Abstract
Camel spiders (Arachnida: Solifugae) are one of the arachnid groups characterised by a prosomal dorsal shield composed of three distinct elements: the pro-, meso- and metapeltidium. These are associated respectively with prosomal appendages one to four, five, and six. What is less well known, although noted in the historical literature, is that the coxae of the 4 th and 5 th prosomal segments (i.e. walking legs 2 and 3) of camel spiders are also separated ventrally by a distinct membranous region, which is absent between the coxae of the other legs. We suggest that this essentially ventral division of the prosoma specifically between coxae 2 and 3 is homolo - gous with the so-called sejugal furrow (the sejugal interval sensu van der Hammen). This division constitutes a fundamental part of the body plan in acariform mites (Arachnida: Acariformes). If homologous, this sejugal furrow could represent a further potential synapomorphy for (Solifugae + Acariformes); a relationship with increasing morphological and molecular support. Alternatively, outgroup comparison with sea spiders (Pycnogonida) and certain early Palaeozoic fossils could imply that the sejugal furrow defines an older tagma, derived from a more basal grade of organisation. In this scenario the (still) divided prosoma of acariform mites and camel spiders would be plesiomorphic. This interpretation challenges the textbook arachnid character of a peltidium (or 'carapace') covering an undivided prosoma.
Highlights
Camel spiders (Arachnida, Solifugae) are a fascinating group of arachnids which, as their name implies, predominantly occur in arid habitats
The name ‘aspidosoma’ can be found in the literature but, as discussed by Weigmann, this term should refer to tergites explicitly associated with the gnathosoma, and there is no evidence that these structures have overgrown the rest of the proterosoma as per the evolutionary scenarios proposed by authors such as Grandjean (1969), Coineau (1974) and Historically, Kittary (1848) differentiated the camel van der Hammen (1989)
As noted by Bernard and Kästner above, an alternative interpretation would be that the body region defined by the propeltidium/sejugal furrow Fig. 1: Camel spider (Solifugae: Galeodes sp.) in ventral view
Summary
Camel spiders (Arachnida, Solifugae) are a fascinating group of arachnids which, as their name implies, predominantly occur in arid habitats. Roewer have an obvious dorsal division of the prosoma, and express a distinct ventral division (Fig. 1), between the coxae of the second and third pair of walking legs (Bernard 1896, Roewer 1932, van der Hammen 1989).
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