Theridiidae

Introduction

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.

Characteristics

Theridiid spiders have a comb of serrated setae on the fourth tarsus. Similar combs can also be found in some related families (Nesticidae, Synotaxidae). The combs are used to throw sticky silk over prey. Theridiids may wrap their prey in silk before applying a poisonous bite (wrap attack), or bite first. The wrap attack allows theridiids to delay direct contact with prey until it is safely immobilized. The sticky silk used in the wrap attack comes from large, specially modified silk spigots that are uniquely elongate.

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Left (top): Comb of serrated setae on fourth tarsus of a Thwaitesia female. Right (bottom): Spinnerets of an adult Latrodectus hesperus female. Note the elongate silk spigots. Images © Jeremy Miller

Most theridiids have a stridulatory (sound producing) organ where elevated setal bases on the front of the abdomen rub against ridges on the back of the carapace. These organs are often more strongly developed in males. During courtship, males signal females with stridulatory sounds and other vibrations; in some species, females signal back.

Theridiid webs are extremely variable but typically consist of a three dimensional mesh with gumfoot lines. Gumfoot lines are under tension and adorned with sticky droplets. Similar webs are made by nesticids. In some theridiids, the mesh part of the web may be a broad sheet or nearly spherical with gumfoot lines radiating out. Gumfoot lines and even sticky silk are absent from some theridiid webs. Some theridiid webs are reduced to a few lines. Hadrotarsines apparently do not build webs.

Discussion of Phylogenetic Relationships

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.

Other Names for Theridiidae

• Cobweb spiders

References

Agnarsson, I. 2004. Morphological phylogeny of cobweb spiders and their relatives (Araneae, Araneoidea, Theridiidae). Zool. J. Linnean Soc. 141:447-626.

Agnarsson, I. 2006. Phylogenetic placement of Echinotheridion (Araneae: Theridiidae) - do male sexual organ removal, emasculation, and sexual cannibalism in Echinotheridion and Tidarren represent evolutionary replicas? Invertebr. Syst. 20:415-429.

Arnedo, M. A., J. A. Coddington, I. Agnarsson, and R. G. Gillespie. 2004. From a comb to a tree: Phylogenetic relationships of the comb-footed spiders (Araneae, Theridiidae) inferred from nuclear and mitochondrial genes. Mol. Phy. Evol. 31:225-245.

Dippenaar-Schoeman, A. S., and R. Jocqué. 1997. African Spiders. An Identification Manual. Plant Protection Research Instute, Handbook No. 9, Pretoria.

Foelix, R. F. 1996. Biology of Spiders. Second Edition. Oxford University Press, New York.

Forster, R. R., N. I. Platnick, and J. A. Coddington. 1990. A proposal and review of the spider family Synotaxidae (Araneae, Araneoidea), with notes on theridiid interrelationships. Bull. American Mus. Nat. Hist. 193:1-116.

Jocqué, R., and A. S. Dippenaar-Schoeman. 2006. Spider Families of the World. Royal Museum for Central Africa, Tervuren.

Knoflach, B. 2004. Diversity in the copulatory behaviour of comb-footed spiders (Araneae, Theridiidae). Denisia 12:161-256.

Levi, H. W. 2005. Theridiidae. Pages 235-243 in Spiders of North America: An Identification Manual (D. Ubick, P. Paquin, C. E. Cushing, and V. Roth, eds.). American Arachnological Society.

Levi, H. W., and L. R. Levi. 1962. The genera of the spider family Theridiidae. Bull. Mus. Comp. Zool. 127:1-71.

Platnick, N.I. 2006. The World Spider Catalog, Version 7.0. http://research.amnh.org/entomology/spiders/catalog/INTRO1.html.

Information on the Internet

• Interactive Key to the World Genera of Cobweb Spiders (Theridiidae) by Ingi Agnarsson. This comprehensive key to theridiid genera was developed using LUCID BUILDER and requires the LUCID PLAYER (available from www.lucidcentral.com/lucid2/player/lucidplayer.htm). LUCID PLAYER software is free but registration is required. Also, an internet connection is required to make use of the images.