Macroxenos Schultze, 1925, 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

Macroxenos Schultze, 1925, stat. res.

Macroxenos Schultze, 1925: 238. Type species: Macroxenos piercei Schultze, 1925, by original designation.

Pseudoxenos Saunders, 1872 (partim!) (synonymy proposed by Bohart 1937).

Diagnosis of female cephalothorax.

Maxilla reduced, not distinctly prominent (Fig. 34E ). Two distinct dark spots present mesally on border between head and prothorax (Fig. 32D ). Thoracic segments conspicuously sclerotized laterally from dorsal side (Fig. 32D ). Lateral parts of abdomen posterior to spiracles always pale (Fig. 32D ). Clypeal region bulging, very distinctly separated from labral area (Fig. 34D ). 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. Nearly as long as wide, or as long as or distinctly longer than wide. Very variable in size, length 0.8-1.82 mm, width 0.64-1.9 mm in midline. Anterior head margin evenly rounded or protruding. Thorax slightly or distinctly widening posteriorly. Cephalothorax with multiple brown shades forming distinct pattern.

Head capsule. Between ⅓ and> ½ × as long as entire cephalothorax including the lateral cephalic extensions. Coloration forming specific pattern with pale and dark shades. Clypeal region very distinctly delimited from labral area (Fig. 34D ), arcuate, or protruding and forming clypeal lobe. Surface smooth or distinctly wrinkled. Sensilla mainly concentrated on clypeal lobe. Border between clypeal area and frontal region clearly indicated by change in cuticular surface. Cuticle of frontal area variable, distinctly wrinkled or covered with papillae. Border between head and prothorax usually distinct on dorsal side, delimited by transverse stripe of distinctive coloration and two distinct dark spots on mesal region (Fig. 32D ).

Supra-antennal sensillary field. Smooth, with dispersed sensilla. Furrow between supra-antennal sensillary field and frontal region absent, or very indistinct and only indicated by change in cuticular sculpture (Fig. 34B ).

Antenna. Preserved as poorly defined area, with several small, rounded plates, antennal sensilla, or cavity, in some cases all three combined. Periantennal area smooth or slightly wrinkled, sometimes indistinct.

Labrum. Ventral field wider than long, elliptic to nearly circular. Dorsal field arcuate, distinctly raised (Fig. 34D ), sometimes very wide and narrow, ~ 5-8 × wider than long in midline. Dorsal field with 14-41 (or more) setae or sensilla inserted in cavities.

Mandible. Anteromedially directed at angle of 30-35° and enclosed in mandibular capsule. Mandibular bulge distinctly raised, with several sensilla. Cuticle smooth or slightly sculptured, sometimes with longitudinal grooves (Fig. 34E ). Tooth narrow or slightly widened, pointed apically or ventrally, more or less distinctly armed with spines.

Maxilla. Almost completely fused with labial area, or slightly raised (Fig. 34E ), not projecting beyond mandible. Cuticle smooth or wrinkled. Vestige of palp present as cavity or poorly defined area; usually located medially on ventral side of maxilla (Fig. 34E ). Submaxillary groove more or less distinctly produced anterolaterally to maxillary base.

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

Mouth opening. Widely arcuate, sclerotized marginally.

Thorax and abdominal segment I. Pro-mesothoracic and meso-metathoracic borders more or less distinct, usually separated by mesal furrows, rarely combined with pigmented stripes or spots on dorsal and ventral side. Border between metathorax and abdomen usually formed by ridge or indicated by change in cuticular sculpture. Cuticle of thoracic segments on ventral side reticulate, with scattered inconspicuous or more distinct pigmented papillae. Dorsal surface of thorax smooth or slightly reticulated. Prosternal extension undifferentiated or distinct, in some cases extremely elongated. Thoracic segments conspicuously sclerotized laterally from dorsal side (Fig. 32D ). Shape of meso- and metathorax unmodified, transverse, or narrowed laterally in species with elongated head. Lateral parts of abdomen posterior to spiracles always pale (Fig. 32D ). Setae present on lateral region of abdominal segment I (Fig. 33E, F ).

Spiracles. Spiracles on posterior ~ ⅓ of cephalothorax slightly elevated, with anterodorsal and anterolateral orientation.

Diagnosis of male cephalotheca.

Male cephalotheca unknown.

Phylogenetic relationships.

The phylogenetic position is unstable. Benda et al. (2019) revealed it as sister to a lineage including Sphecixenos , Tuberoxenos , and Pseudoxenos in our concept. In contrast, Benda et al. (2021) resolved its position as sister to a clade including Sphecixenos , Tuberoxenos , Pseudoxenos , Deltoxenos , and Xenos . In both cases, the support was very weak. Further phylogenomic investigations with robust data are needed to resolve the intergeneric relationships.

Diversity and distribution.

A lineage of Australasian origin, with dispersion into the Indomalayan region ( Benda et al. 2019). The two currently known species are restricted to these two biogeographic regions.

Hosts.

Various genera of Odynerini ( Vespidae : Eumeninae ).

Comments.

The genus Macroxenos was described by Schultze (1925) but the descriptions of male and female was superficial. Later, Bohart (1937) synonymized it with Pseudoxenos . We classify this lineage as a separate genus, based on molecular phylogenies ( Benda et al. 2019, 2021) and morphological characters newly reported here. However, this genus is quite complicated to diagnose because of a high morphological variability of species. More samples are still needed for a better characterization and recognition of this formerly overlooked group.

Figures

Figure 32. Macroxenos cf. piercei, host, female, cephalothorax, photomicrographs A Anterhynchium flavomarginatum stylopized by female of M. cf. piercei, lateral view B detail of host abdomen with adult female inside C ventral side of cephalothorax D dorsal side of cephalothorax. Abbreviations: asI - abdominal segment I, sbhp - segmental border between head and prothorax, tx - thorax.

Figure 33. Macroxenos cf. piercei, 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 right lateral border of abdominal segment I below spiracle, dorsal side.

Figure 34. Macroxenos cf. piercei, 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: dlf - dorsal field of labral area, fr - frontal region, md - mandible, mx - vestige of maxilla, mxp - vestige of maxillary palp, sbcl - segmental border between clypeus and labrum, smxg - submaxillary groove, ssf - supra-antennal sensillary field.