Genera of the Protura of the World: diagnosis, distribution, and key

Abstract Protura are known all over the world with more than 800 described species belonging to three different orders (Acerentomata, Sinentomata, and Eosentomata) and seven families (Hesperentomidae, Protentomidae, Acerentomidae, Fujientomidae, Sinentomidae, Eosentomidae, and Antelientomidae). At present 76 genera are known worldwide. In this paper a description of the diagnostic characters of these genera and an updated key for their identification are reported.


Introduction
Since their discovery (Silvestri 1907), the knowledge of Protura has rapidly increased all over the world thanks to the careful research of many specialists. For example, the number of worldwide genera cited by Tuxen (1964), Nosek (1978a), Yin (1983) and Szeptycki (2007) was 22, 40, 54 and 72, respectively. Four new genera (Nanshanentulus Bu & Yin, 2007, Osientomon Nakamura, 2010, Liaoxientulus Wu & Yin, 2011, Nosekientomon Shrubovych et al., 2014b have been described after the publication of Szeptycki's Catalogue (2007), but since Yin (1983) there has not been published an updated key for the worldwide genera of Protura. In the belief that such a key is an essential tool for those researchers who want to study in detail this still poorly known taxon, this paper describes the diagnostic characters of the 76 genera known to date worldwide, accompanied by a key for their identification.
The diagnostic characters used at a generic level for Protura have changed and increased in number thanks to the contributions from different authors during the last century. In the first papers published on Protura (Berlese 1908a(Berlese , 1908b(Berlese , 1909Silvestri 1909) just four genera (Acerentomon, Acerentulus, Proturentomon, and Eosentomon -the possibility of subgenus Acerella was suggested in Berlese, 1909) were described mainly based on the presence/absence of a tracheal system, presence/absence of a rostrum, shape and size of mouthparts (maxillae, mandibles, maxillary, and labial palps), the number of segments of abdominal appendages and presence/absence of teeth on the lid (comb) covering the large glands on the sides of tergite VIII. In 1921 Ewing redescribed the genera Eosentomon, Acerentomon, and Acerentulus and described three new genera Protentomon, Acerentuloides, and Microentomon (the last one subsequently rejected). In his paper he stressed the importance of the size of abdominal appendages (one or two-segmented) and introduced for the diagnosis of genera the importance of tergal plates, the shape and size of tergal apodemes on the meso-and metanotum and on the first eight urotergites and the presence of transverse rows of dorsal setae on abdominal segments. The same characters, with reference also to the comb of urotergite VIII, were used by this author in his 1936 Synopsis of the genera of Protura. In a more extensive paper Ewing (1940) described genera of North American Protura based on dorsal apodemes, labrum, maxillae, mandibles, the number of transverse rows of setae on abdominal segments, presence of pectinated structures, characteristics and relative size of legs, colour and degree of sclerotization of cuticle (the size of abdominal appendages was elevated as a diagnostic character at a family level). Meanwhile, labial and tarsus ratios (LR and TR), chaetotaxy and details of both foretarsal sensilla/setae and female squama genitalis shape were introduced by Womersley, Ionescu and Condé, respectively, but such characters were used almost exclusively for new species description. In 1959, in their Synopsis of the Japanese Protura, Imadaté and Yosii described the new genus Nipponentomon based on previously used characters but included the number of setae shared by the four species of the genus on some of the abdominal segments and the distribution and shape of the abdominal pectinated structures. In 1964 Tuxen published his valuable monograph on the World Protura. Each genus was identified using characters such as abdominal appendages (size, number of setae and their relative size), shape and relative position of key sensilla on the foretarsus, presence and size of a rostrum, development of maxillary and labial palps, and shape of the maxillary glands. However, in addition to the important diagnostic features Tuxen provided meticulous descriptions of every part of the body morphology: e.g., the number of setae and sensilla on the labial and maxillary palps, the structure of all foretarsal sensilla present (and noting those absent), and the abdominal chaetotaxy. A similar scheme was followed by Nosek (1973) and Imadaté (1974) in their monographs on European and Japanese Protura, respectively, but the last author added the chaetotaxy of nota and, for Acerentomidae, the position of seta P3 on abdominal tergites II-VI (diagnostic character he had suggested in 1964) in his diagnoses of genera. At the same time, Rusek (1974) examined in detail the taxonomy of Acerentomidae, redescribing older genera and introducing five new genera (Vesiculentomon, Imadateiella, Filientomon, Verrucoentomon, and Nosekiella), based on the following diagnostic characters: number and relative length of setae on abdominal appendages, shape of the maxillary glands, shape of sensillum t1 and relative position of some sensilla on the foretarsus (mainly a' respect to t2, d respect to e and c, f respect to g), number of anterior setae on the metanotum and toothed rows on the hind border of the last abdominal segments. Nosek (1978a) published a paper on the genera of Protura of the world. In the diagnosis of each genus he described the labial and maxillary palps, mandibles, maxillary glands, pseudoculi, some main foretarsal sensilla (e.g. t1 and t3), abdominal appendages, chaetotaxy of sternite VIII, striate band (in Acerentomidae), comb VIII, and female squama genitalis; other characters (like the number of anterior setae on metanotum) are provided just in a few cases. Three years later Tuxen (1981) undertook a taxonomical revision of Acerentomidae based on the degree of development of the striate band of abdominal segment VIII, stressing the importance of the hook-shaped design, the number and relative size of setae of abdominal appendages (legs) and other characters (chaetotaxy of metanotum, shape of foretarsal sensillum t1, labial palps and maxillary glands). Based on these features he introduced the new genera Kenyentulus (splitting Gracilentulus) and Amphientulus (from Berberentulus). The next year Tuxen and Yin (1982) revised Protentomidae and introduced the new genus Neocondeellum (from Condeellum) based on shape of pseudoculi, maxillary palps, foretarsal sensilla, abdominal appendages, presence of a serrated line on abdominal segment VIII, comb VIII and chaetotaxy. Characters such as the presence/absence of a tracheal system, thoracic and abdominal chaetotaxy, pseudoculi, maxillary glands, labial palps and shape of foretarsal sensilla were used by Yin (1983) to arrange her phylogeny of Protura, grouping the 54 then known genera into 17 subfamilies and eight families. Zhang and Yin (1984) revised the subfamily Anisentominae and created the new genera Neanisentomon, Pseudanisentomon and Paranisentomon based mainly on the presence/absence and shape of few foretarsal sensilla (t2, f1, b'2) and on the chaetotaxy of abdominal segment VIII. Szeptycki (1988) introduced the term porotaxy to define the distribution of pores on the body of proturans and suggested the possibility of using this character for the identification of genera. Since then porotaxy has been generally used in the description of new species. Szeptycki (1995a) summarized the knowledge about porotaxy of Protura and reiterated the taxonomic importance it could have at the level of genus and family. Szeptycki also (see 1997) introduced the analysis of the shape of accessory setae (especially of meso-and metanotum) as a taxonomically important character (at least for Acerentomata). Unfortunately, porotaxy and the shape of accessory setae of the majority of species are still unknown and, for many genera, their use as diagnostic characters is not practicable. Nevertheless, they are among those characters used by Shrubovych (2014) in her cladistic approach to the Acerentomidae of Northeastern Palearctic. Other useful diagnostic characters are the head chaetotaxy and porotaxy (Rusek et al., 2012); currently they are mainly provided in new species description, but some, like the presence/absence of postpseudocular seta, have been used for the diagnosis of new genera (e.g. Wu and Yin 2011).
For the whole bibliographic references to genera description and distribution, we refer to the World Catalogue by Szeptycki (2007). Genera are listed according to Szeptycki's system (2007).
The diagnostic characters available for each genus are given, following the same general scheme: characters related to the head (especially the labial palps, maxillary glands and pseudoculi), foretarsal sensilla and setae (presence/absence, position, size and shape), notal chaetotaxy, abdominal appendages (number of segments and setae), other abdominal characters (chaetotaxy, striate band, comb VIII) and shape of female squama genitalis. When available and useful for taxonomic diagnosis, other characters considered are the presence of a rostrum and the cephalic chaetotaxy. Those unifying characters reported in the order, family or subfamily diagnosis are not repeated in that of genera. For the characters described in the diagnosis and used in the key, refer to Tuxen (1964), Nosek (1973), Tuxen and Imadaté (1974), Tuxen (1981), Tuxen and Yin (1982), Yin (1999), Rusek et al. (2012) and Shrubovych (2014). For the appearance/shape of some characters (e.g. the foretarsal sensilla and the maxillary glands) would be appropriate to consult directely the type species description or those papers indicated in the remarks of the genera. Order ACERENTOMATA Yin, 1996

