| Black widow | |
|---|---|
 | |
| Eating a prey | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | Chelicerata |
| Class: | Arachnida |
| Order: | Araneae |
| Infraorder: | Araneomorphae |
| Family: | Theridiidae |
| Genus: | Latrodectus |
| Species: | L. mactans |
| Binomial name | |
| Latrodectus mactans | |
Latrodectus mactans, known assouthern black widow or simplyblack widow, and theshoe-button spider,[citation needed] is a venomous species of spider in the genusLatrodectus. The females are well known for their distinctive black and red coloring and for the fact that they will occasionally eat their mates after reproduction. The species is native toNorth America. The venom can cause pain and other symptoms, but is rarely fatal to healthy humans.[2]
Latrodectus mactans was first described byJohan Christian Fabricius in 1775, placing it in the genusAranea.[1][3] It was transferred to the genusLatrodectus in 1837 byCharles Walckenaer and is currently placed in the familyTheridiidae of the orderAraneae.[1] The species is closely related toLatrodectus hesperus (western black widow) andLatrodectus variolus (northern black widow). Members of the three species are often confused with the genusSteatoda, the false widows. Prior to 1970, when the current taxonomic divisions forNorth American black widows were set forth by Kaston,[4] all three species 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 toL. mactans alone. Common usage of the term "black widow" makes no distinction between the three species.
The body length (excluding legs) of the mature female is 8–13 mm (0.31–0.51 in), and 3–6 mm (0.12–0.24 in) for males. Legs are long in proportion to body.[5] Females are shiny and black in color, with a red marking in the shape of anhourglass on theventral (under) side of her very roundedabdomen.[6] 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.[7] Juveniles have a distinctly different appearance from the adults; the abdomen is grayish to black with white stripes running across it and is spotted with yellow and orange.[7] Males are either purple, or closer to the appearance of the juveniles in color.
The web of the black widow spider is a three-dimensional tangled cobweb of exceptionally strong silk.[8]
The southern widow is primarily found in (and is indigenous to) thesoutheastern United States, ranging as far north asOhio and as far west asTexas.[9] The northern black widow(L. variolus) is found primarily in themiddle Atlantic states, though its range overlaps with that ofL. mactans. In theDominican RepublicL. mactans is found throughout the whole country.[10][11]L. mactans is also found throughoutMexico where its range overlaps with that ofLatrodectus hesperus andLatrodectus geometricus.[12]
L. mactans, along withL. hesperus andL. geometricus, is established in theHawaiian Islands (USA).[13][14] One pathway of entry intoHawaii for at least one of these black widow species is imported produce[15] (which is also considered an important potential pathway for widow spiders elsewhere).[16]
When a male is mature, he spins a sperm web, deposits semen on it, and charges hispalpal bulbs with the sperm. Black widow spiders reproduce sexually when the male inserts his palpal bulbs 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 firstmolting, while the rest will perish due tocannibalism, lack of food, and/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 about three to four months.[17] The female may eat the male after mating.[citation needed]
Black widow spiders typically prey on a variety of insects; however, they also consumewoodlice,diplopods,chilopods, and otherarachnids.[18] 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.[19]
There are variousparasites andpredators of widow spiders inNorth America, though apparently none of these have ever been evaluated in terms of augmentation programs for improvedbiocontrol. Parasites of the egg sacs include the flightlessscelionid waspBaeus latrodecti,[20] and members of thechloropid fly genusPseudogaurax. Predators of the adult spiders include thebrown widow spider,Latrodectus geometricus,[21] wasps, most notably theblue mud dauberChalybion californicum, and thespider waspTastiotenia festiva.[citation needed] Other organisms includingmantises orcentipedes also will occasionally and opportunistically take widows as prey, but the preceding all exhibit some significant specific preference forLatrodectus.[citation needed]
Thebrown widow appears to be competing for territory with, and ultimately displacing black widows in areas where they occur together, including predation on black widows.[21][22]
Although the reputation of these spiders is notorious and their venom does affect humans, only mature females are capable of envenomation in humans; theirchelicerae—the hollow, needle-like mouthparts that inject venom—are approximately 1 mm, or .04 in. in length, making them long enough to inject venom into humans, unlike those of the much smaller males. The actual amount injected, even by a mature female, is variable. The venom injected by the female black widow is known asalpha-latrotoxin which binds to receptors at the neuromuscular motor end plate of both sympathetic and parasympathetic nerves, resulting in increased synaptic concentration ofcatecholamines. The symptoms are caused by lymphatic absorption and vascular dissemination of the neurotoxin. The symptoms that result from a black widow spider bite are collectively known aslatrodectism. Deaths in healthy adults fromLatrodectus bites are exceedingly rare, with no deaths despite two thousand bites yearly, and studies within the past several decades have been unable to confirm any fatalities from this or any of the other U.S. species ofLatrodectus (e.g. zero fatalities among 23,409 documentedLatrodectus bites from 2000 through 2008).[23] On the other hand, the geographical range of the widow spiders is vast. Epidemics of mostlyEuropean widow spider bites had been recorded from 1850 to 1950, and during that period deaths were reported from 2 per 1000 bites to 50 per 1000 bites. Deaths from thewestern black widow had been reported as 50 per 1000 bites in the 1920s. At that same time,antivenom was introduced.[24] TheLD-50 ofL. mactans venom has been measured in mice as 1.39 mg/kg,[25] and separately as 1.30 mg/kg (with a confidence interval of 1.20–2.70).[26] In 1933,Allan Blair allowed himself to be bitten by the spider in order to investigate the toxicity of its venom in humans and as a means of convincing skeptics at the time who thought that the spider's venom might not be dangerous to humans.
There are a number of active components in the venom:
The venom isneurotoxic.
