Xenos Rossi, 1794

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

Xenos Rossi, 1794

Xenos Rossi, 1794: 114. Type species: Xenos vesparum (Rossi, 1793), by monotypy.

Acroschismus Pierce, 1908: 79 (synonymized by Bohart 1941). Type species: Acroschismus hubbardi Pierce, 1908.

Schistosiphon Pierce, 1908: 80 (synonymized by Bohart 1941). Type species: Xenos peckii Kirby, 1813.

Vespaexenos Pierce, 1909: 133 (synonymized by Bohart 1941). Type species: Vespaexenos crabronis Pierce, 1909.

Belonogastechthrus Pierce, 1911: 498 (synonymized by Bohart 1941). Type species: Belonogastechthrus zavattarii Pierce 1911.

Clypoxenos Brèthes, 1923: 46 (synonymized by Bohart 1941). Type species: Clypoxenos americanus Brèthes, 1923.

Diagnosis of female cephalothorax.

Differing from other genera by the combination of following characters. Clypeal sensilla distinct, position on clypeal lobe extended onto ventral side, often present near clypeo-labral border (Fig. 53D ). Maxilla variable in shape, almost fused with labial area, or raised from it, but not distinctly prominent anteriorly (Fig. 53E, F ). Reduced forms of maxilla often indistinctly separated from labial area. Cuticle of maxilla in some cases strongly sclerotized like in Brasixenos , but border between clypeus and labrum always distinct (Fig. 53D ). Prosternal extension not differentiated. 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. Extremely variable, ca. as long as wide, slightly wider than long, or distinctly longer than wide. Meso-metathoracic segmental border in some cases distinctly constricted laterally. Extremely variable in size, length 0.8-2.7 mm, maximum width 0.84-2.43 mm. Anterior head margin evenly rounded, protruding, or strongly protruding. Thorax slightly or distinctly widening posteriorly. Cephalothorax uniformly pale or colorful. Coloration with multiple brown (nearly black) and orange shades forming distinct pattern, often with pale anterior part and dark posterior area (Fig. 51C ).

Head capsule. Ca. ⅓ ~ ½ as long as entire cephalothorax including lateral cephalic extension. Coloration forming specific pattern with pale and dark combined. Clypeal region well delimited from labral area, border between clypeus and labrum often distinct (Figs 52F , 53D ). Clypeal area variable in shape, apical margin arcuate, nearly flat, or protruding, forming distinct clypeal lobe. Cuticle smooth or slightly wrinkled. Numerous distinct sensilla present on clypeal surface, between 20 and 60 (or more), mainly concentrated anteriorly, rarely also scattered laterally, on clypeal lobe extending to ventral side, often near indistinct clypeo-labral border (Fig. 53D ). Cuticle of frontal region slightly wrinkled. Segmental border between head and prothorax often indistinct to almost absent, at most indicated by change of color or transverse colored stripe.

Supra-antennal sensillary field. Slightly wrinkled with dispersed sensilla. Not delimited or indistinctly delimited by furrow medially, but border usually still recognizable (Fig. 53B ).

Antenna. Preserved as poorly defined area, usually with several small, rounded plates, antennal sensilla, or cavity (Fig. 52C ), in some cases combined, but antennal vestige in some cases only visible as strongly sculptured cuticle, without any plates or sensilla. Periantennal area wrinkled or reticulated.

Labrum. Ventral field variable, semicircular to nearly circular, elliptic, or subtriangular. Dorsal field slightly arcuate to straight, raised, or flat, ~ 4-5 × wider than long in midline (Fig. 52F ). Dorsal field laterally as long as medially, or laterally narrowed, with ~ 10-20 setae or sensilla inserted in cavities.

Mandible. Anteromedially directed at angle of 30-75° and enclosed in mandibular capsule, exceptionally slightly protruding. Mandibular bulge more or less distinctly raised, with several sensilla. Cuticle of mandible completely or partially sculptured. Tooth narrow or wider, pointed apically, more or less distinctly armed with spines.

Maxilla. Variable in shape, nearly fused with labial area and scarcely distinguishable from it, or raised but not distinctly prominent anteriorly (Fig. 53E, F ). Cuticle smooth, wrinkled or reticulated, in some cases strongly sclerotized. Maxillary apex not projecting beyond mandible anteriorly but in some cases elevated maxillary base very slightly overlapping base of mandible. Vestige of palp inconspicuous, very poorly defined, often forming cavity or completely missing. If recognizable usually located medially or slightly apically on ventral side of maxilla (Fig. 53E ). Maxillary base usually indistinctly produced anterolaterally as a submaxillary groove.

Labium. Labial area more or less recognizable between maxillae, delimited anteriorly by mouth opening and posteriorly by birth opening. Flat, slightly wider than long, as long as wide, or longer than wide. Cuticular surface smooth or reticulated.

Mouth opening. Widely arcuate to nearly straight or bisinuate, in some cases V-shaped, sclerotized along margin.

