<Tr> <Td_colspan="2"> <P> 31 described species, estimated 65 species living </P> </Td> </Tr> <P> 31 described species, estimated 65 species living </P> <P> Latrodectus is a genus of spiders in the family Theridiidae, most of which are commonly known as widow spiders . The genus contains 31 recognized species distributed worldwide, including the North American black widows (L. mactans, L. hesperus, and L. variolus), the button spiders of Africa, and the Australian redback spider . 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 (but not always) hourglass - shaped . </P> <P> The venomous bite of these spiders is considered particularly dangerous because of the neurotoxin latrotoxin, which causes the condition latrodectism, both named after the genus . The female black widow has unusually large venom glands and its bite can be particularly harmful to humans . However, despite the genus' notoriety, Latrodectus bites are rarely fatal . Only female bites are dangerous to humans . </P>

Where is the red marking on a black widow