Cues that Spiders (Araneae: Araneidae, Tetragnathidae) Use to Build Orbs: Lapses in Attention to One Set of Cues because of Dissonance with Others?

The evolutionary diversification of spiders is attributed to spectacular innovations in silk. Spiders are unique in synthesizing many different kinds of silk, and using silk for a variety of ecological functions throughout their lives, particularly to make prey-catching webs. Here, we construct a broad higher-level phylogeny of spiders combining molecular data with traditional morphological and behavioral characters. We use this phylogeny to test the hypothesis that the spider orb web evolved only once. We then examine spider diversification in relation to different web architectures and silk use. We find strong support for a single origin of orb webs, implying a major shift in the spinning of capture silk and repeated loss or transformation of orb webs. We show that abandonment of costly cribellate capture silk correlates with the 2 major diversification events in spiders (1). Replacement of cribellate silk by aqueous silk glue may explain the greater diversity of modern orb-weaving spiders (Araneoidea) compared with cribellate orb-weaving spiders (Deinopoidea) (2). Within the "RTA clade," which is the sister group to orb-weaving spiders and contains half of all spider diversity, >90% of species richness is associated with repeated loss of cribellate silk and abandonment of prey capture webs. Accompanying cribellum loss in both groups is a release from substrate-constrained webs, whether by aerially suspended webs, or by abandoning webs altogether. These behavioral shifts in silk and web production by spiders thus likely played a key role in the dramatic evolutionary success and ecological dominance of spiders as predators of insects.

This review focuses on the evolutionary causes and consequences of limited attention, defined as the restricted rate of information processing by the brain. The available data suggest, first, that limited attention is a major cognitive constraint determining animals’ search for cryptic food, and, second, that limited attention reduces animals’ ability to detect predators while involved in challenging tasks such as searching for cryptic food. These two effects of limited attention probably decrease animal fitness. Furthermore, a simulated evolutionary study provides empirical support for the prediction that focused attention by predators selects for prey polymorphism. The neurobiological mechanisms underlying limited attention have been widely studied. A recent incorporation of that mechanistic knowledge into an ecological model suggests that limited attention is an optimal strategy that balances effective yet economical search for cryptic objects. The review concludes with a set of testable predictions aimed to expand the currently limited empirical knowledge on the evolutionary ecology of limited attention.