A revision of Onychelmis Hinton, 1941 (Coleoptera: Elmidae), with description of new species, DNA barcoding and notes on the geography of the genus

The genus Onychelmis Hinton, 1941 was for a long time regarded as a small taxon with only three known species distributed in the Andes. A study of new material from Ecuador, using morphological and molecular data, has resulted in the discovery of five new species: Onychelmis lenkae sp. nov., O. lobata sp. nov., O. minor sp. nov., O. onorei sp. nov. and O. splendida sp. nov. We also revised the entire genus and redescribed the three known species, O. longicollis (Sharp, 1882), O. leleupi Delève, 1968 and O. whiteheadi Spangler & Santiago, 1991. Habitus photographs of adults are provided, together with line drawings of male and female genitalia, and schematic illustrations of the distribution of femoral tomentum for each species. DNA sequences for barcoding the COI mtDNA fragment were used to support species delimitation and to suggest possible relationships among species. The revision includes a key to adults of all species of Onychelmis and notes on the biogeography of the genus, with an updated distribution map.


Introduction
The Elmidae Curtis, 1830 is a moderately large cosmopolitan family of aquatic beetles, which includes around 1500 known species classifi ed in almost 150 genera . The elmids are common inhabitants of various lotic habitats, and they also form an important component of aquatic assemblages (Cayrou et al. 2000) and serve as valuable ecological indicators (e.g., Compin & Céréghino 2003).
Numerous recently published descriptions of new species document the high diversity of Elmidae in the Neotropical region (e.g., Manzo & Archangelsky 2015;Passos et al. 2015;Čiampor et al. 2016;Shepard & Barr 2016;Barr 2018;Martínez-Román et al. 2019;Polizei et al. 2020), and although our knowledge of the elmid fauna of the tropics improves and interest in riffl e beetle taxonomy grows, the information on the real diversity of the family is still far from complete.
This study examines the type material of two of the three known species of Onychelmis Hinton, 1941 and freshly collected material from Ecuador, the country in South America with the highest biodiversity per km 2 (Sites et al. 2003). Monte & Mascagni (2012) described ten new elmid species and recognized 59 species for Ecuador classifi ed in 19 genera. Since then, an expedition to this country has yielded extensive material, from which several new species ) and one new genus have so far been described (Čiampor et al. 2019). Additionally, Polizei & Barclay (2018 recorded two known species of the genus Hintonelmis Spangler, 1966 and described one new species of Cylloepus Erichson, 1847 from Ecuador. Onychelmis was, for a long time, regarded as a small genus with only three species distributed along the Andes in Central and South America. The genus was erected by Hinton (1941) with the type species Onychelmis longicollis (Sharp, 1882), originally described from a single specimen found in Panama (Sharp 1882). Additional material, collected by M.N. Leleup in Ecuador, was described as a new species, Onychelmis leleupi Delève, 1968. The third species, Onychelmis whiteheadi Spangler & Santiago, 1991, was collected in 1984 Most works that deal with Elmidae taxonomy rely on morphological diagnostic characters. In this study, we employed DNA barcoding, a useful method for species delimitation (e.g., Hayashi et al. 2013;Čiampor et al. 2016). Molecular methods or the knowledge of genital structure are often necessary for accurate identifi cation of minute insects. Without this information, a lot of species can be reliably determined only to the genus level. However, a common practice is to identify them as the known species from a respective region, which can result in undervaluation of local biodiversity and distortion of species distribution data. Moreover, the DNA-based methods often represent a faster and more universal approach for species delimitation than morphology, accelerating our understanding of biodiversity richness. This is especially important with respect to increasing deforestation and habitat loss in tropical regions due to human activities.
In this work, the combination of morphological and molecular data is used, which allows us to identify a surprisingly high diversity of Onychelmis species within a relatively small geographic area. We describe fi ve new species, and almost triple the known number of species of the genus. The three known species are redescribed and relationships within the genus are briefl y discussed. Additionally, we have summarized data on the distribution of the genus and compiled an identifi cation key for adults of all known species.

