Steatoda grossa (False Black Widow)

Summary

The "False Black Widow," Steatoda grossa, belongs to the same family of cobweb weavers (Theridiidae) as true black widows, and is easily mistaken for its dangerous cousins. This species may be native to Europe but it has spread across the globe as a stowaway on cargo. Here in North America it ranges along the Atlantic and Pacific coasts, and around the Great Lakes. In recent years, it is also being found in Colorado, Arizona, New Mexico, and likely a few other inland states. It is common in buildings, but may live outdoors in sheltered spots such as wood piles, under bridges, or in rock walls. Webs are tangled, three-dimensional snares built near the ground. The spider hides in a crack or crevice by day, emerging at night to hang upside down in the web. One study (Barmeyer 1973) suggested that this species may be a specialist on woodlice (“pillbugs,” “roly-polies”). Female False Black Widows may live at least six years, males 12-18 months. After mating, a female produces multiple egg sacs, each one roughly spherical, made of fluffy white silk. Inside are 10-70 eggs. Note that this spider does not have the red hourglass on the underside of the abdomen that true widows do.