Tachytixenos Pierce, 1911, stat. res.

Benda, Daniel, Pohl, Hans, Nakase, Yuta, Beutel, Rolf & Straka, Jakub, 2022, A generic classification of Xenidae (Strepsiptera) based on the morphology of the female cephalothorax and male cephalotheca with a preliminary checklist of species, ZooKeys 1093, pp. 1-134 : 1

Tachytixenos Pierce, 1911, stat. res.

Tachytixenos Pierce, 1911: 501. Type species: Tachytixenos indicus Pierce, 1911, by original designation.

Pseudoxenos Saunders, 1872 (partim!) (synonymy proposed by Hofeneder 1949: 148).

Paraxenos Saunders, 1872 (partim!) (synonymy proposed by Kinzelbach 1971b: 162).

Diagnosis of female cephalothorax.

Differing from the other genera by a specific shape of the mandibular tooth, which is very wide basally and reaches the area of mandibular bulge. Tooth with pointed, ventrally directed apex. Base of tooth ventrally covered with small depressions continuous with several rows of spines (Fig. 14E ). Prosternal extension undifferentiated (compared to similar genus Paraxenos ), evenly arched, without any swelling or color differentiation. Maxillae distinctly prominent as in Pseudoxenos , Tuberoxenos , and some Paraxenos species. Mandible not protruding from capsule. In contrast to Paragioxenos , head and prothorax ventrally delimited by birth opening medially and by suture laterally.

Description of female cephalothorax.

Shape and coloration. Cephalothorax compact, ca. as long as wide, or slightly wider than long, or vice versa. Size varying strongly within genus, length 0.94-1.82 mm, width 0.88-1.88 mm. Anterior head margin evenly rounded or projecting. Thorax slightly widening posteriorly. Coloration comprising multiple brown shades and distinct patterns (Fig. 12C, D ).

Head capsule. Approximately ¼ ~ ½ as long as entire cephalothorax including lateral extensions. Coloration variable, pale, completely dark brown, or forming specific color pattern. Clypeal area well delimited from labral area, arcuate, or slightly protruding anteriorly forming clypeal lobe. Surface of clypeal area smooth or slightly wrinkled. Sensilla (~ 40-55) regularly dispersed over clypeal surface or mainly concentrated on clypeal lobe. Border between clypeal and frontal region present but indistinct. Frontal region smooth or slightly wrinkled. Dorsal segmental border between head and prothorax distinct or only recognizable.

Supra-antennal sensillary field. Smooth with dispersed sensilla, delimited by distinct furrow on medial side (Fig. 13B ).

Antenna. Preserved as poorly defined area with several minute rounded plates, antennal sensilla, or cavity, in some cases all three combined. Periantennal area smooth, flat, or forming incomplete elliptic wall between antenna and supra-antennal sensillary field (Fig. 13C, D ).

Labrum. Ventral field wider than long, elliptic. Dorsal field slightly arcuate, at least 3 × wider than long in midline. Dorsal field bearing ~ 15-30 setae inserted in cavities.

Mandible. Anteromedially directed at angle of 40-65°, enclosed in mandibular capsule. Mandibular bulge not distinctly raised, with several sensilla. Cuticle completely smooth to slightly sculptured. Mandibular tooth very wide on its base, reaching area of mandibular bulge. Tooth ventrally directed and pointed apically. Base with small depressions continuous with several rows of spines (Fig. 14E ).

Maxilla. Well-developed, prominent, and clearly separated from labial area, strongly sclerotized, directed anteriorly or anteromedially. Not or very slightly overlapping with mandible proximally, not projecting beyond mandibular apex anteriorly. Cuticle usually smooth, rarely wrinkled. Vestige of palp distinct, forming small bulge with more or less distinct plates, situated medially on ventral side of maxilla. Submaxillary groove slightly produced posterolaterally.

Labium. Labial area between maxillae distinct, delimited anteriorly by mouth opening and posteriorly by birth opening. Wider than long in midline and flat or convex. Cuticular surface smooth or slightly reticulated.

Mouth opening. Mouth opening arcuate, sclerotized along margin.

Thorax and abdominal segment I. Pro-mesothoracic and meso-metathoracic borders more or less distinct, usually separated by mesal furrows, often combined with pigmented stripes or spots on dorsal side. Border between metathorax and abdomen usually formed by ridge. Cuticle of thoracic segments on ventral side reticulate, with small scattered pigmented papillae. Dorsal side of thorax smooth or slightly reticulated. Prosternal extension undifferentiated, evenly arched. Shape of meso- and metathorax unmodified, transverse. Setae present on lateral region of abdominal segment I. Cuticular surface distinctly sculptured in cases with sparse setation (Fig. 13E ).

