Evolutionary kinematics of spinneret movements for rapid silk thread anchorage in spiders.

Many organisms secrete structural materials from their bodies to enhance protection, foraging or signalling. The function of such secretion products can be further extended by their assembly into complex structures, so-called extended phenotypes, such as shells, nests and biofilms. Understanding the variation in the efficacy of such assembly processes could help to explain why extended phenotypes are common on some lineages and rare in others. Here, I comparatively studied the assembly of sticky silk fibres into thread anchorages by the innate 'printing' behaviour in 92 species of spiders from 45 families, representing the so-far largest comparative study of construction-related motion patterns. I found a global evolutionary trend towards a faster production of silk thread anchorages, in both web builders and hunting spiders. The slowest producers of silk anchors belong to a clade with an ancestral configuration of respiratory organs, suggesting that a major constraint to the evolution of spinning speed is the efficiency of oxygen uptake. Motion patterns were found to contain a high phylogenetic signal, but did not correlate with spinning speeds. These results help to explain the variation in diversity and ecological success among the spider fauna and showcase the value of comparative kinematics in biodiversity studies.