Material and methods
The studied material was collected by net sampling in smaller streams fl owing in primary or degraded forest. Larvae and adults were fi xed in pure alcohol directly in the fi eld. Specimens for the morphological study were cleaned and examined under a Leica M205C stereo microscope at magnifi cations up to 160 ×. Male genitalia and ovipositors were studied as temporary glycerine slides at magnifi cations up to 600 ×, using a Leica DM1000 light microscope. Drawings were made with a drawing tube. Photographs of habitus were made using a Leica M205C with a Nikon D3s digital camera attached. Image stacks were combined using Zerene Stacker software and fi nalised in Adobe Photoshop CS5. The beginning and end of label texts are indicated using double quotation marks (""); a double slash (//) separates the data on diff erent labels. Morphological terms generally follow Kodada et al. (2016).
Metric characters were measured with an ocular grid to the nearest 0.05 mm. Abbreviations used in the text: CL = combined body length (measured from anterior margin of pronotum to elytral apices) EL = elytral length EW = maximum elytral width HW = head width with eyes ID = interocular distance PL = pronotal length PW = maximum pronotal width The DNA was isolated from whole specimens using the DNeasy Blood and Tissue Kit (Qiagen) according to the manufacturer's protocol or by phenol-chloroform extraction. A fragment of the 5′ end of the mitochondrial gene for cytochrome c oxidase subunit I (cox1) was amplifi ed with primers LCO1490 and HCO2198 (Folmer et al. 1994). Amplifi cation products were purifi ed and sequenced from both sides in Macrogen Europe Inc. (Amsterdam, Netherlands) and subsequently assembled and edited in Sequencher ver. 5.1. For the phylogenetic analysis, 20 sequences from larvae and adults were used, one of which was downloaded from GenBank. The measurement of the genetic distance using the K2P model, maximum likelihood tree and bootstrap support (1000 replicates) were performed in MEGA X software ver. 10.0.4 (Kumar et al. 2018). The best-fi tted substitution model (TN93+G) was selected by jModelTest 2 (Darriba et al. 2012). The fi nal tree was edited in FigTree ver. 1.4.2 and Adobe Illustrator. Two sequences of Ictelmis martae Čiampor et al., 2019 and three of Notelmis sp. were used for outgroup rooting. Sequences were sent to GenBank (accession numbers: KR134432; MF322597-MF322604; MF322606-MF322612; MF322614; MT762804-MT762811) and BOLD databases. The dataset DS-ELMONYCH, with all information on sequences, was created in BOLD and was assigned with a DOI: https://doi.org/10.5883/DS-ELMONYCH. All Ecuadorean specimens belong to PUCE, but presently are, for a long-term loan, deposited in CCB.

DNA barcoding and species delimitation
Final cox1 barcoding fragments were 620bp long with no ambiguous sites or indels. The phylogenetic tree ( Fig. 1) produced by maximum likelihood from twenty specimens of Onychelmis revealed seven well recognized molecular taxonomic units (species) in Onychelmis. The intraspecifi c distance was between 0-1.1%, the highest diff erence between conspecifi c samples was recorded in O. leleupi, the species with the widest reported distribution. The interspecifi c distance ranged from 4.4 to 10.5%. The species of Onychelmis were distanced by 11.9-14.9% from Ictelmis and by 18.1-20.4% from Notelmis samples.  Genus Onychelmis Hinton, 1941Onychelmis Hinton, 1941: 66 (type species: Elmis longicollis Sharp, 1882).

