|Hastings is a Biological Field Station of the University of California, Museum of Vertebrate Zoology and UC Natural Reserve System. Gifts made Hastings possible- click here to contribute to our work in research and education.|
Black Widow Spiders At Hastings
The western black widow spider (Latrodectus hesperus), a species native to California, is most famous for its powerful neurotoxic venom. There are 30 other species in the genus Latrodectus, and they occur around the world. Two other species of Latrodectus occur elsewhere in the United States. Unfortunately our fear of these small creatures has often eclipsed our ability to appreciate their interesting biology.
Female black widows can be recognized by their glossy black bodies which feature red ‘hourglass’ markings on the underside of their abdomens (see left). The body is about 0.5-0.6 inches (12-15 mm).
They are usually sedentary and build large webs consisting of two parts: a refuge- a cool dark place like a partially filled gopher hole in the ground (2-5 in. deep) or a crevice in a rock, and a cobweb- a large cloud of messy threads used to catch prey and to send air-borne signals to potential mates. The web can be as large as a meter across- the silk is very fine. Often, the grass is gathered by the spider and as the silk is woven, the heads of the grass are bent inwards to make a dome-shaped assembly of grass stems. We have both a low resolution movie (190K) and a high resolution movie (58 Mb) showing how moving part of the web moves it all. Although we have done extensive work in the grasslands at Hastings for over 50 years, we are not aware of anyone being bitten by the black widows. They hide during the day and are difficult to see or dislodge from their holes during the day. Walking through the webs or sitting in the grass apparently does not evoke much reponse from widows in their holes during the day.
Male black widows look and act very differently from the females (below, left). They can weigh as much as two hundred times less and they have two reproductive organs, called ‘palps’, which they hold out in front of their faces. They spend their short, two or three week long adult life searching for females using the airborne cues put out by the female’s webs. Once they find a female they have to spend up to seven hours courting her before they can mate.
Courtship consists of a ‘dance’ which involves plucking the threads of the female’s web, abdominal vibrations, and drumming on the female with their palps. If the female accepts the male she will let him insert first one, then the other palp into her abdomen and deposit his sperm in her internal sperm storage organs. The female uses the sperm for the next few months, and maybe for the rest of her life, to fertilize eggs that she deposits into an egg sac spun from her own silk. Up to four hundred eggs are laid at one time and spiderlings hatch out after a month inside the sac. They live on their mother’s web until they are large enough to be able to catch prey on their own. They then release a fine line of silk in the shape of a balloon and are carried by the wind to a new location where they will build their own webs and begin the cycle of searching and signalling all over again.
Males are only rarely eaten by the females after breeding, and males may breed with a female several times in their relatively short lifspan. Basic biology- sex ratio of emerging young, density and distribution (habitats) of young or adults, longevity and social behavior are all almost unknown in the western black widow spider at Hastings. Field work by Emily MacLeod, University of Toronto, from Maydianne Andrade's lab is exploring some of these aspects of the biology of black widow spiders. Emily provided the photos here and the text.