Abbreviations
Tracheal system missing. Dilated canal of maxillary glands. Foretarsus typically with three dorsal sensilla (t1-t3), seven external (a-g) and three internal (a'-c'). Abd. app. with no more than four setae. Presence of pectinated structures on the abdomen. Glands on abd. VIII covered by combs. Abd. VIII with striate band (fully developed or reduced). Female squama genitalis without processus sternalis but with acrostylus.

Family Hesperentomidae Price, 1960
Labial palps well developed, with tuft of setae. Maxillary glands long and dilated. Abd. app. I two-segmented, with terminal vesicle and four setae; II-III two-segmented or uni-segmented. Striate band without distinct striae. Comb VIII rectangular with small teeth. Female squama genitalis with pointed acrostyli.

Subfamily Hesperentominae Price, 1960
Maxillary palps with tuft of setae and two sensilla. Labial palps with tuft of setae and three distinct setae. Canal of maxillary glands with a long dilation. Pseudoculi open proximally. All foretarsal sensilla present, with addition of c". Metanotum with median setae and two pairs of A-setae. Abd. app. I and II two-segmented, each with terminal vesicle and four setae; III two-segmented or uni-segmented with two setae.

-Hesperentomon Price, 1960
Labial palps without basal sensillum. Canal of maxillary glands dilated as a long parallel-sided sac for most of its length, narrowing proximally. Pseudoculi pear-like, strongly elevated and with a median S-shaped cleft. Foretarsal sensilla setiform or longish willow leaf-shaped. S seta long, straight and pointed.  Rusek and Stumpp (1989).

Subfamily Huhentominae Yin, 1983
Labial palps well developed with tuft of setae and three distinct setae, without basal sensillum. Metanotum with one or two pairs of A-setae. Abd. app. I two-segmented; II-III uni-segmented, each with three setae, the median apical one approx. twice the length of the others. St. IV-VII each with Pc seta; St. VIII with 2/4 setae.

-Huhentomon Yin, 1977
Canal of maxillary glands extremely long; middle part approx. 3.5 times of hind part, with a tiny racemose appendix distally. Pseudoculi with a median S-shaped opening. Foretarsal sensilla of Hesperentomon type; sensillum t1 filiform; t3 rather long, sensilliform; b', c' and c" distal, close to t3; d nearly halfway between c and e, distal to t2.
Type species. Huhentomon plicatunguis Yin, 1977. Distribution. China, Japan. Remarks. Figures and comparison between the two species of genus Huhentomon are available in Imadaté (1989).

Family Protentomidae Ewing, 1936
Small bodied Protura. Maxillary glands with smooth globular calyx. Foretarsal sensillum t1 sometimes missing (in all the species belonging to genus Protentomon and some of those belonging to Proturentomon). Metanotum with one or two pairs of A-setae. Abd. app. I-II two-segmented, each with terminal vesicle and three or four setae; III two-segmented or uni-segmented with two or three setae.

