Web-building behavior of the odd-clawed spider Progradungula otwayensis (Gradungulidae) and its implication on the evolution of combing behavior in spiders

RAMIREZ, M.J,; MICHALIK, P.

JOURNAL OF ARACHNOLOGY

AMER ARACHNOLOGICAL SOC

Lugar: Nueva York; Año: 2019 vol. 47 p. 299 - 309

0161-8202

This study explores the web-building behavior of Gradungulidae for the first time using the Otway odd-clawed spider Progradungula otwayensis. 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 is highly stereotyped, especially in the initial segments of cribellate capture thread. Two vertical rail lines are made of thin cribellate silk that is not combed with the calamistrum and that are connected by looping segments of combed, much thicker cribellate capture threads. Early 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 iscomposed 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. 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 Progradungula, 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.