Within-group behavioral variation promotes biased task performance and the emergence of a defensive caste in a social spider

The social spider Anelosimus studiosus exhibits a behavioral polymorphism where colony members express either a passive, tolerant behavioral tendency (social) or an aggressive, intolerant behavioral tendency (asocial). Here we test whether asocial individuals act as colony defenders by deflecting the suite of foreign (i.e., heterospecific) spider species that commonly exploit multi-female colonies. We (1) determined whether the phenotypic composition of colonies is associated with foreign spider abundance, (2) tested whether heterospecific spider abundance and diversity affect colony survival in the field, and (3) performed staged encounters between groups of A. studiosus and their colony-level predator Agelenopsis emertoni (A. emertoni)to determine whether asocial females exhibit more defensive behavior. We found that larger colonies harbor more foreign spiders, and the number of asocial colony members was negatively associated with foreign spider abundance. Additionally, colony persistence was negatively associated with the abundance and diversity of foreign spiders within colonies. In encounters with a colony-level predator, asocial females were more likely to exhibit escalatory behavior, and this might explain the negative association between the frequency of asocial females and the presence of foreign spider associates. Together, our results indicate that foreign spiders are detrimental to colony survival, and that asocial females play a defensive role in multi-female colonies

Visually Elicited Escape in Crabs

A review is given on field and laboratory studies, including intracellular recordings from the optic lobes, on the visually elicited escape response of a mangrove and a rocky shore crab. The crabs flee when the angular size of an approaching object has expanded by a critical value. Neurons which respond specifically to this stimulus were encountered in the lobula (internal medulla). Illusiory expansion produced by rotating spirals evokes no reaction. The results suggest a hypothesis on the underlying mechanism for looming detection. The direction of the crab’s escape can be understood as a compromise between a tendency away from the approaching object and towards a probable shelter since it can be described by a weighted vectorial summation of both components