IS GROUP LIVING AN ANTIPREDATOR DEFENSE IN A FACULTATIVELY COMMUNAL WEBSPINNER (EMBIIDINA, CLOTHODIDAE)

I tested whether predator attack rate is a function of expanse of silk for colonies of Antipaluria urichi, a facultatively communal webspinn er. The avoidance effect hypothesis predicts that the probability of a predator detecting a group does not increase proportionately with an increase in expanse of silk, and therefore, larger groups are relative ly less likely to be attacked. I counted the number of holes inflicted on silk over 3 weeks, an indirect measure of predation, for 47 coloni es. Supporting the avoidance effect, holes per perimeter of silk accum ulated at a rate lower than would be predicted by size alone. Further analysis using predation risk, computed as holes per individual per si lk perimeter, revealed that risk was extremely variable for colonies w ith little silk (< 200 cm) but, on average, did not vary as silk expan se increased. Overall, predators of webspinner colonies appear to be i nfluenced by the avoidance effect, but whether the occupants of the si lk are safer is unresolved.