Actinotrichid ancestor
Diagrammatic representation of the actinotrichid ancestor in lateral view.
– Comments. Relative to the mite archetype (not shown but see Fig. 16 in Coineau, 1974), two major deformations of the body structures are assumed to have occurred in the preactinotrichid ancestor (‘S-shaped curvature’ hypothesis sensu Grandjean, 1954a, p. 270, and 1969a, pp. 824, 825; see also Weigmann, 2001).
First, a cambering motion of the anterior part of the body associated with the bringing together of the dorsal regions of both the prosoma and the opisthosoma (vertically hatched and in yellow) has induced the gnathosoma (stippled and in green) to become terminal instead of ventral. An almost complete regression, owing to lack of place, of the dorsal part of the podosoma (in blue) has been likewise provoked by these motions. Consequently, with few exceptions (e.g. in some Palaeosomata in which a thin, dorsal cuticle, or ‘asthenic’ zone, is assumed to pertain to the podosoma), the dorsal region of the podosoma consists today of only a furrow (labelled das; also called ‘dorsosejugal furrow’ dsj). In fact, the furrow das corresponds to the junction of three limits, namely an abjugal limit abj. between the podosoma and the prodorsum (i.e. the dorsal region of the prosoma made up of the dorsal parts of precheliceral, cheliceral and pedipalpal, primitive segments; Grandjean, 1954a, pp. 268, 269; horizontally hatched and in orange), a sejugal limit sej. between the anterior and posterior parts of the podosoma, and a disjugal limit disj. between the podosoma and the opisthosoma.
Second, a downward bending of the posterior part of the opisthosoma has led the anal region AN becoming ventral instead of terminal. This motion has produced simultaneously a contraction, by compression, of the part of the opisthosoma between the anal AN and genital G regions.
Finally, the two narrowings that these movements have induced, i.e. the posteroventral one just mentioned and the anterodorsal one indicated above, have allowed the body to remain straight.
– Annotation. The infracapitulum is a conoid, hollow construction which forms the inner part of the gnathosoma. It bears the pedipalps Pp, the labrum LS and the lateral lips. It contains the mouth and the pharynx.
– Abbreviations. N, naso or unpaired, frontal (or precheliceral) protuberance overhanging the chelicerae and bearing an eye (often bilobate) in some Palaeosomata and Enarthronota (Grandjean, 1958b). Ch, chelicerae. w, capitular angle through which the axis of movement of the infracapitulum-body articulation passes. 1, 2, 3, 4, circumtrochanteral openings of legs I-IV, respectively. (modified from Grandjean, 1969a).
