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Phylogeny and systematics of the orb-weaver family Araneidae (Araneae, Araneoidea).

Final Activity Report Summary - ORB WEAVER PHYLOGENY (Phylogeny and systematics of the orb-weaver family Araneidae (Araneae, Araneoidea))

With 40 000 known species and an estimated total of more than 100 000 species, spiders range as the seventh most diverse group of organism worldwide, only surpassed by insects and mites. The diversity of spiders is particularly interesting, since all spiders are predators whereas the diversification of insects and mites are is often associated with co-evolution with plants and shifts in host plant use. All spiders produce silk from specialised glands in the abdomen and silk is used for many purposes such as egg sacs, line burrows, draglines and traps and the diversification of spiders is often attributed to evolutionary innovations in how spiders use their silk.

Spider silks have remarkable physical properties that allow them to absorb more energy before breakage than most biological and manmade materials. The use of silk in prey capture is epitomised by the spider group called Araneoidea, which construct aerial, wheel-shaped orb webs as prey capture devices. Orb web function depends upon the ability of silk to withstand the kinetic energy of flying prey and then to dissipate that energy such that insects do not escape. This combination of silk properties is achieved primarily through two types of fibres in the orb web. Orb-weaving spiders first construct a support network of non-sticky scaffolding and radii, and then they cover those radii with a spiral of sticky capture threads. The capture treads in orb webs can be of two very different types.

One type is physically adhesive through thousands of minute 'Velcro-like' fibrils that adhere to prey, the other type is adhesive because it is coated with aqueous glue. Based mainly on behavioural evidence, it has been claimed that orb webs evolved only once and that the 'Velcro-like' capture threads were the most basal type of capture thread within orb weavers, from which glue covered capture threads evolved. It has also been suggested that the most basal glue covered web type is an orb web and from those evolved first three-dimensional aerial sheets then three-dimensional cob webs (gumfoot webs).

The aim of our study was to establish a solid phylogenetic background for:
1) testing whether orb webs evolved one or several times within spiders;
2) to study the evolution of silk phenotypes within web building spiders; and
3) to determine whether silk phenotypes influence the diversification of spiders.

Together with colleagues at University of Akron and Smithsonian Institution (both in the United States) we collected morphological, behavioural and molecular data for phylogenetic analyses including an analysis of all data combined. The addition of molecular data for a phylogenetic analysis of this group of spiders has never been done before. Our results clearly support a single origin of orb webs and a transition from ancient 'Velcro-like' orb webs to glue-covered orb webs like the ones seen in the common garden spider (Araneus diadematus).

The innovation of glue-covered capture threads had profound influence on the diversification of orb weaving spiders, as seen by the living number of spiders that produce glue-covered capture threads (approximately 11 600 species) compared to those that still use 'Velcro-like' capture threads (approximately 300 species). Within orb weavers, orb webs have been lost or transformed into other web types several times.

Particularly successful transformations are found within 'dwarf spiders' (linyphioids with 4 400 species) where the orb web has been transformed into a highly three-dimensional aerial sheet web, and within the cob web spiders (theridioids with 2 500 species) where the orb web has been transformed into a highly three-dimensional cob webs (gumfoot webs). Webs have been lost entirely several times within orb weavers, but none of these groups are particular diverse.