days, ballooning away on silk threads.

The web

The web has no pattern at all: a coarse three-dimensional snarl of unusually strong silk in a sheltered pocket close to the ground, with a dense retreat at the top and taut vertical gumfoot lines running down to the soil. Each line is anchored with a droplet of glue and held under tension, so a beetle crossing the anchor sticks fast, the line snaps free and lifts it off its feet, and the vibration brings the widow down. She hangs upside down through the day, which is why the underside — and the hourglass — is what you usually see first. Our guide to spider webs explains how gumfoot snares differ from an orb.

Where the southern black widow lives

Latrodectus mactans is a spider of the warm southeastern and south-central United States, common from Florida and Georgia north through the Carolinas to roughly Ohio, and west through Alabama, Mississippi, Arkansas and Texas. It does not reach southeastern Canada — the widow at the northern edge of the range is the northern black widow — and from the Great Plains westwards it is replaced by the western black widow.

Within that range it specialises in dark, dry, undisturbed cavities within a metre of the ground: woodpiles, stacked bricks, garden furniture, meter and valve boxes, crawl spaces, barns, storm drains, rodent burrows and standing flowerpots. It is not a wandering house spider. Almost every bite on record involves a hand or foot put into one of those cavities without looking first.

Are southern black widow spiders venomous?

Yes, and here the answer matters. Widow venom contains alpha-latrotoxin, a protein that attacks nerve endings directly: it punches pores into the presynaptic nerve terminal and forces the wholesale release of the neurotransmitters stored there until the terminal is emptied. The result is not tissue damage at the bite site but a body-wide storm of nerve signalling, the syndrome doctors call latrodectism.

Only the adult female is medically significant: her venom glands are far larger and her fangs long enough to reach the dermis. Drop for drop the venom is extremely potent, but the quantity injected is minute, which is why most bites are painful and frightening rather than life-threatening. A specific equine-derived Latrodectus antivenom is manufactured and stocked by US hospitals.

Can southern black widow spiders bite?

They can. The widow is not aggressive, and her first response to a hand in the web is to drop, curl her legs and play dead, but she will bite if squeezed, pinned against skin or defending an egg sac.

What a bite feels like. The bite is often felt as a pinprick or barely at all, and the site shows little more than two faint punctures with mild redness — no blister, no dying tissue. Within 30 to 60 minutes the characteristic symptoms begin: intense cramping muscle pain that starts near the bite and spreads to the large muscle groups, rigid board-like abdominal muscles (sometimes mistaken for appendicitis), heavy localised sweating, raised blood pressure, fast heart rate, restlessness, nausea and headache. Pain peaks in the first several hours and settles over one to three days, though aching can persist for a week.

What to do. Wash the site with soap and water, apply a cold pack, keep the limb still and seek medical care — urgently if the person is a young child, elderly, pregnant, has heart disease, or develops severe cramping, chest pain or difficulty breathing. Photograph the spider, or bring it in a sealed container if it can be collected safely, because the identification changes the treatment. Hospital care centres on strong analgesia and benzodiazepines for the muscle spasm, with antivenom for severe cases. Do not cut, suck, apply a tourniquet or apply heat; none of it works and several do harm. Deaths are extremely rare — none has been reliably attributed to a widow bite in the United States in decades.

Ecological importance and behavior of the southern black widow

The southern black widow is a sedentary ambush predator that may spend her whole adult life in one web. Prey is whatever blunders into the gumfoot lines: ground beetles, crickets, cockroaches, ants, woodlice, millipedes, centipedes and scorpions. She handles dangerous prey without grappling with it, combing sticky silk over the struggling animal until it is immobilised. Reports that widows routinely eat mice belong to folklore; small lizards turn up in widow webs occasionally, but they are a curiosity, not a diet.

Natural predators: the blue mud dauber wasp (Chalybion californicum) is a widow specialist and provisions its nests almost entirely with them; spider wasps, centipedes, mantises and predatory ants also take them, and parasitoid wasps attack the egg sacs. In urban habitat the spreading brown widow displaces L. mactans and eats its eggs and juveniles.

The sexual-cannibalism myth: despite the name, the female of this species does not usually kill the male. Post-mating cannibalism was recorded often in early laboratory arenas where the male could not escape, but in the field it is uncommon. The species that made the behaviour famous is the Australian redback, whose male somersaults into the female's fangs.

Season and lifespan: in the deep south adults occur year round; further north females overwinter as large juveniles or adults and produce egg sacs through summer. A female lives one to three years, a male a few months after his final moult.

Relationship with humans: separation beats eradication. Wear gloves to move firewood and stored boxes, and take a torch to a meter box before reaching in. Where a web is in the way, brushing it out and closing the cavity behind it beats spraying, because it is the sheltered gap that keeps attracting new widows.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• Fabricius described the species in 1775 as Aranea mactans; the Latin mactans means "murderous". Walckenaer erected the genus Latrodectus in 1805 and the species was moved into it later.
• Alpha-latrotoxin is a standard tool in neuroscience because it empties a nerve terminal so completely; much of what is known about how synapses release their contents was learned with widow venom.
• The old name "shoe-button spider" comes from the female's abdomen, almost exactly the size, shape and shine of a Victorian jet button.

In summary, the southern black widow is a small, shy, web-bound spider with a genuinely dangerous venom and an undeserved reputation for malice. Learn the glossy black globe and the red hourglass, keep your hands out of dark cavities in the southern states, and treat a suspected bite as a reason to see a doctor rather than a reason to panic.