The identity of Plesialges mimus Trouessart, 1919 and taxonomic notes on the feather mite genus Hemialges Trouessart, 1895 (Acariformes: Analgidae)

The feather mite Plesialges mimus Trouessart, 1919, briefly described from the White-browed Babbler Pomatostomus superciliosus (Vigors & Horsfield) (Passeriformes: Pomatostomidae), is the only species of the genus Plesialges Trouessart, 1919. In this work, I redescribe this mite species based on the type specimens, transfer it into the genus Hemialges Trouessart, 1895, and provide it with the valid name Hemialges mimus (Trouessart, 1919) comb. n. The genus Plesialges syn. n. is synonymized with the genus Hemialges. A new diagnosis, comments on taxonomy and an updated checklist of species are provided for the genus Hemialges. The transfer of P. mimus to the genus Hemialges created a conflict with the previously named Hemialges mimus Trouessart, 1920 from the Trumpet Manucode, Phonygammus keraudrenii (Lesson & Garnot) (Passeriformes: Paradisaeidae). Hemialges mimus (Trouessart, 1919) comb. n. is now the older homonym within this genus and H. mimus Trouessart, 1920 from the Trumpet Manucode is a junior homonym; here I provide the latter with a new name, Hemialges trouessarti nom. n.


Introduction
The feather mite genus Plesialges Trouessart, 1919 (Analgidae: Analginae) was established by for a single species, Plesialges mimus , collected from the museum skin of a White-browed Babbler, Pomatostomus superciliosus (Vigors & Horsfield) (Passeriformes: Pomatostomidae), from Australia . It is necessary to note that this genus and species were described once more as new taxa in another paper, in an issue of "Redia" for 1919, which was actually published two years later (Trouessart and Berlese 1921). Both papers Berlese 1921) provided the same extremely short diagnoses saying only that the genus Plesialges resembles Analges Nitzsch, 1818 in the form of hyperthrophied legs III, but its male differs in having the pretarsus III in a form of bottle without a neck (l'ambulacre renflée en forme la bouteille depourvue de col), and the opisthosoma entire and much wider (l'abdomen entire et plus développé en largeur que chez Analges). Regarding the P. mimus female,  only indicated that it is similar to those of Analges. In the subsequent hundred years, this feather mite species has never been carefully redescribed or illustrated. Atyeo (1982, 1996) included Plesialges in the genus Analges and treated it as a separate subgenus. In the review of the species content of the genus Analges, Mironov (2019) noted that P. mimus does not correspond to the criteria of this genus.
I re-examined the type specimens of P. mimus deposited in the collection of the Museum National d'Histoire Naturelle in Paris, one male and one female constituting the entire type series of this species, and have determined that the genus Plesialges corresponds to the taxonomic criteria of the genus Hemialges Trouessart, 1895 and should be included in this genus. The main mistake that led Trouessart to the suggestions that Plesialges is close to Analges was the incorrect interpretations of the structure of tarsus III in the male. Actually, a large claw on tarsus III in P. mimus is the apical process of the tarsal segment proper, which is one of the main diagnostic features of the genus Hemialges; in males of Analges, the apical "claw" of tarsus III is formed by the modified seta s (Gaud 1974; Mironov 2019. As for pretarsus of legs III in the male of P. mimus, the ambulacral stalk is thinned but retains a cylindrical form and a rudimentary ambulacral disc; in males of Analges, the entire pretarsus is modified into a very thin and transparent claw-like process, tightly adjacent to claw-like seta s. In addition, previous researchers did not pay attention to the structure of epimerites I in the female of P. mimus, which are fused into a Y, while in females of Analges, epimerites I are free.
In the present paper, Plesialges mimus is transferred to the genus Hemialges and redescribed. Additionally, I provide a new diagnosis for the genus Hemialges, comments on its taxonomy, and an updated checklist of species including host associations and localities.

Material and methods
The type specimens of Plesialges mimus were loaned from the Museum National d'Histoire Naturelle (Paris, France). Comparative materials representing various species of the genus Hemialges were examined in the feather mite collection of the Zoological Institute of the Russian Academy of Sciences (Saint Petersburg, Russia). Investigation and redescription of mite specimens were made with the aid of a Leica microscope (DM2500, Leica Microsystems, Inc.) equipped with differential interference contrast (DIC) and a camera lucida.