Thorax and abdominal segment I. Pro-mesothoracic and meso-metathoracic borders more or less distinct, usually indicated by mesal furrows, combined with pigmented stripes or spots on dorsal side. Border between metathorax and abdomen usually formed by ridge or indicated by change of cuticular sculpture. Cuticle of thoracic segments on ventral side reticulate with scattered small or larger pigmented papillae. Dorsal side of thorax smooth or slightly reticulated. Prosternal extension undifferentiated, evenly arched. Meso- and metathorax of standard transverse shape, in few cases constricted laterally. Setae and cuticular spines present on lateral region of abdominal segment I (Fig. 52E ).

Spiracles. Spiracles on posterior third of cephalothorax slightly elevated, with anterodorsal and anterolateral orientation.

Diagnosis of male cephalotheca.

Differing from other genera by the following combination of characters. Diameter of genae between maxillary base and compound eye ~ 2-3 × larger than diameter of vestigial antenna. Paired furrow of supra-antennal sensillary field slightly distinct or indistinct. Cephalotheca usually elliptic (Fig. 54A ). Frontal fissure indistinct or almost absent (Fig. 54D ). Maxilla not distinctly elongated, at most 1.5 × longer than basally wide (Fig. 54E ). Occipital bulge strongly reduced or missing (Fig. 54D ). Cephalotheca mostly dark (Fig. 54A ).

Description of male cephalotheca.

Shape and coloration. In frontal view rounded, elliptic, in lateral view slightly pointed anteriorly or rounded. Coloration with pattern of pale and dark shades but dark color dominant.

Cephalothecal capsule. Compound eyes completely dark or lighter, with dark individual cornea lenses visible. Clypeal lobe straight or slightly arcuate in frontal view, not or slightly prominent in lateral view. Sensilla mainly concentrated on clypeal lobe. Frontal impression inconspicuous or distinct (Fig. 54D ). Occipital bulge indistinct (Fig. 54D ) or absent. Diameter of genae between maxillary base and compound eye ~ 2-3 × larger than diameter of vestigial antenna.

Supra-antennal sensillary field. Kidney-shaped and bulging, without furrows, delimited medially by more or less distinct frontal impression.

Antenna. Of standard shape, with small plates, cavities or sensilla, and complete torulus (Fig. 54C ). Periantennal area not clearly delimited from supra-antennal sensillary field.

Labrum. Labral area distinct, with setae on dorsal field.

Mandible. Anteromedially directed. Mandibular bulge with sensilla, separated from pointed tooth.

Maxilla. Distinct, prominent, dark. Vestige of palp distinct.

Labium and hypopharynx. Dark labium distinctly visible between and below maxillae. Praementum and postmentum separated by indistinct transverse furrow. Hypopharyngeal protuberance present.

Mouth opening. Well visible, not covered by ventral labral field, slightly or distinctly arcuate.

Phylogenetic relationships.

Deeply nested within Xenidae , representing the largest radiation ( Benda et al. 2021), sister to Deltoxenos gen. nov. ( Benda et al. 2019; Straka and Benda unpubl. results)

Diversity and distribution.

The geographic origin is unclear, probably the New World or Afrotropical region ( Benda et al. 2019). The present distribution of 33 described species comprising the Old and New World.

Hosts.

Several tribes of social Vespidae ( Vespini , Polistini , Mischocyttarini , and Ropalidiini ).

Comments.

The first species of Strepsiptera , Xenos vesparum , was superficially described by Rossi (1793), who assigned it to the genus Ichneumon in Hymenoptera . The genus Xenos was introduced later by Rossi (1794). Pierce (1908, 1909, 1911) described several genera ( Acroschismus , Belonogastrechthrus , Schistosiphon , Vespaexenos ) based on his hypothesis of host specialization. These were later synonymized with Xenos by Bohart (1941), and also the genus Clypoxenos described by Brèthes (1923). Kinzelbach (1971b) maintained this concept and extended it to Brasixenos , and considered representatives of Xenos as parasites of social wasps. Benda et al. (2019, 2021) revealed xenids parasitizing social Vespidae as a polyphyletic group. We classify Xenos as a valid genus based on the monophyly revealed by molecular phylogenies ( Benda et al. 2019, 2021) and based on morphological characters newly reported here.

Figures

Figure 51. Xenos peckii, host, female, cephalothorax, photomicrographs A Polistes fuscatus stylopized by two females of X. peckii, lateral view B detail of host abdomen with two adult females inside C ventral side of cephalothorax D dorsal side of cephalothorax. Abbreviations: bcm - brood canal membrane.

Figure 52. Xenos peckii, 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 labral area, dorsal side. Abbreviations: a - vestigial antenna, dlf - dorsal labral field of labral area, sbcl - segmental border between clypeus and labrum.

Figure 53. Xenos peckii, 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, cls - clypeal sensillum, fr - frontal region, mx - vestige of maxilla, mxp - vestige of maxillary palp, sbcl - segmental border between clypeus and labrum, ssf - supra-antennal sensillary field.

Figure 54. Xenos peckii, male, cephalotheca, photomicrographs, SEM micrographs A frontal view B lateral view C vestigial antenna D frontal view E mouthparts. Abbreviations: fi - frontal impression, mx - vestige of maxilla, ob - occipital bulge.