Which family contains black widows and many cobweb spiders?

The family containing black widows and many cobweb spiders is Theridiidae.

Theridiidae is commonly called the tangle-web, cobweb, or comb-footed spider family. Its members typically build irregular three-dimensional webs rather than the neat, wheel-shaped orb webs associated with orb-weaver spiders. The family includes the medically important widow spiders in the genus Latrodectus.

The family was first described by Carl Jakob Sundevall in 1833. It is globally distributed and contains roughly 3,000 species in about 128 genera, although classifications and species totals can change as taxonomists revise spider groups. Theridiids are ecribellate: they produce sticky capture silk rather than the woolly silk made by cribellate spiders.

The name comb-footed refers to serrated bristles on the tarsus of the fourth leg. These bristles help some species manipulate silk and prey. A frequent mix-up is identifying black widows by their common name alone; “black widow” refers to several Latrodectus species, but those species are united taxonomically within Theridiidae. Salticidae, Lycosidae, and Araneidae are separate spider families.