Web-building behavior of the odd-clawed spider Progradungula otwayensis (Araneae: Gradungulidae) and implications for the evolution of combing behavior in spiders

Abstract

This study explores the web-building behavior of Gradungulidae for the first time, using the Otway odd-clawed spider Progradungula otwayensisMilledge, 1997. The web consists of a signal line leading to a retreat, an upper scaffold and a cribellate catching ladder connected to the substrate. The construction sequence and final structure of the catching ladder are highly stereotyped, especially in the initial segments of the cribellate capture thread. Two vertical rail lines - made of thin cribellate silk that is not combed with the calamistrum - are connected by looping segments of combed, much thicker cribellate capture threads. Small juveniles make a relatively larger non-sticky supporting scaffold with some cribellate silk on it. Microscopic analysis of the silk revealed that the cribellate bands have a pair of axial lines and a pair of reserve warps; the cribellate fibrils have nodules. The non-sticky structural lines are made of straight or coiling lines, changing gradually from straight to coiling; each of these structural lines is composed of tightly appressed fibrils, probably associated with the occurrence of multiple ampullate gland spigots. During combing behavior both legs IV are used synchronously. The same combing behavior is performed by the Tasmanian cave spider Hickmania troglodytes (Higgins & Petterd, 1883). We conclude that both type I and type II combing behaviors are monophyletic, although it is unclear which one is ancestral. Given the phylogenetic position of ProgradungulaForster & Gray, 1979, we suggest that the ability to measure with precision and produce repetitive, stereotyped silk structures made of specific silk types may have arisen close to the ancestor of araneomorph spiders.