Mites and ticks can be divided into two well-defined clades, Anactinotrichida and Actinotrichida, for which a recent work formalized a suite of putative autapomorphies and reciprocal differences. Whether they are sister-taxa -forming a monophyletic Acari -is more controversial. Earlier supporters of two independent origins for mites largely failed to demonstrate convincing synapomorphies between either of the two lineages and other arachnid orders; although recent work on reproductive biology revealed explicit characters of this nature. Furthermore, some of the characters proposed in support of a monophyletic Acari do not stand up to detailed scrutiny when compared with Arachnida in general. Effective morphological comparisons between mites and other arachnids are hindered by incompatible nomenclature and long-standing, mite-specific characters which are difficult to score for other arachnids. Furthermore, taxon-specific characters restricted to individual mite groups have sometimes been treated erroneously as ÔtypicalÕ for all Acari. Here, previous hypotheses of mite affinities are reviewed. Historically, authors have debated whether mites are basal arachnids or highly derived. Excluding weakly supported early hypotheses, mites have been resolved -in whole or in part -as sister-group of all other Arachnida (based on tagmosis), closely related to Opiliones (based mostly on genital morphology), Palpigradi (based on controversial interpretations of limb morphology), Solifugae (based mostly on the mouthparts, but now perhaps also reproductive characters) and Ricinulei (based on hexapodal larvae and perhaps mouthparts). We cannot provide a final resolution here, but we aim to highlight important character sets which should be included in subsequent phylogenetic analyses, as well as useful areas for future investigations: particularly tagmosis and the nature of the gnathosoma.

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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.

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