PHYSICAL CONSTRAINTS ON GROUP FORAGING AND SOCIAL EVOLUTION - OBSERVATIONS ON WEB-SPINNING SPIDERS

Ultraviolet (UV)-reflecting visual displays that attract prey and ensure frequent insect-web encounter may be an important factor in the evolution of obligate, group foraging and feeding among spiders. Manipulation of the UV-reflecting properties of webs spun by Argiope argentata (Fabricius) and presence of the spider (also UV reflecting) show that in spatially heterogeneous habitats, spiders foraging in groups attract more insects than solitary ones. Once drawn to a spider cluster, UV-reflecting web decorations lure insects to them and away from undecorated webs spun by neighbouring spiders. Phylogenetic comparisons show that group foraging has evolved frequently among primitive web-spinning spiders that produce silks characterized by high reflectance in the ultraviolet region but infrequently among the derived web spinners that produce silks characterized by low reflectance in the UV or that are spectrally flat. The experimental data presented here, together with data on the spectral properties of silks spun by obligate group foragers and facultative group foragers, suggest that the ability to spin UV-reflecting silks that attract prey and that ensure high rates of insect-web encounter has allowed the evolution of obligate foraging even in sites where prey densities are low or unpredictable. In contrast, group foraging has evolved among spiders that spin silks characterized by low reflectance in the UV, those that do not attract prey, only when insect abundance is high.