-Hinomotentomon Imadaté, 1973
Maxillary palps with tuft of setae and two sensilla. Labial palps with tuft of setae, three distinct setae and basal sensillum. Canal of maxillary glands with heart-shaped calyx with verrucose appendices and a proximal dilation. Pseudoculi with a large triangular prolongation in the proximal part. Foretarsal sensillum a' short; b' and c' missing. Metanotum with two pairs of A-setae. St. IV-V without Pc seta. St. VIII with four setae. Comb VIII with 10 small teeth.

Subfamily Condeellinae Tuxen & Yin, 1982
Labial palps with tuft of setae. Metanotum with median setae and two pairs of A-setae. Abd. app. I-II two-segmented, each with four setae; III uni-segmented with two or three setae. Female squama genitalis with pointed acrostyli.

-Neocondeellum Tuxen & Yin, 1982
Two short and blunt sensilla on maxillary palps. Labial palps without basal sensillum. Canal of maxillary glands, with a globular bladder, in connection with the canal, but not with a simple calyx. Pseudoculi small (PR = 14-18), with a short lever. Four dorsal setae between the pseudoculi (d4 and sd4); lateral setae l3 missing. Foretarsi with only four sensilla in the exterior and interior side, respectively: a, b, f and a'. Abd. app. III uni-segmented with three setae, the median apical one longer than the lateral. Anterior row of Tg. II-VII with at most six setae. Tg. VIII with a single serrate line (striate band) rarely with indication of an anterior border. St. IV-V without Pc seta. St. VIII with six setae. Comb VIII with approx. ten very small teeth.

-Paracondeellum Yin, Xie & Zhang, 1994
Calyx of maxillary glands globular and smooth. Pseudoculi circular without lever. Foretarsal sensilla of the exterior side fully developed; b' and c' missing. Abd. app. III unisegmented with two setae, the median apical longer than the lateral one.  Yin et al. (1994).

-Proturentomon Silvestri, 1909
Labial palps with or without basal sensillum. Canal of maxillary glands with discshaped dilation and sac-like proximal dilation, with sharp proximo-lateral corner, terminating in doubled globule. Pseudoculi with a large triangular proximal prolongation of the same length of pseudoculi and of almost the same width distally. Foretarsi sometimes missing sensillum t1; c always present; sensilla on exterior and interior side of foretarsi are foliaceous or bacilliform, respectively. Seta S blunt, relatively short. Metanotum with one pair of A-setae (A2). Comb VIII with distinct teeth. The hind border of Tg. IX-XI serrated. Type species. Acerentomon minimum Berlese, 1908. Distribution. Holarctic. Remarks. For figures, see Rusek (1975 -with a key to nine Proturentomon spp.), Szeptycki (1988) and Yin (1999).

Family Acerentomidae Silvestri, 1907
Mesonotum with a pair of median setae and two or three pairs of A-setae; metanotum with a pair of median setae and two to four pairs of A-setae. Abd. app. I two-segmented with four setae; II and III each uni-segmented with one to three setae. Abd. VIII with striate band.

Subfamily Berberentulinae Yin, 1983
Labial palps generally reduced (without tuft of setae) with two to four setae and one sensillum. Maxillary glands with generally smooth and often heart-shaped calyx. Foretarsal sensillum t2 usually setiform/filiform. Meso-and metanotum each with two pairs of A-setae (A2, A4). Striate band on abd. VIII often reduced.

-Acerentuloides Ewing, 1921
Maxillary palps with two spindle-like sensilla. Labial palps with a terminal tuft of setae and a sausage-shaped basal sensillum. Canal of maxillary glands with proximal part as long as the proximal part of fulcrum, ending with a row of small globules which increase in size proximally and end with two globules placed beside each other. Pseudoculi small. All foretarsal sensilla present, of average length; sensillum t1 claviform; t3 long lancet-like or finger-like; d closer to c than to e; a' close to the base of t1. Abd. app. II-III each with three setae, the median apical one reduced to little more than its pit. Tg. II-VI with seta P3 in anterior position. St. I-VII with three setae in anterior row. St. VIII with 4/2 setae. Striate band well developed. Comb VIII with a row of approx. ten equal small teeth. Some setae modified as short, thickened sensilla (sd5 on head; β1 and δ4 on foretarsi; setae P4 on metanotum, M2 on prosternum and A2 on thoracic sterna; accessory setae on Tg. and St. I-VI). Female squama genitalis tripartite with pointed acrostyli as in Acerentulus.

-Acerentulus Berlese, 1908
Maxillary and labial palps with two and one sausage-shaped sensilla, respectively, and with a tuft of setae. Canal of maxillary glands with simple dilation, calyx sometimes weakly verrucose, proximal part rather long. Pseudoculi oval or circular with divided lid and short fork-shaped prolongation on the posterior margin of the lid. On foretarsi sensillum t1 claviform; t3 willow leaf-shaped; a' usually broad; b' always present.  Shrubovych et al. (2012b) and Shrubovych et al. (2014d), respectively.

-Amazonentulus Yin, 1989
Labial palps reduced to three setae and a sensillum. Canal of maxillary glands of middle length, proximally bipartite. Foretarsal sensillum t1 claviform; t3 very long and awlshaped; b' absent. Abd. app. II-III each with two setae, the apical one shorter than half length the subapical. Tg. I-VII with uneven number of anterior setae. St. VIII with 4/0 setae. Striate band on abd. VIII reduced, without or with very short striae. Comb VIII with five to eight small teeth. Female squama genitalis with sharp pointed acrostyli.

-Andinentulus Tuxen, 1984
Labial palps reduced to three setae and a sausage-like sensillum. Proximal part of canal of maxillary glands almost as long as proximal branch of fulcrum. Foretarsi with all sensilla present; t1 claviform; t3 knob-shaped; a' extremely long, situated between the insertions of t1 and t2; b' present; d near the base of c.

