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Latrodectus (Black Widow Spider)
Latrodectus is a genus of spider, in the family Theridiidae, which contains 32 recognized species. The black widow spider is perhaps the best-known member of the genus. The venomous bite of these spider species is considered dangerous because of the neurotoxin, latrotoxin, which causes the condition latrodectism, both named for the genus. 
The female black widow has unusually large venom glands and her bite is particularly harmful to humans; however, Latrodectus bites rarely kill if proper medical treatment is provided.
The prevalence of sexual cannibalism, a behaviour in which the female eats the male after mating in some species of Latrodectus has inspired the common name – ‘widow spiders’. The female’s venom is at least three times more potent than that of the males, making a male’s self-defense bite ineffective.
Research at the University of Hamburg in Germany suggests this ultimate sacrifice strategy has evolved to promote the survival odds of the offspring; however females of some species only rarely show this behavior, and much of the documented evidence for mate cannibalism has been observed in laboratory cages where the males could not escape.
Females of most Latrodectus species are dark or black in colour usually exhibiting a red or orange hourglass on the ventrum underside or bottom of the abdomen — some may have a pair of red spots or have no marking at all. They often exhibit various red or red and white markings on the dorsal or top side of the abdomen, ranging from a single stripe to bars or spots.
Females of a few species are paler brown shades and some have no bright markings. Juveniles and adult male Latrodectus are half the size of the females, and are often grey or brown and usually lighter in colour than females; while they may sometimes have an hourglass marking on their ventral abdomen, it may be yellow or white, not red. Variation in specifics by species and by gender is great.
Spiders of the genus Steatoda (also of the Theridiidae family) are often mistaken for widow spiders, and are known as “false widow spiders”; they are significantly less harmful to humans.
In common with other members of the Theridiidae family, the widow spiders construct a web of irregular, tangled, sticky silken fibers. The spider very 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 bite it and wrap it in silk. If the spider perceives a threat, it will quickly let itself down to the ground on a safety line of silk.
As 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.
While some species are more aggressive, most are not; many injuries to humans are due to defensive bites delivered when a spider gets unintentionally squeezed or pinched.
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 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.
The southern black widow, as well as the closely related western and northern species which were previously considered the same species, has a prominent red hourglass figure on the underside of its abdomen. Many of the other widow spiders have red patterns on a glossy black or dark background, which serve as a warning. Spiders found in multiple regions are listed in their predominant native habitat.
Widow spiders can be found on every continent of the world except Antarctica. In North America, the black widows commonly known as southern (Latrodectus mactans), western (Latrodectus hesperus), and northern (Latrodectus variolus) can be found in the United States, as can the “grey” or “brown widow spiders” (Latrodectus geometricus) and the “red widow spiders” (Latrodectus bishopi).
The most prevalent species occurring in Australia is commonly called the redback (Latrodectus hasselti). African species of this genus are sometimes known as button spiders. (Source: Wikipedia)
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