H
. Narrower than pronotum, retractile, rounded, dorsally shiny. Labrum wider than long, anterolateral angles broadly arcuate with numerous golden, recumbent hair-like setae. Clypeus shorter and wider than labrum, about 3.5 times as wide as long, anterior margin slightly concave, anterolateral angles rounded. Frontoclypeal suture visible. Mandible short, moderately broad, symmetrical; apex curved with three obtuse teeth, molar part with pores. Prostheca large, entirely membranous. Maxilla with cardo and stipes moderately short. Maxillary palp 4-segmented, segments 1-3 short and wide, terminal segment almost twice as long as segments 1-3 combined. Galea elongate, as wide as maxillary palp. Lacinia about twice as wide as galea, longer than wide, subrectangular. Apex and inner side with dense, long, curved setae. Labium with submentum broad, subpentagonal; mentum transverse, as wide as submentum, very short; prementum elongate, anterior half widened, margin densely setigerous. Labial palp 3-segmented, fi rst segment short and wide; second segment more than twice as long as fi rst, distally widened; third segment suboval, equally wide as segments 1-2, about twice as long as second segment. Antennae fi liform, 11-segmented; pedicel about twice as long as scape, remaining segments about 4 times as long as scape and pedicel combined, segments 3-10 subrectangular, subequal in length; terminal segment longest, suboval, with pointed apex. Eyes well developed, hemispherical.

T
. Pronotum subparallel, widest behind middle; surface shiny, with narrow reticulation along basal margin and posterolateral angles; sublateral carinae never well-developed (in several species, e.g., O. whiteheadi and O. leleupi, with a very fi ne raised line in basal ⅙ or ⅛); disc convex, divided by a broad, deep transverse impression before middle; two prescutellar foveae separated by a raised line extending from base to apical discal half, connecting both halves and merging into them; anterior margin arcuate; posterior margin bisinuate; sides of pronotum convex before and after transverse constriction; lateral margins narrowly rimmed; posterolateral angles orthogonal; anterolateral angles slightly produced. Hypomeron fi nely microreticulate, widest in middle. Prosternum moderately long in front of procoxae, with anterior margin concave; sides raised around procoxae, forming carinae, not reaching anterior margin, prosternal process long, moderately broad and with posterior margin broadly rounded. Mesoventrite coarse, short and wide, with deep triangular groove for reception of prosternal process; posterior margin around mesocoxae raised. Metaventrite slightly wider than long, shiny with setigerous punctures; disc convex with medial, triangular depression in posterior half; discrimen in basal ⅔; with one prebasal fovea with setal tufts on each side of metaventrite ( Fig. 5; foveae or depression indistinct in a few species, e.g., O. onorei sp. nov. and O. minor sp. nov.). Elytra convex, widest in anterior ⅔; sides strongly declivous; surface shiny, with dense tiny punctures; elytral margin narrowly rimmed; humeri indistinctly to broadly produced; epipleuron tapering posteriorly. Prominent sublateral carina on sixth interval reaching ⅘ of elytron length present or absent. Elytra with ten more or less visible rows of punctures, impressed coarsely and deeply to completely absent. Scutellum subovate, fl at. Legs moderately long; femora clavate, markedly widened in middle; tibiae longest, apically with cleaning fringe. Tarsi 5-segmented, fi rst four segments each with one fi ne pale, recumbent seta, fi fth segment slightly shorter than remaining segments combined; claws with a large subbasal and smaller basal teeth.

Female
Externally similar to male, except metaventrite without distinct medial, triangular depression in posterior half and without two prebasal foveae, fi fth ventrite is more elongate (Fig. 4B) and extension of femoral tomentum is usually greater. Ovipositor with short, noncylindrical stylus.

C
. Head, pronotum and elytra black; venter dark brown with reddish tinge; trochanters, tibiae and tarsi brown; fi rst two segments of antennae and tarsal claws pale brown.

H
. Partly retractable into prothorax. Antennae with only 2 segments remaining; pedicel about twice as long as scape. Labrum with anterior margin almost straight; anterolateral angles broadly arcuate with numerous golden, recumbent hair-like setae; clypeus shorter and wider than labrum, about 3.5 times as wide as long; anterior margin slightly concave, anterolateral angles rounded; frontoclypeal suture almost straight. Eyes well developed, HW: 0.29 mm, ID: 0.14 mm, suboval in lateral view, protruding from head outline in dorsal view, circumocular surface raised. Frons convex between eyes.