Spiracles. Located on posterior ~ ⅓ of cephalothorax, slightly elevated, with lateral, anterolateral, or dorsal orientation.

Diagnosis of male cephalotheca.

Genus characterized by combination of distinct paired furrow of supra-antennal sensillary field (Fig. 15A, D ) and shape of mandibular tooth. Mandibular tooth very wide on its base and reaching area of mandibular bulge. Tooth base with small depressions continuous with several rows of spines (Fig. 15E , see also 14E). Diameter of genae between maxillary base and compound eye at least 2 × as large as diameter of vestigial antenna.

Description of male cephalotheca.

Shape and coloration. Shape of cephalotheca rounded laterally in frontal view, widely elliptic. Anteriorly pointed in lateral view. Coloration forming pattern of pale and dark shades.

Cephalothecal capsule. Compound eyes with darker individual ommatidia well visible on pale background. Clypeal lobe straight in frontal view, distinctly prominent in lateral view. Sensilla mainly concentrated on clypeal lobe. Frontal region with paired furrow of supra-antennal sensillary field, lacking frontal impression. Diameter of genae between maxillary base and compound eye large, ~ 3 × as large as diameter of vestigial antenna. Occipital bulge absent.

Supra-antennal sensillary field. Kidney-shaped and bulging, delimited medially by distinct furrow. Furrows relatively wide and not interconnected anteriorly (Fig. 15A, D ).

Antenna. Of standard shape, small, with complete torulus. Periantennal area not distinctly delimited. Sensilla present (Fig. 15C ).

Labrum. Labral area distinct. Setae on dorsal field present.

Mandible. Mandible anteromedially directed. Mandibular tooth very wide on its base and reaches area of mandibular bulge. Tooth base with small depressions continuing in several rows of spines (Fig. 15E ). Mandibular bulge bears several sensilla.

Maxilla. Maxilla distinct, prominent, completely dark. Vestige of maxillary palp distinct.

Labium and hypopharynx. Well-developed between and below maxillae, completely dark. Praementum and postmentum slightly separated by furrow. Hypopharyngeal protuberance absent.

Mouth opening. Mouth opening well visible, not covered by ventral labral field, slightly arcuate.

Phylogenetic relationships.

One of the earliest diverging lineages of Xenidae . Forming a clade of Palearctic origin with its sister genus Paraxenos ( Benda et al. 2019).

Diversity and distribution.

Monotypic, restricted to the Old World.

Hosts.

Tachytes spp. ( Crabronidae : Crabroninae ).

Comments.

The monotypic genus Tachytixenos was described by Pierce (1911) but only superficial descriptions of the female and male without illustrations were provided. Hofeneder (1949) synonymized it with Pseudoxenos , but it was later classified as Paraxenos by Kinzelbach (1971b). We restored Tachytixenos from synonymy and classify it as a valid genus based on monophyly revealed by the molecular phylogeny ( Benda et al. 2019, 2021) and based on morphological characters newly reported here

Note.

Cook (1919) noted that Bohart synonymized Tachytixenos with Pseudoxenos but it was done laterally by Hofeneder (1949).

Figures

Figure 12. Tachytixenos cf. indicus Pierce, host, female, cephalothorax, photomicrographs A Tachytes sp. stylopized by female of T. cf. indicus, lateral view B detail of host abdomen with adult female inside C ventral side of cephalothorax D dorsal side of cephalothorax.

Figure 13. Tachytixenos cf. indicus Pierce, female, cephalothorax, SEM micrographs A ventral side B dorsal side C left vestigial antenna, dorsal side D right vestigial antenna, dorsal side E left lateral border of abdominal segment I below spiracle, dorsal side F detail of anterior border of cephalothorax, dorsal side. Abbreviations: fssf - furrow of supra-antennal sensillary field, paa - periantennal area.

Figure 14. Tachytixenos cf. indicus Pierce, female, cephalothorax, SEM micrographs A anterior part of cephalothorax, ventral side B anterior part of cephalothorax, dorsal side C mouthparts, ventral side D detail of anterior border of cephalothorax, ventral side E right mandible and maxilla, ventral side F left mandible and maxilla, ventral side. Abbreviations: bcm - brood canal membrane, md - mandible, mdt - mandibular tooth, mdts - spine of mandibular tooth.

Figure 15. Tachytixenos cf. indicus Pierce, male, cephalotheca, photomicrographs, SEM micrographs A frontal view B lateral view C vestigial antenna D frontal view E mouthparts. Abbreviations: fssf - furrow of supra-antennal sensillary field, gn - gena, mdt - mandibular tooth.