Comparative study of spinning field development in two species of araneophagic spiders (Araneae, Mimetidae, Australomimetus)

Abstract

External studies of spider spinning fields allow us to make inferences about internal silk gland biology, including what happens to silk glands when the spider molts. Such studies often focus on adults, but juveniles can provide additional insight on spinning apparatus development and character polarity. Here we document and describe spinning fields at all stadia in two species of pirate spider (Mimetidae: Australomimetus spinosus, A. djuka). Pirate spiders nest within the ecribellate orb-building spiders (Araneoidea), but are vagrant, araneophagic members that do not build prey-capture webs. Correspondingly, they lack aggregate and flagelliform silk glands (AG, FL), specialized for forming prey-capture lines in araneoid orb webs. However, occasional possible vestiges of an AG or FL spigot, as observed in one juvenile A. spinosus specimen, are consistent with secondary loss of AG and FL. By comparing spigots from one stadium to tartipores from the next stadium, silk glands can be divided into those that are tartipore-accommodated (T-A), and thus functional during proecdysis, and those that are not (non-T-A). Though evidence was more extensive in A. spinosus, it was likely true for both species that the number of non-T-A piriform silk glands (PI) was constant (two pairs) through all stadia, while numbers of T-A PI rose incrementally. The two species differed in that A. spinosus had T-A minor ampullate and aciniform silk glands (MiA, AC) that were absent in A. djuka. First instars of A. djuka, however, appeared to retain vestiges of T-A MiA spigots, consistent with a plesiomorphic state in which T-A MiA (called secondary MiA) are present. T-A AC have not previously been observed in Australomimetus and the arrangement of their spigots on posterior lateral spinnerets was unlike that seen thus far in other mimetid genera. Though new AC and T-A PI apparently form throughout much of a spider's ontogeny, recurring spigot/tartipore arrangements indicated that AC and PI, after functioning during one stadium, were used again in each subsequent stadium (if non-T-A) or in alternate subsequent stadia (if T-A). In A. spinosus, sexual and geographic dimorphisms involving AC were noted. Cylindrical silk gland (CY) spigots were observed in mid-to-late juvenile, as well as adult, females of both species. Their use in juveniles, however, should not be assumed and only adult CY spigots had wide openings typical of mimetids. Neither species exhibited two pairs of modified PI spigots present in some adult male mimetids.

Hers and his: Silk glands used in egg sac construction by female spiders potentially repurposed by a ‘modern’ male spider

“…In studies of Mimetus puritanus Chamberlin, 1923 17 , Mimetus notius Chamberlin, 1923 17 , and Australomimetus spinosus Heimer, 1986 19 , CY spigots first appeared in female 3 rd instars, though often not the full set of four CY spigots in A. spinosus. By the 4 th stadium, however, all four CY spigots were invariably present, as they were in the only examined female 4 th instar of Australomimetus djuka Harms and Harvey, 2009 19 and in female 4 th instars of six other species of Australomimetus we have examined. In contrast, assuming our identifications of two 4 th stadium A. maculosus as males are correct (see Methods), it appears that the PMS CY spigots of males do not make their first appearance until the 5 th stadium (Table 1, Fig.…”

“…Drawing of cY silk by juveniles. We cannot assume that CY spigots in juvenile females are used in the drawing of CY silk 19 since CYs exhibit little silk synthesis prior to the onset of vitellogenesis in adults 7,8,21,27,28,51,52 . Likewise, we cannot assume a priori that silk is drawn from CY spigots of juvenile male A. maculosus, though it www.nature.com/scientificreports www.nature.com/scientificreports/ does at least seem more probable than in juvenile females.…”

“…one 2° MaA tartipore, with a primordium of this tartipore presumably present in 1 st instars 17,19,36 (none examined). Adults of both sexes differed from juveniles in possessing only a nonfunctional vestige of the 2° MaA spigot, called a 2° MaA nubbin.…”

“…Additional observations may expand the range of stadia that potentially include adults, but at a minimum, females may reach adulthood after six or seven molts, males after six, seven, or eight molts (Table 1). Variation in the number of molts to maturity has been observed in a wide range of araneomorph spiders 22,26,[37][38][39][40] , including other pirate spiders (Mimetus 17 , Australomimetus 19 ), while in other taxa constancy has been reported in one or both sexes [37][38][39]41 . Both genetic and environmental factors may be responsible for variation 37,38,[42][43][44] .…”

“…Adult females of most mimetids (except Gelanor) are known for having enlarged and stout CY spigots with dome-shaped shafts displaying wide apertures [16][17][18][19][30][31][32]45 . Again, adult females of A. maculosus conformed to this description (Fig.…”