Changes in spinning anatomy and thread stickiness associated with the origin of orb-weaving spiders

The cribellum is an oval spinning field whose spigots produce silk fibrils that form the outer surfaces of the primitive prey capture threads found in aerial spider webs. A comparison of the cribella and cribellar capture thr eads of 13 species of spiders representing seven families (Amaurobiidae, De sidae, Dictynidae, Filistatidae, Neolanidae, Oecobiidae, and Uloboridae) co nfirms that the stickness of a cribellar thread is directly related to the number of spigots on a spider's cribellum. This comparison also demonstrate s that the origin of orb-weaving spiders from ancestors that constructed le ss highly organized webs was associated with increases in both the weight-s pecific number of cribellum spigots and the weight-specific stickiness of c ribellar prey capture threads. In contrast to other cribellate spiders, the number of cribellum spigots of orb-weaving species of the family Uloborida e scales to spider mass. Thus, the origin of orb-weaving spiders involved n ot only behavioural changes that stylized and restricted the placement of c ribellar threads, but also included morphological changes that increased th e stickiness of these capture threads by endowing them with more cribellar fibrils. (C) 1999 The Linnean Society of London.