-Australentulus Tuxen, 1967
Labial palps reduced to three setae and a sensillum. Canal of maxillary glands with heart-shaped calyx and a proximal tripartite dilation. Foretarsal sensillum t3 shaped as a bud rounded at apex, parallel-sided two to five times as long as broad; t1 claviform; a' broad; b' present.  François (1994).

-Baculentulus Tuxen, 1977
Labial palps reduced to two or three setae and a sensillum. Canal of maxillary glands simple, without appendages, with heart-shaped calyx. Foretarsal sensillum t1 baculiform; t3 jar-shaped; b' present or absent. Abd. app. II-III each with two setae, the median apical very short, less than half the length of the subapical. Seta P3 on Tg. II-VI anterior to other P-setae row. St. VIII with 4/0 setae. Abd. VIII with reduced striate band. Comb VIII more or less oblique. Female squama genitalis with pointed acrostyli. Type species. Berberentulus becki Tuxen, 1976. Distribution. Cosmopolitan. Remarks. A recent key to the species of genus Baculentulus with foretarsal sensillum b' is available in Bai & Bu (2013).

-Berberentulus Tuxen, 1963
Labial palps reduced to two or three setae and a willow leaf-shaped or sausage-like sensillum. Maxillary palps with two sensilla. Canal of maxillary glands simple, with heartshaped calyx and long or short proximal part, rarely with excrescences. Pseudoculi circular or oval. Foretarsal sensillum t1 claviform; t3 shaped as a willow leaf, or as a small jar or, sometimes, as a rounded knob; d very close to c; b' usually missing. Empodium short. Abd. app. II-III each with two setae, the median apical less than 1/3 rd the length of the subapical. Seta P3 on Tg. II-VI placed anteriorly to the row of the other P-setae. St. I-VII with three A-setae; St. VIII with 4/0 setae. Abd. VIII with reduced striate band formed by two rather close transverse lines, the more distal serrated, the proximal one more or less undulated with small teeth pointing backwards. Subcuticular parallel tubes are visible in correspondence to the missing striae. Comb VIII reduced with very small dispersed teeth. Female squama genitalis with long pointed acrostyli and broad styli.

-Bolivaridia Bonet, 1942
Labial palps reduced to two setae and a sensillum. Maxillary palps with slender willow leaf-shaped sensilla. Canal of maxillary glands with heart-shaped calyx; the short proximal part with distinct lateral protuberance and with an ending "head of femur"-shaped dilation. Pseudoculi longitudinally divided, somewhat broader than long. Foretarsal sensillum t1 claviform; a' sword-shaped; a and e long and broad, spindle-shaped. Abd. app. II-III each with a single subapical seta. Tg. II-VI with Ac seta. St. I-VII with three A-setae; St. VIII with 4/0 setae. Striate band on abd. VIII reduced, with only short, blind striae extending from the posterior margin. Comb VIII concave, with small teeth. Female squama genitalis with long, broad and pointed acrostyli; basal apodeme short.

-Brasilentulus Nosek, 1973
Labrum slightly prolonged into a rostrum. Labial palps reduced to three setae and a sensillum. Maxillary palps with two setiform sensilla. Canal of maxillary glands with heart-shaped calyx and long proximal part. All foretarsal sensilla present; long spindleshaped sensillum t3; t1 claviform (in B. huetheri Nosek the distal half bent forwards in a rather sharp angle); b placed close to c. Abd. app. II-III each with a long subapical seta and a short apical one (1/3 rd of the subapical  Tuxen (1979).

-Brasilidia Nosek, 1973
Labial palps reduced to three setae and a sensillum. Maxillary palps with two setiform sensilla. Canal of maxillary glands with heart-shaped calyx and relatively short proximal part, proximally tripartite. Pseudoculi longitudinally divided with short posterior fork-shaped prolongation. On the head seta l3 is absent; seta d6 (formerly "additional") present. Foretarsal sensillum d situated nearer to c than to e (but not very close to c); b' present; t1 distinctly clavate (nearly "baculiform"); t3 relatively long, cylindrical; setae β1 and δ4 not differentiated (of same shape as δ1 -δ3). Seta P2a on meso-and metanotum nearer to P3 than to P2; P4a on metanotum not modified (of "normal" shape). Abd. app. II-III each with three setae. Seta P3 on Tg. II-V anterior to other P-setae row. St. VIII with 4/0 or 4/2 setae. Modified setae (meaning accessory setae on nota, Tg. and St. I-VII, and modified setae on head and on thoracic sterna) uniform over the whole body, as small, hair-like microchaetae. Striate band on abd. VIII reduced (with no distinct striae but just signs of them). Comb VIII with fine teeth. Female squama genitalis tripartite with pointed acrostyli. Type species. Brasilidia tropica Nosek, 1973. Distribution. South America.
Remarks. Figures and a recent key to the species are provided by Szeptycki and Bedano (2003).

-Chosonentulus Imadaté & Szeptycki, 1976
Labial palps reduced to three setae and a broad sensillum. Canal of maxillary glands with racemose appendices, its proximal part rather short. Foretarsal sensillum t1 filiform; b in row with c; d close to c; e nearer to f than to d. Abd. app. II-III each with two setae, the apical one nearly half the length of the subapical. Striate band on abd. VIII reduced, with no distinct striae (but a minute serration is present near its anterior edge). Seta P3 on Tg. II-VI anterior to other P-setae row. St. VIII with 4/0 setae. All the tergal accessory setae sensilliform. Female squama genitalis with pointed acrostyli.