T
. Pronotum widest behind middle, PW: 0.41 mm, PL: 0.43 mm; surface shiny, with narrow reticulation along basal margin and posterolateral angles, with dense tiny punctures; sublateral carinae European Journal of Taxonomy 739: 1-35 (2021) absent, fi ne raised line indistinct; disc convex, divided by a broad, deep transverse impression before middle; two prescutellar foveae separated by a raised line extending from base to apical discal half, connecting both halves and merging into them; anterior margin arcuate; posterior margin bisinuate; sides of pronotum convex before and after transverse constriction; lateral margins narrowly rimmed; posterolateral angles orthogonal; anterolateral angles slightly produced. Hypomeron fi nely microreticulate, widest in middle. Prosternum moderately long in front of procoxae, with anterior margin concave; sides raised around procoxae, forming carinae, not reaching anterior margin, prosternal process long, moderately broad and with posterior margin broadly rounded. Mesoventrite coarse, short and wide, with deep triangular groove for reception of prosternal process; posterior margin around mesocoxae raised. Metaventrite slightly wider than long, rugose, more or less shiny with setigerous punctures; disc convex with deep, medial, triangular depression in posterior half; discrimen in basal ⅔; with one prebasal fovea on each side of metaventrite. Elytra (EL: 0.85 mm, EW: 0.60 mm) convex, widest in anterior ⅔; sides strongly declivous; surface shiny, with dense tiny punctures; elytral margin narrowly rimmed; humeri feebly developed; epipleuron tapering posteriorly. Prominent carina on sixth interval well developed, reaching ⅘ of elytron. Elytra with ten rows of small punctures, moderately impressed, separated by 1-2 times puncture diameter, diminishing toward lateral margins and elytral apex. Scutellum subovate, fl at. Legs moderately long; femora clavate; tibiae longest, apically with cleaning fringes. Tarsi 5-segmented, fi rst four segments each with one fi ne pale, recumbent seta, fi fth segment slightly shorter than remaining segments combined; claws with a large subbasal and smaller basal teeth.

A
. With 5 ventrites. First ventrite with basal margin broadly rounded; fi fth ventrite longest, apically setose, with posterior margin broadly arcuate. Aedeagus ( Fig. 7C-D) elongate. Penis without fi bula; corona in apical half; in ventral view convex, from apical half tapering toward broad rounded apex; in lateral view slender, evenly narrowed from base to slightly curved apex. Parameres absent. Phallobase about 1.5 times as long as penis, subparallel, in ventral view straight, slightly curved in lateral view.

Female
Unknown.

Distribution
The species is reported from three countries (Fig. 12B), Colombia (González-Córdoba et al. 2016), Nicaragua (Shepard pers. com.) and Panama (Sharp 1882). However, the holotype, fi rst illustrated herein, diff ers from the non-type O. longicollis from Colombia depicted in Spangler & Santiago (1991) and later followed by González-Córdoba et al. (2016), and it is possible that they in fact represent diff erent species, and therefore the correct distribution of O. longicollis needs to be reviewed.

Remarks
As stated by Sharp (1882), the type specimen is in poor condition and some characters like tibial fringes, which Hinton (1941) incorrectly interpreted as plastron, are diffi cult to properly observe. Antennal segments 3-11, the left hind leg and the tarsus of the left mid leg are missing, the left elytron is broken, and the left foreleg is glued to the mounting point. Spangler & Santiago, 1991 Figs 2C-D, 6H, 7A-B, 12B

C
. Head, pronotum and elytra black; venter dark brown to black; coxae, femora and tarsi dark brown with reddish tinge; trochanters and tibiae brown; basal segments of antennae and tarsal claws pale brown.

