The sejugal furrow in camel spiders and acariform mites

Camel spiders (Arachnida: Solifugae) are one of the arachnid groups characterised by a prosomal dorsal shield composed of three distinct elements: the pro-, mesoand 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 4th and 5th 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 homologous 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.

In camel spiders, schizomids (Schizomida) and palpigrades (Palpigradi) the peltidium is not a single plate, but is divided into a series of discrete dorsal sclerites. These are conventionally referred to as the pro-, mesoand metapeltidium. In fact the camel spider propeltidium seems to be even more complex and composed of multiple elements (KÄSTNER 1932, ROEWER 1932.
Authors such as BERNARD (1896,1897) and Kästner (1932) interpreted this basic tagmosis pattern in camel spiders as plesiomorphic, presumably reflecting a grade of organisation which predates the traditional arachnid prosoma. Other workers explicitly treated a 'divided carapace' as a derived character state (WEYGOLDT &PAULUS 1979, SHULTZ 1990, 2007. Irrespective of polarity, the camel spider condition has interesting parallels with certain mites (Acari), which also express a dorsal sclerite again associated with the chelicerae, pedipalps and the first two pairs of walking legs (COINEAU 1974, EVANS 1992, ALBERTI & Coons 1999, WeigMANN 2001. This whole body region down to the second pair oflegs has been termed the proterosoma and the dorsal sclerite covering it is usually called the prodorsum (e.g. WEIGMANN 2001). The name 'aspidosoma' can also 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 30 JA. Dunlop, J. Krüger & G. Alberti such as Grandjean (1969), Coineau (1974) and VAN DER Hammen (1989). In general, issues remain among mites with respect to questions of segmental homology and the use of a standard terminology.
These observations also reflect two recurrent problems in arachnid comparative morphology (see e.g. Dunlop 2000). The first is the use of divergent terminologies for essentially the same structures in mites and non-mite taxa. The second is the use of the same term, e.g. carapace', for non-homologous structures across different arthropod groups. Such discrepancies in nomenclature can mask potential synapomorphies.
Here, we draw attention to an olderalbeit largely overlookedobservation that camel spiders not only have an obvious dorsal division of the prosoma, but also express a distinct ventral division ( Fig. 1 Furthermore, there is no superficial evidence of a 'break' between the successive coxal pairs. In fact the dividing line elaborated here is best revealed by simply taking a specimen and gently bending the prosoma backwards or sideways. The ventral surface naturally opens up between the second and third pair of leg coxae (Fig. 1); precisely because they are separated by a pale, flexible membrane; superficially similar to the pedicel (or petiolus) of a spider. In gross morphology this membrane is similar in form to an arthrodial membrane between adjacent limb articles and does not reveal any embedded sclerites. It forms a distinct narrowing, with a maximum width about a third of the width of the adjacent coxal pairs, and can be followed as a dividing line up the lateral sides of the animalwhere it merges smoothly into the dorsal membrane dividing the propeltidium from the mesopeltidium.
Significantly, physical manipulation of the prosoma reveals that none of the other coxal pairs can be teased apart in this way to the same extent. In other words, the coxae of the pedipalps, plus legs 1 and 2, essentially form an anterior functional unit. The coxae of legs 3 and 4 form a corresponding posterior functional unit. We interpret this as clear ventral evidence of tagmosis; whereby the soft, membranous suture ( Fig.   1: mb) defines an anterior body region bearing the chelicerae, pedipalps and first two pairs of walking legs: the same body region that is dorsally associated with the propeltidium.

Discussion
Here we confirm and illustrate previous observations about the flexibility of the camel spider body between the second and third pair of walking legs. The body region defined dorsally by the propeltidium in Solifugae is also delimited ventrally by a membranous region (Fig. 1

Poecilophysidea
The presence ofwhat we interpret as a sejugal furrow in camel spiders further emphasises their morphological similarity to certain mites (Figs. 2-5) (see also Dunlop , 2000. Specifically, the sejugal furrow is another potential synapomorphy for a relationship of the form (Solifugae + Acariformes). Most authors have recovered camel spiders as the sister group of pseudoscorpiones (WEYGOLDT &c PAULUS 1979, VAN DER Hammen 1989, Shultz 1990, 2007 (BANKS 1915). Mites, solifuges (and also pseudoscorpions) have a mouth on a projecting 'beak', or rostrum in some terminologies, and also However, but it may be better to treat them as parts of a single character complex relating to tagmosis.
In addition to this morphological data, recent molecular (DABERT et al. 2010) and combined (Pe-PATO et al. 2010)  Adopt sea spiders as the outgroup and the 'divided carapace' / sejugal furrow could be interpreted as a plesiomorphic state; retained from an earlier grade of organisation. This is essentially the argument put forwards by BERNARD (1886) and KÄSTNER (1932,1952) who thought that the divided camel spider prosoma revealed the original arachnid morphology.
Authors such as RemANE (1962: 214) have argued that the arachnid prosoma fundamentally consists of a four-segmented head regionbearing the chelicerae, palps and legs 1 and 2plus two additional segments bearing leg pairs 3 and 4 respectively. Further discussion can be found in KRAUS (1976), who again favoured the idea that separate prosomal elements reflect a '4+2' arachnid groundplan, or WEYGOLDT 8c Paulus (1979) who preferred instead to interpret these divisions as derived and homoplastic features, possibly adapted for increasing prosomal mobility.
Finally, we should mention a series of early Palaeozoic arthropods expressing raptorial anterior limbs the 'great appendage' arthropods, or Megacheira in some schemeswhich some authors interpret as stem-group Chelicerata (CHEN et al. 2004). These fossils also appear to preserve an anterior body tagma bearing four pairs of appendages which authors such as Waloszek and co-workers have termed the 'euarthropod head' (see also ReMANE's 1962 hypothesis) and which they interpret as a fundamental part of the body plan in early arthropods (cf Chen 2009: Fig.   11). Using megacheirans as an outgroup would again polarise the tagmosis pattern of mites, camel spiders (and palpigrades?) as a plesiomorphic, groundplan, character state for arachnids. In this scenario, a unitary prosomal dorsal shield (or peltidium) emerges as a derived character state; perhaps even homoplastic across Arachnida.