In John Wyndham's novel Web (published in 1979, some ten years after Wyndham's own death), a group of settlers attempting to establish a utopian society on a remote Pacific island find themselves besieged by spiders. Contrary to the usual solitary habits of their kind, the spiders of Web have evolved a social structure like that of wasps or ants, and form roving packs that can overwhelm and devour animals many times their size. Fortunately for us, no such rapacious beasts exist in real life. But there are social spiders, even if they do not present a threat to anything much larger than a big insect.
The social habit has evolved in spiders on a number of occasions, but is perhaps best developed in some species of the genus Stegodyphus. This is a genus of the family Eresidae, commonly known as the velvet spiders. Eresids are small spiders, distinguished from most others by their subrectangular carapace with the front edge produced into a hood above the chelicerae (Miller et al. 2012). They have the full spider complement of eight eyes, with the posterior median eyes generally enlarged and directed forwards. Together with their covering of plush fur (hence the name 'velvet' spiders), this gives them an appearance distinctly reminiscent of some sort of carnivorous muppet.
There are nine currently recognised genera of eresids, though only Stegodyphus includes social species. The family is mostly restricted to the Old World, with a single species Stegodyphus manaus known from Amazonian Brazil. A second species, S. annulipes, was originally described as Brazilian, but has since been collected from Israel and appears to have been mislabelled (Miller et al. 2010). Members of the temperate Eurasian genus Eresus are commonly known as 'ladybird spiders' as males often have a striking abdominal colour pattern of black spots on a red background. Most eresids live in silken tubes under objects such as stones or underground, whereas Stegodyphus species construct their webs in vegetation (Miller et al. 2012).
The communal webs of social Stegodyphus species may extend for several metres. When an animal becomes trapped in the web, as many spiders as are able to reach it swarm over, all biting and salivating as they can. As a result, the members of the colony are able to kill and digest much larger prey than they could otherwise handle alone. Sociality in Stegodyphus appears to have arisen as an extension of the parental care found in other eresids. Females of both Stegodyphus and Eresus will regurgitate food for newly hatched young (Kullmann 1972). In social Stegodyphus, young are cared for communally and females will feed the young of their nest-mates as well as their own. Eventually, the young begin feeding directly on the caring female herself, draining her haemolymph to the point of rapid death. Again, in social Stegodyphus, this fate awaits all mature adults in the colony, and there is no overlap between generations (Schneider 2002). After the death of their mother, juvenile Eresus and non-social Stegodyphus remain in a group until they are closer to maturity; social behaviour could have arisen through a simple delay in the time of dispersal.
REFERENCES
Kullmann, E. J. 1972. Evolution of social behavior in spiders (Araneae; Eresidae and Theridiidae). American Zoologist 12: 419–426.
Miller, J. A., A. Carmichael, M. J. Ramírez, J. C. Spagna, C. R. Haddad, M. Řezáč, J. Johannesen, J. Král, X.-P. Wang & C. E. Griswold. 2010. Phylogeny of entelegyne spiders: Affinities of the family Penestomidae (NEW RANK), generic phylogeny of Eresidae, and asymmetric rates of change in spinning organ evolution (Araneae, Araneoidea, Entelegynae). Molecular Phylogenetics and Evolution 55: 786–804.
Miller, J. A., C. E. Griswold, N. Scharff, M. Řezáč, T. Szűts & M. Marhabaie. 2012. The velvet spiders: an atlas of the Eresidae (Arachnida, Araneae). ZooKeys 195: 1–144.
Schneider, J. M. 2002. Reproductive state and care giving in Stegodyphus (Araneae: Eresidae) and the implications for the evolution of sociality. Animal Behaviour 63: 649–658.
That's family values for you!
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