H
. Partly retractable into prothorax, dorsally shiny. Antennae fi liform, 11-segmented; pedicel about twice as long as scape; remaining segments about 4 times as long as scape and pedicel combined, segments 3-10 subrectangular, subequal in length; terminal segment longest, suboval, with pointed apex. Labrum with anterior margin very slightly emarginate medially; anterolateral angles broadly arcuate with numerous golden, recumbent hair-like setae; clypeus shorter and wider than labrum, about 3.5 times as wide as long, anterior margin slightly concave, anterolateral angles rounded; frontoclypeal suture almost straight. Eyes well developed, HW: 0.31-0.32 mm, ID: 0.15-0.16 mm, suboval in lateral view, protruding from head outline in dorsal view, circumocular surface raised. Frons convex between eyes.
T . Pronotum widest behind middle, PW: 0.47-0.48 mm, PL: 0.51-0.54 mm; surface shiny, with narrow reticulation along basal margin and posterolateral angles, with dense tiny punctures; sublateral carinae never well-developed, but a short, very fi ne raised line in basal ⅙ present; disc convex, divided by a broad, deep transverse impression before middle; two prescutellar foveae separated by a raised line extending from base to apical discal half, connecting both halves and merging into them; anterior margin arcuate; posterior margin bisinuate; sides of pronotum convex before and after transverse constriction; lateral margins narrowly rimmed; posterolateral angles orthogonal; anterolateral angles slightly produced. Hypomeron fi nely microreticulate, widest in middle. Prosternum moderately long in front of procoxae, with anterior margin concave; sides raised around procoxae, forming carinae, not reaching anterior margin, prosternal process long, moderately broad and with posterior margin broadly rounded. Mesoventrite coarse, short and wide, with deep triangular groove for reception of prosternal process; posterior margin around mesocoxae raised. Metaventrite slightly wider than long, wrinkled, more or less shiny with setigerous punctures; disc convex with deep, medial, triangular depression in posterior half; discrimen in basal ⅔; in males with one prebasal fovea on each side. Elytra (EL: 0.12-0.15 mm, EW: 0.79-0.81 mm) convex, widest in anterior ⅔; sides strongly declivous; surface shiny, with dense tiny punctures; elytral margin narrowly rimmed; humeri protruding from outline; epipleuron tapering posteriorly. Prominent carina on sixth interval in ⅘ of elytron. Elytra with ten rows of small, shallowly impressed punctures, separated by 2-4 times puncture diameter, diminishing toward lateral margins and elytral apex. Scutellum subovate, fl at. Legs moderately long; femora clavate; tibiae longest. Protibiae with anterior cleaning fringe on apical ⅓; mesotibiae with two cleaning fringes -anterior on apical ⅕ and posterior on apical ⅓; metatibiae with posterior cleaning fringe on apical ⅓. Tarsi 5-segmented, fi rst four segments each with one fi ne pale, recumbent seta, fi fth segment slightly shorter than remaining segments combined; claws with a large subbasal and smaller basal teeth.

A
. With 5 ventrites. First ventrite with basal margin broadly rounded, fi fth ventrite longest, apically setose, with posterior margin broadly arcuate. Aedeagus (Fig. 7A-B) elongate. Penis without fi bula; corona below apex, well developed; in ventral view concave, from apical half widening toward laterally projected, apically nearly acute sides, apical margin medially with wide arcuate projection; in lateral view thick, evenly narrowed from base to curved apex, with apical sides pointing backwards. Parameres absent. Phallobase about 1.5 times as long as penis, subparallel, distal portion markedly narrowed, in ventral view straight, curved in lateral view.

Female
Unknown.

C
. Head, pronotum and elytra black; venter dark brown to black with reddish tinge; coxae, femora and tarsi dark brown with reddish tinge; trochanters and tibiae brown; basal segments of antennae and tarsal claws pale brown.

