Orb spiders are commonly used to study many ecological and behavioral questions, due in part to the ease of working with their webs. The frequency distributions of prey captured by spiders are often compared among species or individuals to search for evidence of competition, effects of experimental manipulations, etc. In most instances, smaller insects are extremely common compared to larger prey, presenting a critical flaw in how such comparisons are analyzed because natural selection acts upon web evolution through the biomass, rather than total number, of prey captured. The “rare, large prey” hypothesis, developed for Zygiella x-notata (Clerk 1757), suggests that orb spiders derive the bulk of their energy not from common prey, but rather from a small subset of the largest possible insects. If correct, then orb webs should be designed to facilitate the capture of these insects, which are essential for spider fitness, even though they rarely fly into webs. Here, I test the generality of the “rare, large prey” hypothesis by comparing the frequency and biomass of large prey in 38 studies of the diets of 31 species of orb spiders in four different families. I define large prey as insects at least 66% as long as the spiders capturing them. The 38 studies included both large and small species of spiders, living in both tropical and temperate habitats. Large insects accounted for only 17% of the total number of prey captured by spiders, but contributed 85% of the total consumable biomass. The “rare, large prey” hypothesis thus can apparently be generalized across orb spiders. Future experiments need to account for the disproportionate influence of these large insects on spider fitness and on how to effectively measure these rare events. More importantly, the “rare, large prey” hypothesis provides a new framework in which to better understand variation in the web spinning behaviors of spiders.
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Vol. 39 • No. 2