Descriptions of mite taxa follow the modern format proposed for genera and species of Analgidae (Mironov 2009, 2019; Dabert et al. 2018; Pedroso and Hernandes 2018. General morphological terminology and idiosomal chaetotaxy follow Gaud and Atyeo (1996) with minor corrections by Norton (1998), and the leg chaetotaxy is after Grandjean (1939). All measurements are in micrometres (μm). Classification and scientific names of birds follow Gill et al. (2021).
Male. Strongly variable in general appearance among species and within species; intraspecific polymorphism mainly expressed in size and form of body and in structure of legs III. Idiosoma variable in shape, from roughly rhomboid (in heteromorph males of megamerus and pappus groups) to moderately ovate; lateral margins of opisthosoma gradually attenuate posteriorly or almost parallel-sided. Opisthosoma, in most species, with a pair of triangular opisthosomal lobes bearing terminal lamellae of various form (angular, tongue-shaped, leaf-like) and with interlobar membrane between lobes; rarely without distinct lobes, truncate or rounded, without lamellae (furcula group; homeomorph males of some species, e.g. H. effeminatus, H. hologaster, H. pilgrimi). Lateral margin of humeral shields with or without spine-like extensions. Lateral membranes of opisthosoma absent. Hysteronotal shield occupying median area of hysterosoma with anterior end usually extending to level of humeral shields or trochanters III; rarely developed only in posterior half of opisthosoma. Setae c2 situated on anterior ends of humeral shields and usually represented by macrosetae. Setae d2, e2 vary in size, represented by macrosetae or relatively short filiform setae; these pairs distant from each other, d2 situated at level of trochanters III, and e2 at midlength of opisthosoma. Setae ps2, h2 and h3 arranged in a short row on lateral margin of opisthosomal lobes or, when lobes not developed, on margin of opisthosoma. Setae ps1 short filiform. Supranal concavity present.
Coxal field III closed, completely sclerotized and fused with corresponding humeral shields and rarely with coxal fields II forming large combined shields on both sides of venter. Genital apparatus at level of anterior trochanters III. Epiandrum usually absent; paragenital apodemes flanking genital apparatus laterally present. Genital shield absent. Adanal shield represented by a pair of large longitudinal plates occupying area between genital apparatus and anal area; in some species, additional semicircular fragment of this shield partly encircles anal field (e.g. H. pilgrimi). Anal suckers circular, with smooth corolla. Cupules ih well developed in most species.
Legs III hypertrophied, much thicker and longer than legs IV; trochanter, femur, genu and tibia always enlarged, inner margin of femur III can bear 1-2 spine-like processes of various lengths and shapes. Tarsus III well developed, narrower than corresponding tibia, with large claw-like apical process (in some species half as long as entire segment), with or without basal spine-like extension on inner margin; all setae of tarsus III filiform; ambulacral stalk well developed as in other legs or slightly thinner (H. forcipes, H. spinicrus); ambulacral disc circular or reduced and shaped as small oval (H. mimus). Tarsus IV elongate, similar in length to tibia, with or without small apical process; modified setae d and e button-like; pretarsus IV developed as on legs I, II.
In heteromorph males, trochanter, femur, genu and tibia III strongly enlarged; inner margin of femur usually with spine-or claw-like process, rarely with 2 processes (furcula group); in cases of strongly developed claw-like process and relatively short genu, tibia and tarsus, whole leg III shaped as a chela (H. forcipes, H. mimus). In homeomorph males, trochanter, femur, genu and tibia III moderately enlarged, inner margin of femur with much smaller spines than in heteromorphs or lacking them.

Figure 2
Hemialges mimus , male lectotype, ventral view. from posterior tips of epimerites I, II. Lateral flaps of oviporus poorly sclerotized. Epimerites IIIa strongly reduced or absent. Anal opening ventral, copulatory opening posterior to anal opening.