-Gracilentulus Tuxen, 1963
Labial palps reduced to three setae and a small elongated sensillum. Maxillary palps with two sensilla. Canal of maxillary glands with rather large heart-shaped calyx. Pseudoculi small, broader than long. Foretarsi with small knob-like sensillum t3 (except in G. sarmaticus Shrubovych and Szeptycki where t3 is large, subequal in length to t1); t1 claviform; a and a' usually broader than the other sensilla (in G. sarmaticus b is broad and long as in

-Kenyentulus Tuxen, 1981
Labial palps reduced to three setae and a sensillum. Calyx smooth and proximal canal of maxillary glands with two or three beady widenings. Sensillum t1 on foretarsi bacu-liform; b very short. Metanotum with a small P2a seta quite close to P3. Abd. app. II-III each with a short median apical seta and a long subapical one. Seta P1a on Tg. II-VI very close to P1. St. VIII with 4/0 setae. Striate band reduced, with striae only faintly visible using phase contrast. Comb VIII with small pointed teeth. Acrostyli of female squama genitalis pointed.

-Maderentulus Tuxen, 1963
Maxillary and labial palps with tuft of setae and sausage-like sensilla (two and one, respectively). Canal of maxillary glands with long and slender calyx and short proximal part; the terminal dilation as in Acerentulus. Pseudoculi small. Foretarsal sensillum t3 bacilliform; t1 claviform; b, c and d in a row at the same level. Abd. app. II-III each with two setae, the median apical one more than half the length of the subapical. Tg. II-VI with seta P3 in anterior position to P-setae row. Remarks. For figures, see Tuxen (1982).

-Neobaculentulus Yin, 1984
Maxillary palps with two sensilla: one setiform, the other torch light-shaped. Labial palps reduced to two setae and a sensillum. Calyx of maxillary glands simple. Foretarsi with all the sensilla; a to g and a' to c' present; t1 baculiform; t3 short (lanceolate  Bu and Xie (2006).

-Notentulus Yin, 1989
Labial palps without tuft, with two to four setae and one sensillum. Maxillary glands with heart-shaped calyx with lobed distal excrescences; proximal part without dilations. Foretarsal sensillum t1 baculiform; b' absent. Abd. app. II-III each with two setae, the lateral apical one half the length of the subapical. Tg. VII with Ac seta. St. VIII with 4/0 setae. Striate band with short, irregular striae, and arranged in a waved line. Comb oblique and slightly concave, with sparsely sharp teeth. Female squama genitalis with pointed acrostyli. Type species. Notentulus zunyinicus Yin, 1989. Distribution. China, Central America. Remarks. For figures, see Yin (1999).

-Proacerella Bernard, 1975
Labial palps reduced to four setae and a sensillum. Distal end of canal of maxillary glands bulbous; faint expansions proximal to the calyx. Foretarsal sensillum t1 claviform; g nearly at the same level of t3 and of approx. the same length; b' absent. Abd. app. II-III each with two setae, the lateral apical one more than half the subapical. St. VIII with 4/0 setae. Striate band well developed, with hook-shaped design in P. reducta Bernard (after Tuxen, 1981). Comb VIII with approx. 12 teeth. Female squama genitalis with pointed stylus, passing the apex of acrostyli. Type species. Proacerella reducta Bernard, 1975. Distribution. North America, West Europe. Remarks. For figures, see Bernard (1975), Aldaba (1983).

-Silvestridia Bonet, 1942
Labial palps reduced to two or three setae and a sensillum. Sensillum of maxillary palps willow leaf-shaped. Canal of maxillary glands with heart-shaped calyx and short or longer proximal part not exceeding the proximal branch of the fulcrum; more or less bipartite in the proximal part. Pseudoculi circular with fork-shaped prolongation. All sensilla present on foretarsi; t1 claviform; t3 willow leaf-shaped; a' shaped as an old Roman vase, close to t1; b spindle-shaped, reaching seta γ4. Seta P3 on Tg. II-VI is slightly anterior to the other P-setae. Abd. app. II-III each usually with one seta only, the subapical one (rarely also a delicate median apical seta is present  Tuxen (1980), Yin and Dallai (1985).

-Tasmanentulus Tuxen, 1985
Labial palps reduced to four setae and a sensillum. Canal of maxillary glands with heartshaped calyx, sometimes with distal dilations of variable size and shape. Sensillum t1 on foretarsi claviform; t3 more or less parallel-sided; d closer to c than to e; b' present or absent. Abd. app. II-III each with two setae, the lateral apical one more than half as long as the subapical. Striate band partly reduced (the striae may be seen more or less distinctly, but never as distinct) with hook-shaped design.  Szeptycki and Broza (2004).

-Vindobonella Szeptycki & Christian, 2001
Labial palps without tuft, with three setae and one long, thick, parallel-sided sensillum. Calyx of maxillary glands ovoid, smooth. Head with sd4 seta. Foretarsi with sensillum b'; d nearer to c than to e; t1 claviform; t2 spindle-like; t3 cylindrical. Seta P2a on meso-and metanotum nearer to P3 than to P2. Abd. app. II-III each with a median apical seta less than half the length of the subapical. Seta P3 on Tg. II-VI anterior to line P2-P4. St. VIII with 4/0 setae. Striate band on abd. VIII well developed. Comb VIII with straight hind margin and nearly ten slender teeth. Female squama genitalis short, with short subapical bidentate acrostyli.

-Yinentulus Tuxen, 1985
Labial palps reduced to three setae and a sausage-shaped sensillum. Maxillary palps with tuft of setae and two pointed sensilla. Canal of maxillary glands with heart-shaped calyx, distal extensions and the proximal part less than half as long as proximal branch of fulcrum. The head in profile has a prominent "front". All foretarsal sensilla present; t1 baculiform; t3 expanded distally; sensillum b distal to c-d, extremely long. Abd. app. II-III each with two setae, the lateral apical one shorter than the subapical. Striate band on abd. VIII reduced. Comb VIII with many very small teeth. Female squama genitalis with pointed acrostyli. Type species. Yinentulus paedocephalus Tuxen, 1986. Distribution. New Zealand. Remarks. For figures, see Tuxen (1986a).

