Gnathosoma, Krefft, 1871

Fajfer, Monika, 2019, Systematics of reptile-associated scale mites of the genus Pterygosoma (Acariformes: Pterygosomatidae) derived from external morphology, Zootaxa 4603 (3), pp. 401-440 : 407

Gnathosoma

In the genus Pterygosoma the gnathosoma is relatively well-developed, at least three times shorter than the idiosoma and typically situated terminally or subterminally ( Figs 1 and 2 ).

Chelicerae. The fixed cheliceral digit (digitus fixus) is strongly reduced, and may be rounded, membranous, spiniform or branched with few or numerous tines in the apical part ( Fig. 4 A ̄D). The movable digit (digitus mobilis) in most species has twisted so that the terminal point and the basal spur are directed laterally, thus acting as a barb or holdfast, in a similar way to the chelicerae of ticks. The basal spur is absent only in the subgenus Gerrhosaurobia and the ligare group of Pterygosoma s. str. (see Fajfer & Gonzalez-Acuna 2013).

Hypostome. In all pterygosomatids, the hypostome forms a flat or slightly rolled plate in which the chelicerae slide and is situated ventrally, except for mites of the genus Ixodiderma where the hypostome lies dorsally to the chelicerae and is curved round slightly below them. In the genus Pterygosoma the hypostomal apex takes different shapes in various species ( Fig. 4E ̄I). It can be rounded, triangular, flat or with a small depression, and its tip may be smooth or with different-sized teeth. Fused palpal coxae are situated basally to the hypostome which form a subcapitulum equipped with one pair of proximal subcapitular setae n.

Palpal segmentation. The palpal tibia bears a well-developed claw and strongly reduced tarsus ( Fig. 3 ). The tibial claw is more or less curved and differs in length among different species, only in three species of Pterygosoma ( P. gracilipalpis Jack, 1962 , P. tuberculata Jack, 1962 and P. caucasica Jack, 1962 ) the tibial claw is absent and the eupathidion (ζ) is present. In most Pterygosoma species the palpal tarsus is partly or completely fused with the tibia, whereas in the subgenus Gerrhosaurobia and the ligare group of Pterygosoma s. str. it is rounded, well-articulated and displaced to the ventral surface of the tibia.

Palpal setation. In all Pterygosoma species the palpal setae are strongly reduced in number. The palpal femur and genu bears one mid-dorsal seta dF and dG, respectively ( Fig. 3A ). The palpal tibia bears three setae: two lateral setae (l’Ti, l”Ti) and one dorsal (dTi) seta (only in P. gracilipalpis , P. caucasica and P. tuberculata an additional seta, vTi, in the form of a eupathidion may be present as a replacement of the tibial claw).

Among Pterygosoma species, the subgenus Gerrhosaurobia has the richest tarsal chaetotaxy and possesses four non-modified tactile setae (ba, bp, lp and va), one or two terminal eupathidia (ul’, sul) and solenidion ω ( Fig. 3D ). In representatives of the ligare group and P. aegyptiaca Mostafa, 1974 ( Baker 2017) five setae (ba, bp, lp, va, ul’) and solenidion ω ( Fig. 3E ) are present whereas in the remaining species of Pterygosoma four (ba, bp, lp, va) ( Fig. 3C ) or three (ba, bp and va) simple setae occur.

The respiratory structures, like stigmae and peritremes are situated dorsally. The stigma openings are located between the swollen cheliceral parts in the midline of the chelicerae and the segmented peritremes run from each stigma antero-laterally.

Figures

FIGURE 1. Pterygosoma agamae Peters, 1849, general scheme. A, dorsal view; B, ventral view. Abbreviations: perit, peritreme; ant, anterior setae; lat, lateral setae; post, posterior setae; per, peripheral setae; dm, dorso-median setae; vm, ventromedian setae; ps, pseudanal setae; g, genital setae; sca, subcapitulum; c.fs, coxal fields; Tr, trochanter; F, femur; G, genu; Ti, tibia; Ta, tarsus.

FIGURE 2. Male of Pterygosoma sp., general scheme. A, dorsal view; B, ventral view. Abbreviations: ae, aedeagus; ant, anterior setae; c.fs, coxal fields; g.c, genital cone; dm, dorso-median setae; vm, ventro-median setae; perit, peritreme; post.-lat, postero-lateral setae; sca, subcapitulum.

FIGURE 3. Gnathosoma of Pterygosoma aqabensis Fajfer and Melnikov, 2014. A, dorsal view; B, ventral view. Palpal tarsi of: C, Pterygosoma persicus Hirst, 1926; D, Gerrhosaurobia bicolor (Lawrence, 1936); E, Pterygosoma levissima Fajfer and González-Acuña, 2013.

FIGURE 4. Fixed cheliceral digit. A, spiniform; B, elongated; C, branched; D, rounded. Hypostomal apex. E, rounded; F, triangular; G, flattened; H, with depression; I, with teeth.