Differential diagnosis -Combinations of characters differentiating Hemialges from remaining genera of Analginae. Both sexes. Epimerites I fused into a Y, ventral cuff-like processes of tarsi I, II in most species with minute denticles and striation on margin. Male. Pairs of setae d2 and e2 distant from each other, posterior end of opisthosoma with triangular opsthosomal lobes provided with variously shaped terminal lamellae, or opisthosoma can be without lobes and lamellae, truncate or rounded. Opisthosoma without lateral membranes. Paragenital apodemes small, flanking genital apparatus laterally. Adanal shield represented by a pair of longitudinal plates between genital apparatus and anal field. Legs III hypertrophied with trochanter, femur, genus and tibia enlarged. Tarsus III with large claw-like or spine-like apical process, all setae of tarsus simple filiform; ambulacral stalk well developed, almost as in other legs; ambulacral disc circular or ovate. Female. Hysteronotal shield absent, epigynum distant from posterior tips of epimerites I, II, setae ps1 present.
Remarks -Authority and date. The name Hemialges was established by Trouessart (1895) as a subgenus of the genus Megninia Berlese, 1881 for two species, Megninia pappus Trouessart and Neumann, 1888 (type species of the subgenus) and M. (Hemialges) magnifica Trouessart, 1895. In the next paper concerning this taxon, Trouessart (1899) described four more species in the content of this subgenus (in pages 27-28), but in the second part of his paper (Deuxième Note, page 57), he treated Hemialges as a full genus and described one more species, H. emarginata Trouessart, 1899. In his revision of feather mite genera, "Sarcoptides plumicoles", Trouessart (1916: 218) treated Hemialges as a full genus and established a new genus Hyperalges with a single species Hyperalges magnificus (Trouessart, 1895) transferred from Hemialges. It is interesting to note, in that general paper, Trouessart (1916) for the first time gave the erroneous date "1888" as the year of establishing the genus Hemialges.
Finally, in the monograph with revisions of the genera Hemialges and Hyperalges, Trouessart (1920: 302) made two erroneous statements: (a) that the taxon Hemialges was established in 1888, and (b) that it was elevated to the rank of full genus in 1916. Actually, Trouessart published only two papers in 1888, both co-authored by G. Neumann, and one of these papers (Trouessart and Neumann 1888b) included the description of Megninia pappus, but did not contain any mention of the taxon Hemialges. It is only possible to guess that Trouessart accidentally or deliberately applied the date 1888 to this genus, based on the year of the original description of M. pappus. The second paper by Trouessart and Neumann (1888a) dealt with skin-inhabiting feather mites, the families Dermationidae and Epidermoptidae in modern sense, and did not concern feather-inhabiting mites. It is also necessary to add that in his latest papers, Trouessart (1916Trouessart ( , 1920 attributed only his name as the authority of M. pappus, although the original paper (Trouessart and Neumann 1888b) did not have any comments that he was the sole author of the new taxa.
These incorrect and conflicting statements in the papers by Trouessart entangled subsequent researchers. In the same paper but on different pages, Gaud and Atyeo (1982) applied to Hemialges the authorities "Trouessart, 1888" (p. 299) and "Trouessart and Neumann, 1888" (p. 303). In their world review of supraspecific taxa of feather mites, Gaud and Atyeo (1996) attributed to this genus two authors, Trouessart and Neumann, but in different pages provided different dates, 1888 (p. 53) and 1916 (p. 182).
Species content. In the revision of the genus Hemialges, Trouessart (1920) considered 25 species, including 19 newly described, and arranged them in seven species groups designated in capital letters A-G. These groups were based on characters of heteromorph males, such as the general shape of the body, structure of the prodorsal shield, opisthosoma and legs III. In Table 1, all groups retained to date in the genus are provided with new names taken from the first species mentioned in corresponding groups of Trouessart, and the letter designations by Trouessart are shown in brackets. Species within each group are listed according to their sequences in the revision of Trouessart (1920).
The transfer of Plesialges mimus  to the genus Hemialges in the present work makes the species Hemialges mimus Trouessart, 1920 described from Phonygammus keraudrenii (Lesson, R & Garnot) (Paradisaeidae), a junior homonym. The latter mite species is given here a new valid name, Hemialges trouessarti nom. nov. Taking into account all aforementioned taxonomic changes, the genus Hemialges currently includes 25 species (Table  1).
