Abstract
Spiders are the most important terrestrial predators among arthropods. Their ecological success is reflected by a high biodiversity and the conquest of nearly every terrestrial habitat. Spiders are closely associated with silk, a material, often seen to be responsible for their great ecological success and gaining high attention in life sciences. However, it is often overlooked that more than half of all Recent spider species have abandoned web building or never developed such an adaptation. These species must have found other, more economic solutions for prey capture and retention, compensating the higher energy costs of increased locomotion activity. Here we show that hairy adhesive pads (scopulae) are closely associated with the convergent evolution of a vagrant life style, resulting in highly diversified lineages of at least, equal importance as the derived web building taxa. Previous studies often highlighted the idea that scopulae have the primary function of assisting locomotion, neglecting the fact that only the distal most pads (claw tufts) are suitable for those purposes. The former observations, that scopulae are used in prey capture, are largely overlooked. Our results suggest the scopulae evolved as a substitute for silk in controlling prey and that the claw tufts are, in most cases, a secondary development. Evolutionary trends towards specialized claw tufts and their composition from a low number of enlarged setae to a dense array of slender ones, as well as the secondary loss of those pads are discussed further. Hypotheses about the origin of the adhesive setae and their diversification throughout evolution are provided.
Highlights
Spiders are, besides insects and mites, the most successful terrestrial arthropods and are of superior ecological importance as predators [1,2]
(1) Adhesive setae are present in the derived Mygalomorphae, including Nemesiidae, Cyrtaucheniidae, Paratropidae, Barychelidae, Theraphosidae, and Idiopidae
Similarities in the fine structure of the adhesive setae and the occurrence of intermediate forms indicate that claw tufts previously evolved from scopulae in the mygalomorph lineage, Tengellidae, Desidae, Zoropsidae, Ctenidae, and higher Dionycha, excluding Liocranidae, Clubionidae, and Anyphaenidae
Summary
Besides insects and mites, the most successful terrestrial arthropods and are of superior ecological importance as predators [1,2]. The evolution of viscid silk and orb webs radically reduced the web associated costs and led to an enormous radiation and success of those lineages [9,10]. Another strategy was at least successful: Many spider lineages never developed a silk-dependent prey capture mode or have independently abandoned web building and explored alternative hunting strategies [9], resulting in more than one half of all Recent spider species hunting without webs [10]
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