H
. Partly retractable into prothorax, dorsally shiny. Antennae fi liform, 11-segmented; pedicel about twice as long as scape; remaining segments about 4 times as long as scape and pedicel combined; segments 3-10 subrectangular, subequal in length; terminal segment longest, suboval, with pointed apex. Labrum with anterior margin very slightly emarginate medially; anterolateral angles broadly arcuate with numerous golden, recumbent hair-like setae; clypeus shorter and wider than labrum, about 3.5 times as wide as long, anterior margin slightly concave, anterolateral angles rounded; frontoclypeal T . Pronotum widest behind middle, PW: 0.41-0.43 mm, PL: 0.66-0.68 mm; surface shiny, with microreticulation only along basal margin and posterolateral angles, with dense tiny punctures; sublateral carinae never well-developed, but a short, very fi ne raised line in basal ⅙ present; disc convex, divided by a broad, deep transverse impression before middle; two prescutellar foveae separated by a raised line extending from base to apical discal half, connecting both halves and merging into them; anterior margin arcuate; posterior margin bisinuate; sides of pronotum convex before and after transverse constriction; lateral margins narrowly rimmed; posterolateral angles orthogonal; anterolateral angles slightly produced. Hypomeron fi nely microreticulate, widest in middle. Prosternum moderately long in front of procoxae, with anterior margin concave; sides raised around procoxae, forming carinae, not reaching anterior margin, prosternal process long, moderately broad and with posterior margin broadly rounded. Mesoventrite coarse, short and wide, with deep triangular groove for reception of prosternal process; posterior margin around mesocoxae raised. Metaventrite slightly wider than long, wrinkled, more or less shiny; disc convex with deep, medial, triangular depression in posterior half; discrimen in basal ⅔; with one prebasal fovea on each side. Elytra (EL: 0.96-0.99 mm, EW: 0.68-0.69 mm) convex, widest in anterior ⅔; sides strongly declivous; surface shiny, with dense tiny punctures; elytral margin narrowly rimmed; humeri protruding from outline; epipleuron tapering posteriorly. Prominent carina on sixth interval reaching ⅘ of elytron. Elytra with ten rows of small, shallowly impressed punctures, separated by 2-4 times puncture diameter, diminishing toward lateral margins and elytral apex. Scutellum subovate, fl at. Legs moderately long; femora clavate; tibiae longest. Protibiae with anterior cleaning fringe on apical ⅓; mesotibiae with two cleaning fringes -anterior on apical ⅕ and posterior on apical ⅓; metatibiae with posterior cleaning fringe on apical ⅓. Tarsi 5-segmented, fi rst four segments each with one fi ne pale, recumbent seta, fi fth segment slightly shorter than remaining segments combined; claws with a large subbasal and smaller basal teeth.

A
. With 5 ventrites. First ventrite with basal margin broadly rounded; fi fth ventrite longest, apically setose, with posterior margin broadly arcuate. Aedeagus (Fig. 8A-B) elongate. Penis without fi bula; corona at apex, well developed; in ventral view parallel-sided, apically extended, apex rounded with medial projection; in lateral view slender, evenly narrowed from base to slightly curved apex. Parameres absent. Phallobase slightly longer than penis, parallel-sided, in ventral view straight, curved in lateral view.

Female
Externally similar to male, except metaventrite without distinct medial, triangular depression in posterior half and without two prebasal foveae, fi fth ventrite more elongate and extension of femoral tomentum slightly greater. Ovipositor (Fig. 10E) with very short, laterally curved stylus; preterminal segment long, narrow, widened at base, well sclerotized, about 10 times as long as stylus, with sparse curved blunt and acuminate spines; basal segment membranous, about as long as preterminal and distal segments combined, baculus curved, well sclerotized.

Etymology
The species is named after its larva (to be described in a separate study), which possesses distinct cuticular lobes. Other material ECUADOR • 1 ♂, 5 ♀♀, 1 larva; same collection data as for holotype; CCB.

H
. Partly retractable into prothorax, dorsally shiny. Antennae fi liform, 11-segmented; pedicel about twice as long as scape; remaining segments about 4 times as long as scape and pedicel combined; segments 3-10 subrectangular, subequal in length; terminal segment longest, suboval, with pointed apex. Labrum with anterior margin very slightly emarginate medially; anterolateral angles broadly arcuate with numerous golden, recumbent hair-like setae; clypeus shorter and wider than labrum, about 3.5 times as wide as long, anterior margin slightly concave, anterolateral angles rounded; frontoclypeal suture almost straight. Eyes well developed, HW: 0.27-0.30 mm, ID: 0.12-0.15 mm, suboval in lateral view, protruding from head outline in dorsal view, circumocular surface raised. Frons between eyes convex.