Hosts -All presently known Hemialges species, except one, are associated with oscine passerines of the infraorder Corvida and distributed in the Old World, mainly in the Indo-Malayan and Australian realms ( Table 1). The record of H. hologaster Trouessart, 1899 from the Double-eyed Fig Parrot, Cyclopsitta diophthalma (Psittaciformes: Psittaculidae), is most likely the result of accidental contamination, as was noted even by the author of this mite species (Trouessart 1899(Trouessart , 1820. It is interesting to note that this mite was described a new species in both cited papers of Trouessart. The majority of Hemialges species associated with oscine passerines were recorded from whistlers (Pachycephalidae), 9 species from 13 host species, and from birds-of-paradise (Paradisaeidae), 4 species from 8 host species; and a few or only one species were found of birds from the families Meliphagidae, Monarchidae, Orthonychidae, Petroicidae, Pomatostomidae, Rhagologidae, Rhipiduridae and Vangidae. Considering the wide distribution of the genus Hemialges on representatives of Corvida and the number of potential hosts from its families that have not been yet examined for feather mites, it would be reasonable to expect a vast number of undescribed species of this genus in the Indo-Malayan and Australian realms.
Ventral cuff-like processes of tarsi I, II with minute denticles and striations on margins. Femur II with rounded lateral margin ( Figure 5B). Legs III strongly widened and roughly chela-shaped, femur especially strongly widened and bearing large bidentate extension on inner margin, genu and tibia shaped as truncated cones (frustums); tarsus with large claw-like apical process bearing setae e, d and f and with basal angular process on inner margin bearing setae w and s. All setae of tarsus III filiform, not longer than this segment; ambulacral stalk cylindrical, not extending beyond tip of apical claw; ambulacral disc reduced, small ovate and partly retracted into ambulacral stalk. Legs IV with distal part of tarsus extending beyond posterior margin of opisthosoma; tarsus IV 45 long, with small bidentate apical process; modified setae d and e small button-like, situated at midlength and apical process of this segment, respectively. Tibial solenidion φIII slightly shorter than tarsus III, solenidion φIV about 1.5 times longer than tarsus IV. Length of genual solenidia: σ2I 45, σII 10, σIII 35.
Epimerites I fused in a Y as in male. Coxal fields I, II without extensive sclerotization. Epigynum bow-shaped, without suprategumental processes, 25 long and 65 wide (Figure 4). Apodemes of oviporus not sclerotized. Genital papillae of each side fused to each other, situated posterior to setae g. Setae 1a short, not reaching epigynum. Setae 4b 55 long, situated on tips of epigynum, and extending to midlength between levels of setae g and 4a; setae g 55 long, situated at posterior ends of oviporus flaps extending slightly beyond level of setae 4a; setae 4a 120-130 long not extending to anterior end of anus. Setae ps3 about 60 long, extending slightly beyond posterior margin of opisthosoma. Distances between ventral setae: 4b:3a 17, 4b:g 42, g:4a 55. Copulatory opening situated asymmetrically, at level of posterior end of anus. Spermatheca and spermaducts in paralectotype specimen indistinct.
Differential diagnosis -Based on the subdivision of the genus Hemialges by Trouessart (1920) into species groups (Table 1), H. mimus  should be placed in the furcula group, the males of which are characterized in having femur III with two large spines and the posterior end of opisthosoma without distinct opisthosomal lobes. These two features clearly discriminate the furcula group from remaining species groups of Hemialges, always having well developed triangular lobes with variously shaped terminal lamellae, and femora III with one spine or without it. Hemialges furcula was described from a single heteromorph male without any illustrations and was the only species previously constituting the furcula group. The male of H. mimus differs from that of H. furcula in having the following features: in H. mimus, the large extension of the inner margin of femur III bears two relatively short spines similar in shape and size, and the posterior end of opisthosoma is trapezoidal in form, with almost straight posterior margin. According to the brief description of H. furcula, femur III bears two unequal spines situated one after another, among which the posterior one is smaller and shaped as a claw, and the posterior end of opisthosoma is slightly concave. Additionally, H. mimus is much smaller, with idiosoma 365 long; while H. furcula is a very large species, about 800 long (Trouessart 1920).