Does group closure exist in the social spider Anelosimus eximius? Behavioural and chemical approach

The exclusion of a conspecific intruder by the members of a colony is a well-known phenomenon in social insects. This phenomenon is called group closure. It may exist in social spiders as well. ¶Experiments were conducted to test inter-colony tolerance in Anelosimus eximius, a social spider of South America. Two types of information were taken into account: the behavior of the intruder introduced in a group of 10 individuals from same colony and the chemical characteristics of the cuticular products of the spiders. ¶Spiders were collected from four natural colonies in French Guyana; two colonies were less than 5 km apart, while the others were separated by more than 40 km. Two weeks after collection, an intruder (from the same colony or from another colony) was introduced into the box. In all cases, the intruder was accepted by the members of the group after a minimum of 24 h. Thus, in Anelosimus eximius, the aggressive behavior necessary for group closure is nonexistent. ¶The presence of an intruder (of the same colony or a different one) temporarily affected the structure of the group. Twenty-four hours after the introduction, there was an increase in the nearest neighbor distances between members of the group. ¶The analysis of the chemical products of the cuticle showed volatile and non-volatile products. There was no qualitative difference between the spiders of the different colonies, only quantitative differences. These differences were not correlated with the distance between colonies. Close colonies had greater differences in the ratios of several chemical cuticular products than did distant colonies. ¶We conclude that there is no active group closure in Anelosimus eximius, although there may be differences in the “odors” of the individuals.