Latrodectus mactans, or Southern black widow or simply black widow, is a highly venomous species of spider in the genus Latrodectus. They are well known for the distinctive black and red coloring of the female of the species that will occasionally eat her mate after reproduction. The species is native to North America. The venom is seldom fatal to healthy humans.
Latrodectus mactans was first described by Johan Christian Fabricius in 1775, placing it in the genus Aranea. It is currently placed amongst the Theridiidae family of the order Araneae. The species is closely related to Latrodectus hesperus (western black widow) and Latrodectus variolus (northern black widow). Members of the three species are often confused with the genus Steatoda, the False Widows. Prior to 1970, when the current taxonomic divisions for North American black widows were set forth by Kaston, all three varieties were classified as a single species, L. mactans. As a result, there exist numerous references which claim that "black widow" (without any geographic modifier) applies to L. mactans alone. Common usage of the term "black widow" makes no distinction between the three species.
The mature female is around 1.5 in (38 mm) long and 0.25 in (6.4 mm) in diameter. She is shiny and black in color, with a red marking in the shape of an hourglass on the ventral (under) side of her very rounded abdomen. There is much variation in female size, particularly in egg-carrying (gravid) females. The abdomen of a gravid female can be more than 1.25 cm (~0.5 in) in diameter. Many female widows also have an orange or red patch just above the spinnerets on the top of the abdomen.
The male is either black, or closer to the appearance of the juveniles in color, and is much smaller with a body of less than 0.75 cm (< 0.25 in).
Juveniles have a distinctly different appearance to the adults, the abdomen is grayish to black with white stripes running across it and is spotted with yellow and orange.
The southern widow is primarily found in (and is indigenous to) the southeastern United States, ranging as far north as Ohio and as far west as Texas. The northern black widow (L. variolus) is found primarily in the northeastern United States and southeastern Canada, though its range overlaps with that of L. mactans. In Canada, black widows range in the southern parts of British Columbia, Alberta, Saskatchewan, Manitoba and Ontario. In the Dominican Republic it is found throughout the whole country.
Latrodectus mactans, along with Latrodectus hesperus and Latrodectus geometricus (the "brown widow spider"), is established in the Hawaiian Islands (USA). One pathway of entry into Hawaii for at least one of these black widow species is imported produce (which is also considered an important potential pathway for widow spiders elsewhere).
When a male is mature, he spins a sperm web, deposits semen on it, and charges his palpi with the sperm. Black widow spiders reproduce sexually when the male inserts his palpus into the female's spermathecal openings. The female deposits her eggs in a globular silken container in which they remain camouflaged and guarded. A female black widow spider can produce four to nine egg sacs in one summer, each containing about 100–400 eggs. Usually, eggs incubate for twenty to thirty days. It is rare for more than a hundred to survive this process. On average, thirty will survive through the first molting, because of cannibalism, lack of food, or lack of proper shelter. It takes two to four months for black widow spiders to mature enough to breed, however full maturation typically takes six to nine months. The females can live for up to three years, while a male's lifespan is much shorter. The female may eat the male after mating. The male will approach the female to ensure she knows he is not food. He will inseminate her, after wrapping her in a thin layer of web. If he can not get away fast enough the female may eat him.
Black widow spiders typically prey on a variety of insects, but occasionally they do feed on woodlice, diplopods, chilopods and other arachnids. The spider's web is even strong enough to catch animals as large as mice. When the prey is entangled by the web, Latrodectus mactans quickly comes out of its retreat, wraps the prey securely in its strong web, then bites and envenoms its prey. The venom takes about ten minutes to take effect; in the meantime, the prey is held tightly by the spider. When movements of the prey cease, digestive enzymes are released into the wound. The black widow spider then carries its prey back to its retreat before feeding.
There are various parasites and predators of widow spiders in North America, though apparently none of these have ever been evaluated in terms of augmentation programs for improved biocontrol. Parasites of the egg sacs include the flightless scelionid wasp Baeus latrodecti, and members of the chloropid fly genus Pseudogaurax. Predators of the adult spiders include a few wasps, most notably the blue mud dauber, Chalybion californicum, and the spider wasp Tastiotenia festiva. Other species including Mantis or Centipede also will occasionally and opportunistically take widows as prey, but the preceding all exhibit some significant specific preference for Latrodectus.
Although these spiders are not especially large, their venom is extremely potent. Compared to many other species of spiders, their chelicerae are not very large or powerful. In the case of a mature female, the hollow, needle shaped part of each chelicera, the part that penetrates the skin, is approximately 1.0 millimeter (about 0.04 in) long, long enough to inject the venom to a point where it can be harmful. The males, being much smaller, inject far less venom with smaller chelicerae. The actual amount injected, even by a mature female, is very small in physical volume. When this small amount of venom is diffused throughout the body of a healthy, mature human, it usually does not amount to a fatal dose (though it can produce the very unpleasant symptoms of latrodectism). Deaths in healthy adults from Latrodectus bites are relatively rare in terms of the number of bites per thousand people. Sixty-three deaths were reported in the United States between 1950 and 1959. On the other hand, the geographical range of the widow spiders is very great. As a result, far more people are exposed, worldwide, to widow bites than to bites of more dangerous spiders, so the highest number of deaths worldwide are caused by members of this genus. Widow spiders have more potent venom than most spiders, and prior to the development of antivenom, 5% of reported bites resulted in fatalities. The venom can cause a swelling up to 15 cm. The LD-50 of L. mactans venom has been measured in mice as 1.39 mg/kg, and separately as 1.30 mg/kg (with a confidence interval of 1.20-2.70).
There are a number of active components in the venom:
- A number of smaller polypeptides - toxins interacting with cation channels which display spatial structure homology - which can affect the functioning of calcium, sodium, or potassium channels.
The venom is neurotoxic.
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