The generic taxonomy and host specialization of Xenidae have been understood differently by previous authors. Although the recent generic classification has implied a specialization on the level of host families or subfamilies, the hypothesis that each xenid genus is specialized to a single host genus was also previously postulated. A critical evaluation of the classification of the genera of Xenidae is provided here based on morphology in accordance with results of recent molecular phylogenetic studies. External features of the female cephalothoraces and male cephalothecae were documented in detail with different techniques. Diagnoses and descriptions are presented for all 13 delimited genera. The earliest diverging genera are usually well characterized by unique features, whereas deeply nested genera are usually characterized by combinations of characters. Three new genera are described: Sphecixenosgen. nov., Tuberoxenosgen. nov., and Deltoxenosgen. nov. Five previously described genera are removed from synonymy: Tachytixenos Pierce, 1911, stat. res.; Brasixenos Kogan & Oliveira, 1966, stat. res.; Leionotoxenos Pierce, 1909, stat. res.; Eupathocera Pierce, 1908, stat. res.; and Macroxenos Schultze, 1925, stat. res. One former subgenus is elevated to generic rank: Nipponoxenos Kifune & Maeta, 1975, stat. res.Monobiaphila Pierce, 1909, syn. nov. and Montezumiaphila Brèthes, 1923, syn. nov. are recognized as junior synonyms of Leionotoxenos Pierce, 1909, stat. res.Ophthalmochlus Pierce, 1908, syn. nov., Homilops Pierce, 1908, syn. nov., Sceliphronechthrus Pierce, 1909, syn. nov., and Ophthalmochlus (Isodontiphila) Pierce, 1919, syn. nov. are recognized as junior synonyms of Eupathocera Pierce, 1908, stat. res. A preliminary checklist of 119 described species of Xenidae with information on their hosts and distribution is provided. The following 14 species are recognized as valid and restituted from synonymy: Tachytixenos indicus Pierce, 1911, stat. res.; Brasixenos acinctus Kogan & Oliveira, 1966, stat. res.; Brasixenos araujoi (Oliveira & Kogan, 1962), stat. res.; Brasixenos bahiensis Kogan & Oliveira, 1966, stat. res.; Brasixenos brasiliensis Kogan & Oliveira, 1966, stat. res.; Brasixenos fluminensis Kogan & Oliveria, 1966, stat. res.; Brasixenos myrapetrus Trois, 1988, stat. res.; Brasixenos zikani Kogan & Oliveira, 1966, stat. res.; Leionotoxenos hookeri Pierce, 1909, stat. res.; Leionotoxenos jonesi Pierce, 1909, stat. res.; Leionotoxenos louisianae Pierce, 1909, stat. res.; Eupathocera luctuosae Pierce, 1911, stat. res.; Eupathocera lugubris Pierce, 1909, stat. res.; Macroxenos piercei Schultze, 1925, stat. res. New generic combinations are proposed for 51 species: Leionotoxenos arvensidis (Pierce, 1911), comb. nov.; Leionotoxenos bishoppi (Pierce, 1909), comb. nov.; Leionotoxenos foraminati (Pierce, 1911), comb. nov.; Leionotoxenos fundati (Pierce, 1911), comb. nov.; Leionotoxenos huastecae (Székessy, 1965), comb. nov.; Leionotoxenos itatiaiae (Trois, 1984), comb. nov.; Leionotoxenos neomexicanus (Pierce, 1919), comb. nov.; Leionotoxenos prolificum (Teson & Remes Lenicov, 1979), comb. nov.; Leionotoxenos robertsoni (Pierce, 1911), comb. nov.; Leionotoxenos tigridis (Pierce, 1911), comb. nov.; Leionotoxenos vigili (Brèthes, 1923), comb. nov.; Eupathocera argentina (Brèthes, 1923), comb. nov.; Eupathocera auripedis (Pierce, 1911), comb. nov.; Eupathocera bucki (Trois, 1984), comb. nov.; Eupathocera duryi (Pierce, 1909), comb. nov.; Eupathocera erynnidis (Pierce, 1911), comb. nov.; Eupathocera fasciati (Pierce, 1909), comb. nov.; Eupathocera fuliginosi (Brèthes, 1923), comb. nov.; Eupathocera inclusa (Oliveira & Kogan, 1963), comb. nov.; Eupathocera insularis (Kifune, 1983), comb. nov.; Eupathocera mendozae (Brèthes, 1923), comb. nov.; Eupathocera piercei (Brèthes, 1923), comb. nov.; Eupathocera striati (Brèthes, 1923), comb. nov.; Eupathocera taschenbergi (Brèthes, 1923), comb. nov.; Eupathocera westwoodii (Templeton, 1841), comb. nov.; Macroxenos papuanus (Székessy, 1956), comb. nov.; Sphecixenos abbotti (Pierce, 1909), comb. nov.; Sphecixenos astrolabensis (Székessy, 1956), comb. nov.; Sphecixenos dorae (Luna de Carvalho, 1956), comb. nov.; Sphecixenos erimae (Székessy, 1956), comb. nov.; Sphecixenos esakii (Hirashima & Kifune, 1962), comb. nov.; Sphecixenos gigas (Pasteels, 1950), comb. nov.; Sphecixenos kurosawai (Kifune, 1984), comb. nov.; Sphecixenos laetum (Ogloblin, 1926), comb. nov.; Sphecixenos orientalis (Kifune, 1985), comb. nov.; Sphecixenos reticulatus (Luna de Carvalho, 1972), comb. nov.; Sphecixenos simplex (Székessy, 1956), comb. nov.; Sphecixenos vanderiisti (Pasteels, 1952), comb. nov.; Tuberoxenos altozambeziensis (Luna de Carvalho, 1959), comb. nov.; Tuberoxenos sinuatus (Pasteels, 1956), comb. nov.; Tuberoxenos sphecidarum (Siebold, 1839), comb. nov.; Tuberoxenos teres (Pasteels, 1950), comb. nov.; Tuberoxenos tibetanus (Yang, 1981), comb. nov.; Deltoxenos bequaerti (Luna de Carvalho, 1956), comb. nov.; Deltoxenos bidentatus (Pasteels, 1950), comb. nov.; Deltoxenos hirokoae (Kifune & Yamane, 1992), comb. nov.; Deltoxenos iwatai (Esaki, 1931), comb. nov.; Deltoxenos lusitanicus (Luna de Carvalho, 1960), comb. nov.; Deltoxenos minor (Kifune & Maeta, 1978), comb. nov.; Deltoxenos rueppelli (Kinzelbach, 1971a), comb. nov.; Xenos ropalidiae (Kinzelbach, 1975), comb. nov.Xenos minor Kinzelbach, 1971a, syn. nov. is recognized as a junior synonym of X. vesparum Rossi, 1793. Ophthalmochlus duryi Pierce, 1908, nomen nudum and Eupathocera lugubris Pierce, 1908, nomen nudum are recognized as nomina nuda and therefore unavailable in zoological nomenclature. The species diversity of Xenidae probably remains poorly known: the expected number of species is at least twice as high as the number presently described.
