Fine structure of spermatozoa of some labidognath spiders (Filistatidae, Segestriidae, Dysderidae, Oonopidae, Scytodidae, Pholcidae; Araneae; Arachnida) with remarks on spermiogenesis

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Abstract
The present study reports on the spermiogenesis and spermatozoa of seven labidognath spiders: Filistata insidiatrix (Filistatidae), Segestria senoculata (Segestriidae), Dysdera sp., Harpactea hombergi (Dysderidae), Oonops domesticus (Oonopidae), Scytodes thoracica (Scytodidae), and Pholcus phalangioides (Pholcidae). Filistata insidiatrix is the first cribellate spider whose spermatology is described electron microscopically. A common characteristic of the spermatozoa of the cribellate spider and the remaining species, often referred to as haplogyne spiders, is the coiling process that occurs at the end of spermiogenesis. As a result of coiling, an elongated spermatid is converted into a lens-shaped structure with the flagellum bearing a 9 × 2 + 3 axoneme becoming incorporated into the cell body. Remarkable differences regarding the main components (shape of nucleus, acrosomal vacuole, implantation fossa, and centriolar complex) probably reflect systematic relationships. The formation of sperm capsules and sperm balls is described for the first time in detail. Sperm capsules occur in Filistata, in which numbers of individual spermatozoa are grouped together by a common secretory envelope established in the distal vas deferens. In contrast, in the sperm balls, two (Harpactea) or four (Segestria, Dysdera, Seytodes) spermatids fuse completely at the end of spermiogenesis. These sperm balls, considered unique in the animal kingdom, are also provided with an envelope. A further peculiarity not reported previously is the occurrence of a large vesicular area in the sperm balls of Dysdera and Harpactea; this area is also found in Oonops, which, however, possesses individual spermatozoa. Components of the spermatozoa such as the acrosomal vacuole, part of the nucleus, and the axoneme protrude into this area and are thus secondarily covered with a membrane. A detailed study of the individual spermatozoa of Pholcus phalangioides completes earlier investigations and stresses the exceptional position of the genus in comparison to that of other spiders.