Theridiidae  

View Theridiidae Tree

Tree simplified from Agnarsson (2006).

Despite major advances in the past few years, theridiid phylogeny is still a work in progress. Recent studies have analyzed data from morphology (Agnarsson, 2004), molecules (Arnedo et al., 2004), and both in combination (Agnarsson, 2006). These studies largely agree on the composition of subfamilies. The phylogenetic hypothesis illustrated above is simplified from Agnarsson (2006), the only published study combining molecular and morphological data.

The spider family Theridiidae (cobweb weavers or combfooted spiders) has a worldwide distribution and includes 2387 described species, ranking it among the few most species-rich spider families (Platnick 2014). According to Levi (2005), 234 theridiid species are known from North America north of Mexico. One of these species, the Common House Spider (Parasteatoda tepidariorum) may be the most common spider in the world, living in abundance in association with humans on every continent except Antarctica, occurring both indoors and outdoors (Bradley 2013).

Theridiids are sedentary and typically construct an irregular web with sticky strands attached to the substrate. The strands break when prey touches the line, pulling the prey toward the center of the web. The web is usually built beneath some sort of cover, such as a rock face, a branch, or even a single leaf. Some theridiids build webs consisting of just a few lines of silk. Members of some theridiid genera (Rhomphaea, Argyrodes) are found in the webs of other spiders, especially orb weavers, and may feed on their hosts, their host's eggs, or their host's prey. Other "atypical" feeding habits are knowns as well. For example, Dipoena and Euryopis are ground-dwelling ant predators. Latrodectus (the "widow" spiders) are well known for being venomous to humans, although they will choose to retreat rather than bite if given the option. (Levi 2005; Bradley 2013)

Theridiids typically have a rounded abdomen tapering to a point at the spinnerets. This shape gives the abdomen a characteristic teardrop shape when the spider is hanging upsidedown in its web. Some theridiids hang a leaf or other debris in the web and rest beneath this cover. Many theridiids are conspicuous and colorful. Like most spiders, theridiids have eight eyes. The fourth tibiae of most theridiids sport curved serrated bristles which the spider uses to draw out and fling sticky silk during a wrapping attack. (Levi 2005; Bradley 2013)

Levi (2005) reviews key aspects of the taxonomic history of the Theridiidae and provides some key references.

Cobweb spiders (Theridiidae) are one of the most diverse spider families, not only in terms of species numbers (>2200 described and many undescribed species), but also for the range of behavior, ecology, and morphology represented within the group. Theridiids include the widow spiders (genus Latrodectus), known for their potent venom and sexual cannibalism (females of some species typically eat the males during or shortly after mating). Sexual cannibalism is actually much more common in Tidarren and Echinotheridion, two genera where juvenile males amputate one of their two pedipalps (sperm transfer organs). Like Latrodectus, Tidarren and Echinotheridion have females much larger than males, a phenomenon known as sexual size dimorphism. Theridiids also include the majority of social spider species, which live in large colonies composed mostly of juvenile and adult females that cooperate in hunting, web building, and brood care. They include the largest group of kleptoparasitic spiders, which steel prey from other species.