-Zangentulus Yin, 1983
Labial palps reduced to two setae and a sensillum. Canal of maxillary glands with smooth and simple kidney bean-shaped calyx; end of proximal canal slightly tripartite. Pseudoculi circular. Foretarsal sensillum t1 claviform; b' present; all the exterior sensilla extremely long. Abd. app. II-III each with two setae, the median apical one less than half the length of the subapical. Striate band on abd. VIII well developed, with striae slightly varied in shape and length. Comb VIII with straight posterior margin. Female squama genitalis with pointed acrostyli. Type species. Zangentulus sinensis Yin, 1983. Distribution. China. Remarks. For figures, see Yin (1999).

Subfamily Acerentominae Silvestri, 1907
Maxillary palps with apical tuft of setae and two basal sensilla. Labial palps generally well developed with tuft of setae, two or three distinct setae and one sensillum. Mesoand metanotum each with two pairs of A-setae, or three pairs of A-setae on the mesonotum and four on the metanotum. Striate band well developed or reduced.

-Acerentomon Silvestri, 1907
Labrum almost always prolonged into a rostrum. Labial palps with tuft of setae, three setae and one sensillum lancet-like or knife-shaped. Calyx of maxillary glands simple, small; short proximal part. Foretarsal sensillum t1 claviform; t2 long, setiform; t3 leaflike, large; b' absent; a' displaced distally, close to c' and δ5; f and g close to each other; b and c at the same level; very long seta δ4 which contrasts with the length of other δ-setae.

-Huashanentulus Yin, 1980
Labial palps with tuft of setae, three distinct setae and a large sensillum. Maxillary glands of the Acerentulus-type, with a smooth, simple enlarged calyx; the proximal part long, with a terminal dilation. Pseudoculi circular, enclosed in an elliptical cavity. Foretarsal sensillum t1 filiform; t1, b and c at the same level; d distal to t2; e near to d; a' between the level of t1 and t2; b' missing; c' proximal to t3. Meso-and metanotum each with two pairs of A-setae (A2, A4). Seta P3 on Tg. II-VI situated anterior to other P-setae row. Abd. app. II-III each with two setae, the lateral apical one longer than half the subapical. St. I-VII with three A-setae. Striate band on abd. VIII well developed. Comb VIII slightly protruding backwards.
Type species. Huashanentulus huashanensis Yin, 1980. Distribution. China. Remarks. For figures and a key to species, see Bu and Yin (2010).

-Orinentomon Yin & Xie, 1993
Labial palps with tuft of setae, two or three distinct setae and a thick sensillum. Canal of maxillary glands relatively short, the ellipsoidal calyx with smooth, elliptic appendix on its dorsal side.  Nosek (1980) and Yin (1999).

-Sugaentulus Imadaté, 1978
Labial palps with tuft of setae. Calyx of maxillary glands simple or slightly granulated with an extra appendix; posterior filament short with apical dilation. Foretarsal sensillum t1 claviform; t2 thin; t3 small, lanceolate; b and c nearly at the same level; d fairly close to e, a bit proximal respect to seta γ3; f and g very close to each other; a' fairly distal to t1, at approx. the same level of t2; b' absent; seta β1 short. Meso-and metanotum each with two pairs of A-setae (A2, A4). Abd. app. II-III each with two setae, the median apical one 2/3 rd the length of the subapical.  (2010).

-Tuxenentulus Imadaté, 1973
Labial palps with tuft of setae. Canal of maxillary glands with globular calyx; the proximal part relatively short. Foretarsal sensilla similar to those of Acerentulus both for shape and position; a' close to t2; b' present; t1 claviform. Meso-and metanotum each with two pairs of A-setae (A2, A4). Abd. app. II-III each with two setae, the median apical one 2/3 rd the length of the subapical. Seta P3 on Tg. II-VI slightly anterior to the other P-setae. Striate band on abd. VIII reduced as in Berberentulus. Comb VIII square, not oblique. Pleural pectines and rotary wheels on abd. present, but sometimes indistinct. Female squama genitalis with pointed acrostyli.

-Wenyingia Imadaté, 1986
Labial palps with tuft of setae, and a slender sensillum. Canal of maxillary glands simple, smooth, calyx not globular. Foretarsi with all sensilla; t1 filiform; t3 slender and rounded apically; c and b at approx. the same level; d closer to c than to e, a bit more proximal than t2; a' slightly distal to t1; b' present, at approx. the same level of δ3. Meso-and metanotum each with two pairs of A-setae (A2, A4). Abd. app. II-III each with two setae, the lateral apical one strong and longer than half the subapical. On Tg.  Bu and Wu (2012).

-Liaoxientulus Wu & Yin, 2011
Head without differentiated sensory setae. Postpseudocular (sd4) seta present. Labial palps reduced, with three setae and no sensillum. Canal of maxillary glands simple, with smooth, ovate calyx and long, simple posterior filament. Pseudoculi nearly circular. All foretarsal sensilla present; dorsal sensillum t1 claviform; t2 thin and pointed; t3 broad jar-shaped; c and d close to each other; f distinctly nearer to e than to g; a' slightly proximal to t1; b' at approx. the same level of δ3; ventral seta β1 and interior seta δ4 both short and sensilliform. Meso-and metanotum each with two pairs of A-setae (A2, A4); seta P2a nearer to P3 than to P2. Abd. app. II-III each with two setae, the lateral apical one more than half as long as the subapical. On Tg. II-VI seta P3 slightly anterior to other P-setae row; accessory setae very short, sensilliform.  Wu and Yin (2011).

Subfamily Nipponentominae Yin, 1983
Labial palps fully developed with tuft of setae. Maxillary glands with two racemose appendices or a distinct vesicle on the calyx. Meso-and metanotum each with two pairs of A-setae, or three pairs of A-setae on the mesonotum and three or four pairs of A-setae on the metanotum. Abd. app. II-III each with two setae. Striate band on abd. VIII well developed.

