Cobweb Spiders of the Family Theridiidae

From the harmless house spiders to the venomous widows, the family Theridiidae includes a large and diverse group of arachnids. Chances are there's a cobweb spider somewhere in your house right now.

Spiders of the family Theridiidae are also called comb-foot spiders. Theridiids have a row of setae, or bristles, on their fourth pair of legs. The setae help the spider wrap its silk around captured prey.

Cobweb spiders are sexually dimorphic in size; females are larger than males. Female cobweb spiders have spherical abdomens and long, slender legs. Some species practice sexual cannibalism, with the female eating the male after mating. The black widow gets its name from this practice.

Cobweb spiders build irregular, 3-dimensional webs of sticky silk. Not all spiders within this group build webs, however. Some cobweb spiders live in social communities, with spiderlings and adult females sharing the web. Others practice kleptoparasitism, stealing prey from other spiders' webs.

Kingdom – Animalia
Phylum – Arthropoda
Class – Arachnida
Order – Araneae
Family – Theridiidae

Cobweb spiders feed on insects, and occasionally other spiders. When an insect becomes ensnared in the sticky strands of the web, the spider quickly injects it with venom and wraps it tightly in silk. The meal can then be consumed at the spider's leisure.

Male cobweb spiders roam in search of mates. In many species, the male uses a stridulatory organ to signal his interest in females. Though some Theridiid males do get eaten after mating, most survive to find another mate.

The female cobweb spider wraps her eggs in a silk case and attaches it to a point near her web. She guards the egg sac until the spiderlings hatch.

With dozens of genera in the Theridiidae family, adaptations and defenses are as diverse as the cobweb spiders. Argyrodes spiders, for example, live along the edges of other spiders' webs, dashing in to grab a meal when the resident spider isn't around. Some Theridiids mimic ants, either to trick potential ant prey or to fool possible predators.

Cobweb spiders live throughout the world, with more than 2200 species described to date. Well over 200 Theridiid species live in North America.