Social Spiders

Several spider species have learned to live with large numbers of their own kind - plus a few impostors.

Once they reach maturity, most spiders only seek out another adult for one of two reasons: to mate, or to eat each other. However, several species have evolved a more tolerant attitude toward their kin. Living in groups of up to several hundred individuals, these social spiders work together to capture prey and construct a communal nest, which they build out of silk, plant matter, and prey remains in the middle of their web.

Academy entomologist Charles Griswold first studied social spiders in the genus Stegodyphus while conducting research in South Africa in the 1980's. To his surprise, he found impostors from a separate species living undetected among the Stegodyphus spiders, eating their food and enjoying the protection of their nest without contributing to the workload. Because of their behavior, these impostors are called kleptoparasites.

In January and February of this year, Griswold traveled to Madagascar, where he found new populations of social spiders in the Stegodyphus genus. This time, however, he didn't find any kleptoparasites. Madagascar has been separated from the African continent for about 120 million years, so its Stegodyphus spiders must have either evolved before the landmasses split or traveled across the ocean at a later time. The apparent absence of kleptoparasites in Madagascar points toward the latter scenario, since the kleptoparasites are less likely to be present if their hosts had to make an ocean voyage.

Left: Academy entomologist Charles Griswold in the Vatoharanana forest, near Ranomafana, Madagascar. Photo: Dave Kavanaugh
Right: A Stegodyphus nest in the veld near Spioenkop,
South Africa. There may be more than 100 individuals in a single nest, and their main predators are wasps and birds. Photo: Teresa Meikle

 

Mass attack by social spiders (Stegodyphus) of a carabid beetle. Photo by Teresa Meikle
Reseach in the field took place at Canyon de Singe, Isalo, in southern Madagascar at 750 meters elevation. Photo: Charles Griswold
Map: Colleen Sudekum