-Alaskaentomon Nosek, 1977
Labial palps with a sensillum. Canal of maxillary glands with verrucose appendices on calyx and helmet-like dilation on the cover. Foretarsal sensillum a extraordinarily long, reaching seta γ4; t1 filiform. Meso-and metanotum each with two pairs of A-setae (A2, A4

-Imadateiella Rusek, 1974
Labial palps with two lateral setae and a foliaceous sensillum. Pseudoculi almost circular or broader than long. Canal of maxillary glands with racemose appendices on the calyx and, usually, an additional appendix on the cover.  Shrubovych et al. (2015).

-Nipponentomon Imadaté & Yosii, 1959
Labrum prolonged into a short rostrum. Labial palps with three setae and a foliaceous sensillum. Canal of maxillary glands with verrucose calyx, often produced into side-flaps, the proximal part usually long and with long proximal dilation. Pseudoculi small, usually, broader than long. Foretarsal sensillum t1 long and spindle-shaped; t3 long, lancet-like; d close to e; b and c subequal in length; a' a little bit more distal than t2; b' missing. Mesonotum with three pairs of A-setae (A2, A3, A4); metanotum with four pairs of A-setae (A1, A2, A3, A4). Lateral apical and subapical seta on abd. app. II-III of equal length. Tg. I-VI with 8-10 Asetae. Seta P3 on Tg. II-VI on the same row of the other P-setae. St. I-VI with three to five A-setae; VII with three A-setae; VIII with 4/2 setae. Comb VIII with approx. ten long and strong teeth. Posterior margin of abd. IX-XI with pectinated structures. Pleural pectines often present on segments II-VIII. Female squama genitalis with mostly square acrostyli with uneven apex or produced into three teeth; sometimes, long and pointed acrostyli. Type species. Acerentomon nippon Yosii, 1938. Distribution. Northeast Asia, North America. Remarks. For a key to species, see Bu et al. (2013).

-Noldo Szeptycki, 1988
Maxillary palps with equal, thin and nearly setiform basal sensilla. Labial palps with broad leaf-like sensillum. Calyx of maxillary glands small, granulated; posterior filament with granulated thikening. Pseudoculi circular, with relatively long lever. Foretarsal sensillum b' missing; a' slightly more proximal than t2; t1 filiform; t3 leaf-like; b and c nearly at the same level; d nearer to e than to c; setae β1 and δ4 sensillum-shaped. Meso and metanotum each with three pairs of A-setae (A2, A3, A4); seta P2a on nota nearer to P3 than to P2. Prosternum with seta A2 present. Lateral apical and subapical seta on abd. app. II-III nearly of equal length. Seta P3 on Tg. II-VI in line with other P-setae. St. VIII with 4/2 setae. Comb VIII with straight hind margin. Abd. IX-X without striate band. Hind margin of Tg. X-XI with very subtle serration. Female squama genitalis with relatively short, pointed acrostyli. Type species. Noldo submontanus Szeptycki, 1988. Distribution. Russia and Ukraine (Crimea). Remarks. For figures and the distinction between the two known species of genus Noldo, see Shrubovych and Szeptycki (2006).

-Nosekiella Rusek, 1974
Labial palps with a broad sensillum. Canal of maxillary glands with two racemose appendices on the calyx and an additional appendix on the cover. Pseudoculi broader than long. Foretarsi with baculiform sensillum t1; t2 long, setiform; t3 lanceolate; a' at the same level of or anterior to t2 base. Meso-and metanotum each with three pairs of A-setae (A2, A3, A4). Lateral apical and subapical seta on abd. app. II-III nearly of equal length. Seta P1a present on Tg. I-VI. Tg. II-VI with 10 A-setae; Tg. VIII with four or six A-setae. Prosternum with or without seta M2. St. III with five P-setae; St. I-VII with three A-setae; St. VIII with 4/0 or 4/2 setae. Pleural pectines visible. Hind margin of segments IX-XI smooth. Female squama genitalis short, with blunt, bilobed acrostyli.

-Paracerella Imadaté, 1980
Maxillary and labial palps with tuft of setae and two and one sensilla, respectively. Maxillary glands with distinct calyx with racemose appendices on its surface. Foretarsal sensillum t1 filiform; sensillum d placed at approx. halfway between c and e or a little closer to e; a' fairly distal to t1, at approx. the same level of t2. Meso-and metanotum each with three pairs of A-setae (A2, A3, A4). Subapical seta on abd. app. II-III slightly longer than lateral apical. Seta P3 on Tg. II-VI in line with other P-setae. St. I-VII with three A-setae. St. VIII with 4/0 or 4/2 setae. Comb on abd. VIII with irregular teeth. Type species. Acerella shirataki Imadaté, 1964. Distribution. China (after Bu et al., 2016, Japan and North America. Remarks. Figures and a key to species are provided in Shrubovych and Smykla (2012).

-Verrucoentomon Rusek, 1974
Labial palps with a four-branched terminal tuft and a sensillum. Maxillary glands with racemose surface of calyx and without wide racemose appendices. Foretarsal sensillum t1 filiform; t3 leaf-like; d near e; a' at the level or distal to the base of t2; seta β1 setiform; δ4 short and blunt. Meso-and metanotum each with three pairs of A-setae (A2, A3, A4). Lateral apical and subapical seta on abd. app. II-III nearly of equal length.  Shrubovych and Bernard (2012). For a recent review of the North American species, see Shrubovych et al. (2015).

