Developmental changes in spider spinning fields: a comparison between Mimetus and Araneus (Araneae: Mimetidae, Araneidae)

Abstract

As part of an effort to understand the changes that piriform (Pi) and aciniform (Ac) silk glands undergo throughout the molt–intermolt cycle, spinnerets from all instars of two Mimetus Hentz, 1832 and one Araneus Clerck, 1757 species were examined by scanning electron microscopy. Especially informative were scans of spinning fields from the same individual over two or more consecutive stadia. These allowed us to determine that, in both genera, the Pi and Ac glands in use during a given stadium could be divided into two categories: (1) those with ducts accommodated by tartipores during proecdysis and (2) those not so accommodated and thereby apparently not functional during proecdysis. A silk gland in the former category, once developed, could presumably be used, at most, in every other stadium, whereas one of the latter category could potentially be used in every stadium. Unexpectedly, Pi glands of the latter category were constant in number [two per anterior lateral spinneret (ALS); four in total] throughout the ontogenies of both genera, and their spigots occupied the same location in the Pi spinning field in both genera. By contrast, tartipore-accommodated Pi glands increased in number from one stadium to the next in both genera. Of the Ac glands that had their spigots located on the posterior median spinnerets, none were tartipore-accommodated in either genera but, of the Ac glands with spigots on the posterior lateral spinnerets (PLS), some were tartipore-accommodated and some were not. All of them increased in number during the ontogenies of both genera. Adult males of three Mimetus species had two unusual spigots of unknown function on each ALS, which we interpret as modified Pi (MoPi) spigots. Their number and position raise the possibility that they serve (presumably modified) versions of the Pi glands that were not accommodated by tartipores during proecdysis. No spigots fully matching the description of Mimetus MoPi spigots were observed in adult males of four Australomimetus Heimer, 1986 or five Ero C. L. Koch, 1836 species. However, two of the Ero species did show a pair of Pi spigots on each ALS that, although not as conspicuous as Mimetus MoPi spigots, had wider openings than other Pi spigots. A minority of juvenile Mimetus specimens, and no adults, had a protuberance on one or both of their PLS, in a position similar to that of the aggregate (Ag)-flagelliform (Fl) spigot triad of araneoids, which may represent a phylogenetic vestige of a Fl or Ag spigot or one of their homologues. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 343–383.