Trhypochthoniellus (Weigmann, 1997)

Leg setation in Trhypochthoniellus View in CoL

Seniczak et al. (1998) presented the setal ontogeny of T. longisetus (Berlese, 1904) under the synonym T. crassus ( Warburton & Pearce, 1905) . [See Weigmann (1997a) for discussion of synonymies in this group.]. Notable traits include: the presence of four larval setae (including cʺ) on tibia I, with no additions; the absence of cʺ from larval tibia II, with no additions, for a final complement of three; the absence of several fundamental tarsal setae, including the primilateral pair (pl) from tarsus I, primiventral setae (pv) from tarsi I–III, and fastigial seta ftʹ from tarsus III. Of antelateral setae, only aʹ forms on tarsus IV. No accessory setae form on any tarsus.

Illustrations of adult legs were provided by Weigmann (1997a); they are unlabelled, and slight variations in leg orientation make setal identifications somewhat challenging, but they accord well with the ontogenetic table. The absence of eustasic pair (pv) from tarsus I, as well as II–III, and the absence of any proximal accessory setae, conspicuously leaves the unpaired seta s as the most proximal seta in the ventral region of these tarsi. Eustasic pair (a) forms as usual on these tarsi, occupying a clearly lateral position, slightly higher than usual in oribatid mites and slightly distal to the level of s.

For most other Trhypochthoniellus species leg setation is unknown or analysed based only on adults. In T. chilensis Ermilov & Weigmann, 2015 and T. malaconothriformis Ermilov et al., 2017 , the tarsal setation seems to differ from that of T. longisetus in two ways. First, both species seem to form pair (pv) on tarsus I; their homology could only be confirmed with knowledge of early instars, but it seems likely they are (pv), since no proximal accessory setae are known in this genus. The second difference is that the partial regression of pair (a) on tarsus IV of T. longisetus seems to extend also to the other tarsi in T. chilensis and T. malaconothriformis , such that only the abaxial (antiaxial) member of the pair is formed (aʺ on I/II, aʹ on III/IV). These authors misidentified the antelateral setae as accessory iteral setae (itʺ, itʹ), which do not develop on any known member of Trhypochthoniidae ; antelateral setae can be unusually high in this genus, close to or even distal to setae tc. Tarsi I–III of T. churincensis Ojeda et al., 2020 also lack pair (pv) and probably one antelateral seta; the latter point is equivocal, as their illustrations are difficult to interpret and they wrongly applied many setal notations (including it); tarsus IV is like that of all the other Trhypochthoniellus species. The only other data we have comes from a past study of adult type specimens of T. ramosus Hammer, 1982 (R.A.N., unpublished, 1996); it has leg setation identical to that of adult T. longisetus except for lacking seta vʹ from trochanter III. Overall, the data allow the following generalizations Trhypochthoniellus species lack the primiventral pair (pv) on tarsi II, III and in some species also on I; antelateral setae are present as a pair on tarsi I–III or only the abaxial seta forms; leg IV always forms pair (pv) and seta aʹ; no proximal accessory setae form on any tarsus.