-Nanshanentulus Bu & Yin, 2007
Maxillary palps with two sensilla, parallel-sided and equal in length. Labial palps with tuft of setae and a broad sensillum. Calyx of maxillary glands with two lateral racemose appendices and with posterior filament slightly split into two leaves.
Pseudoculi ovate. Foretarsi with no sensillum b'; t1 claviform; t3 small and lanceolate; c slightly lower than level of b; d nearer to e than to c; c and e short and broad; a' at the level of t1; setae β1 and δ4 of normal shape. Claw with single inner flap. Mesoand metanotum with three pairs of A-setae (A2, A3, A4); seta P2a on nota near to P3 than to P2. Prosternum with no seta A2. Subapical seta on abd. app. II-III slightly longer than lateral apical. Seta P1a on Tg. I-VI absent; seta P3 on Tg. II-VI in posterior position. St. VIII with 4/0 setae. Striate band on Tg. IX and X absent. Comb on abd. VIII with straight hind margin. Female squama genitalis robust and with short blunt acrostyli.

-Yavanna Szeptycki, 1988
Maxillary palps with thin, nearly setiform basal sensilla. Labial palps with broad basal sensillum. Pseudoculi circular, with short lever. Calyx of maxillary glands simple, large, with small granulation. Foretarsi with claviform sensillum t1; t3 leaf-like; b and c nearly at the same level; d nearer to e than to c; a' more distal than t2; b' absent; seta β1 of normal shape, δ4 sensillum-shaped. Meso-and metanotum each with three pairs of A-setae (A2, A3, A4); seta P2a on nota nearer to P3 than to P2. Prosternum with or without seta M2. Subapical seta on abd. app. II-III slightly longer than lateral apical. Seta P1a present on Tg. I-VII or on Tg. VII only. Tg. II-VI with 10 anterior setae and seta P3 in line with other P-setae; Tg. VIII with six A-setae (A2, A4, A5  .

Subfamily Acerellinae Yin, 1983
Labial palps with terminal tuft of setae, two setae and an oval or foliaceous sensillum. Canal of maxillary glands with lateral verrucose appendices on calyx. Meso-and metanotum each with three pairs of A-setae (A2, A3, A4). Abd. app. II-III each with two setae (lateral apical and subapical) subequal in length.

Order SINENTOMATA Yin, 1966
Abdomen with pectinated structures. Median setae absent on meso-and metanotum. Abd. app. I always two-segmented with terminal vesicle and four setae. Female squama genitalis with long acrostyli.

Family Fujientomidae Tuxen & Yin, 1982
Pale body. Spiracles absent. Foretarsi with additional sensillum x on the external side. Abd. app. all two-segmented with four setae and terminal vesicle. Comb VIII with small teeth.

-Fujientomon Imadaté, 1964
Rostrum somewhat protruded. Maxillary palps with tuft of setae and two sensilla. Labial palps reduced with an apical thick seta and two basal setae. Canal of maxillary glands simple, ending with a long sac-or sausage-like dilation. Pseudoculi circular without lever. Foretarsal sensilla b' and c' absent; sensillum t1 extremely broad; t2 broad; t3 long and slender; exterior sensilla a and x broad, foliaceous. Empodium long, a little shorter than claw. Seta S remarkably longer than claw. Metanotum with two or three pairs of A-setae (A3 is very short, sensilliform, often absent). Tg. I-VII with seta Ac. St. VIII with 4/4 setae. Comb VIII with approx. 15 small teeth, and a few small teeth on its anterior margin. Female squama genitalis with rod-like acrostyli. Type species. Fujientomon primum Imadaté, 1964. Distribution. China, Japan. Remarks. Redescription by Nakamura (2014) was followed.

Family Sinentomidae Yin, 1965
Body reddish-brown. Tracheal system present. Pseudoculi large with 7-13 transverse striae. Foretarsi with additional sensillum c" on the internal side. Abd. app. II-III unisegmented, each with two setae, a long distal and a short proximal one. Basal part of the female squama genitalis bucket-like, with large and stout acrostyli.

-Zhongguohentomon Yin, 1979
Integument highly sclerotized, earth-yellow coloured. Labial palps with tuft of setae, two distinct setae and a sausage-like sensillum. Central seta present on antero-dorsal surface of the head (similar to that of Sinentomon). Pseudoculi with a large triangular prolongation on the anterior margin. Foretarsal sensillum t1 clavate; t3 stout; e and g short, slightly spatulate; b1' and b2' present; c' fairly long. Abd. VIII with a pair of rudimentary combs on abdominal glands. Pectinated patches on St. XI-XII. Female squama genitalis characterized by a simple stylus with a nipple-shaped processus on its posterior end and the incurvated processus filum. Type species. Zhongguohentomon magnum Yin, 1979. Distribution. China. Remarks. For figures and a key to species, see Yin (1999).

-Styletoentomon Copeland, 1978
Labrum longer than mandibles and very narrow, forming a rostrum. Mandibles very long, extremely slender and sharp pointed (stylet-shaped). Canal of maxillary glands reaching prothorax. Foretarsal sensilla e and g clavate. Female squama genitalis with sharp-pointed processus sternalis and evenly bent giving a stoop-shouldered appearance.
Tg. II-VI with 10 or eight A-setae; eight or six A-setae on Tg. VII; St. VIII always with seven P-setae (2/7 or 0/7). Tg. X or XI of some species bearing a pair of peculiar setae or spines (Anisentomon).

Family Antelientomidae Yin, 1983
Eosentomata without open spiracles on meso-and metanotum. Sensilla on foretarsus similar to those of genus Eosentomon.

Key to genera
This key directly identifies genera, without intermediate steps to families and subfamilies (indicated in the diagnosis part) for greater convenience and to keep it valid regardless of the system followed. Parts concerning the Acerentominae and Nipponentominae genera are taken from Shrubovych and Rusek (2010) and Shrubovych et al. (2014b), respectively. The previous key for Acerentomidae genera by Tuxen (1984) was also used as a reference. For Protentomidae and Hesperentomidae genera, we referred to the key to subfamilies and genera by Tuxen and Yin (1982). Numbers in brackets after the names of genera indicate their position in the diagnostic section of this paper.