Behaviour of colonial and solitary spiders of the theridiid species Anelosimus eximius

Most Anelosimus eximius live in colonies, but a few females emigrate short distances and establish small, individual webs. Field studies were conducted on one colony and nearby smaller webs in order to describe communal activities and division of labour, and to note costs and benefits of remaining in the parental colony and emigrating. Adult and juvenile females repaired the web and captured prey. Adult females, rather than juveniles, cared for egg sacs and fed young. When colonial egg sacs were abundant, females moved from sac to sac. Several colonial females regurgitated food to spiderlings which had recently emerged from a particular sac. In smaller webs inhabited by two or three females, the mother cared for the sac but all females fed the young by regurgitation. Males rarely participated in communal activities. Advantages of colony living include protection from predators, the availability of large prey and, in the event of a female's death, the care of her egg sac and feeding of her young. Cannibalism of the egg sac is a potential cost of coloniality, affecting about 10–20% of colonial sacs. Cannibalism was not observed in the smaller webs. However, the costs of emigrating are very high: most of the solitary females disappeared, leading to interspecific predation on their eggs and young.