Black widow spider

Latrodectus is a broadly distributed genus of spiders, which is composed of both black widow spiders and brown widow spiders. A member of the Theridiidae family, this genus contains 31 species, including the North American black widows (L. mactans, L. hesperus, and L. variolus), the European black widow (L. tredecimguttatus), the Australian redback black widow (L. hasseltii) and the button spiders of Southern Africa. Species vary widely in size. In most cases, the females are dark-coloured and readily identifiable by reddish markings on the abdomen, which are often hourglass-shaped. These small spiders have an unusually potent venom containing the neurotoxin latrotoxin, which causes the condition latrodectism, both named after the genus. Female widow spiders have unusually large venom glands and their bite can be particularly harmful to large vertebrates, including humans. Only the bites of the females are dangerous to humans. Despite their notoriety, Latrodectus bites rarely cause death or produce serious complications. Female widow spiders are typically dark brown or a shiny black in colour when they are full grown, usually exhibiting a red or orange hourglass on the ventral surface (underside) of the abdomen; some may have a pair of red spots or have no marking at all. The male widow spiders often exhibit various red or red and white markings on the dorsal surface (upper side) of the abdomen, ranging from a single stripe to bars or spots, and juveniles are often similar to the male pattern. Females of a few species are paler brown and some have no bright markings. The bodies of black widow spiders range from 3–10 mm (0.12–0.39 in) in size; some females can measure 13 mm (0.51 in) in their body length. The prevalence of sexual cannibalism, a behaviour in which the female eats the male after mating, has inspired the common name 'widow spiders'. This behaviour may promote the survival odds of the offspring; however, females of some species only rarely show this behaviour, and much of the documented evidence for sexual cannibalism has been observed in laboratory cages where the males could not escape. Male black widow spiders tend to select their mates by determining if the female has eaten already to avoid being eaten themselves. They are able to tell if the female has fed by sensing chemicals in the web. Like other members of the Theridiidae, widow spiders construct a web of irregular, tangled, sticky silken fibres. Black widow spiders prefer to nest near the ground in dark and undisturbed areas, usually in small holes produced by animals, or around construction openings or wood piles. Indoor nests are in dark, undisturbed places such as under desks or furniture or in a basement. The spider frequently hangs upside down near the centre of its web and waits for insects to blunder in and get stuck. Then, before the insect can extricate itself, the spider rushes over to envenomate and wrap it in silk. To feed, the mouth pulses digestive juices over the prey which is liquified and then internalized by capillary action, allowing the slurry to be sucked back up into the mouth. Their prey consists of small insects such as flies, mosquitoes, grasshoppers, beetles, and caterpillars. If the spider perceives a threat, it quickly lets itself down to the ground on a safety line of silk. As with other web-weavers, these spiders have very poor eyesight and depend on vibrations reaching them through their webs to find trapped prey or warn them of larger threats. When a widow spider is trapped, it is unlikely to bite, preferring to play dead or flick silk at the potential threat; bites occur when they cannot escape. Many injuries to humans are due to defensive bites delivered when a spider gets unintentionally squeezed or pinched. The blue mud dauber (Chalybion californicum) is a wasp that, in western North America, is the primary predator of black widow spiders. The ultimate tensile strength and other physical properties of Latrodectus hesperus (western black widow) silk were found to be similar to the properties of silk from orb-weaving spiders that had been tested in other studies. The tensile strength for the three kinds of silk measured in the Blackledge study was about 1,000 MPa. The ultimate strength reported in a previous study for Nephila edulis was 1,290 ± 160 MPa. The tensile strength of spider silk is comparable to that of steel wire of the same thickness. However, as the density of steel is about six times that of silk, silk is correspondingly stronger than steel wire of the same weight. Spiders of the genus Steatoda (also of the Theridiidae) are often mistaken for widow spiders, and are known as 'false widow spiders'; they are significantly less harmful to humans.