T
. Pronotum widest behind middle, PW: 0.41-0.42 mm, PL: 0.47-0.48 mm; surface shiny, with narrow reticulation along basal margin and posterolateral angles, with dense tiny punctures; sublateral carinae never well-developed, fi ne raised sublateral lines indistinct; disc convex, divided by a broad, deep transverse impression before middle; prescutellar foveae separated by a raised line extending from base to middle, connecting pre-and postimpression portions and merging into them; anterior margin arcuate; posterior margin bisinuate; sides of pronotum convex before and after transverse constriction; lateral margins narrowly rimmed; posterolateral angles orthogonal; anterolateral angles slightly produced. Hypomeron fi nely microreticulate, widest in middle. Prosternum moderately long in front of procoxae, with anterior margin concave; sides raised around procoxae, forming carinae, not reaching anterior margin, prosternal process long, moderately broad, with posterior margin broadly rounded. Mesoventrite coarse, short and wide, with deep triangular groove for reception of prosternal process; posterior margin around mesocoxae raised. Metaventrite slightly wider than long, shiny with setigerous punctures; disc convex with deep, medial, triangular depression in posterior half; discrimen in basal ⅔, very thin, indistinct; with prebasal fovea on each side of metaventrite. Elytra (EL: 0.88-0.91 mm, EW: 0.62-0.68 mm) convex, widest in anterior ⅔, sides strongly declivous; surface shiny, with dense tiny punctures; elytral margin narrowly rimmed; humeri protruding from outline; epipleuron tapering posteriorly. Prominent carina on sixth interval reaching ⅘ of elytron. Elytra with ten rows of small, shallowly impressed punctures, separated by 2-4 times puncture diameter, diminishing toward lateral margins and elytral apex. Scutellum subovate, fl at. Legs moderately long; femora clavate; tibiae longest. Protibiae with anterior cleaning fringe on apical ⅓; mesotibiae with two cleaning fringes -anterior on apical ⅕ and posterior on apical ⅓; metatibiae with posterior cleaning fringe on apical ⅓. Tarsi 5-segmented, fi rst four segments each with one fi ne pale, recumbent seta, fi fth segment slightly shorter than remaining segments combined; claws with a large subbasal and smaller basal teeth.

Female
Externally similar to male, except metaventrite without distinct medial, triangular depression in posterior half and without two prebasal foveae, fi fth ventrite more elongate (Fig. 4B) and extension of femoral tomentum greater.

Biology
Collected from small stream directly below waterfall surrounded by remnants of primary forest, with water fl owing between larger stones (Fig. 11B).

Etymology
This species is dedicated to Prof. Giovanni Onore from the Pontifi cial Catholic University of Ecuador (Quito), entomologist, enthusiastic nature conservationist and a very good friend.
Other material ECUADOR • 1 ♂; same collection data as for holotype; CCB.

C
. Head, pronotum and elytra black; venter dark brown to black with reddish tinge; coxae, femora and tarsi dark brown with reddish tinge; trochanters and tibiae brown; basal segments of antennae and tarsal claws pale brown.

H
. Partly retractable into prothorax, dorsally shiny. Antennae fi liform, 11-segmented; pedicel about twice as long as scape; remaining segments about 4 times as long as scape and pedicel combined; segments 3-10 subrectangular, subequal in length; terminal segment longest, suboval, with pointed apex. Labrum with anterior margin almost straight; anterolateral angles broadly arcuate with numerous golden, recumbent hair-like setae; clypeus shorter and wider than labrum, about 3.5 times as wide as long, anterior margin slightly concave, anterolateral angles rounded; frontoclypeal suture almost straight. Eyes well developed, HW: 0.26-0.28 mm, ID: 0.14-0.16 mm, suboval in lateral view, protruding from head outline in dorsal view, circumocular surface raised. Frons between eyes convex.

T
. Pronotum widest behind middle, PW: 0.38-0.41 mm, PL: 0.47-0.49 mm; surface shiny, with narrow reticulation along basal margin and posterolateral angles, with dense tiny punctures; sublateral carinae never well-developed, but a short, very fi ne raised sublateral line in basal ⅛ present; disc convex, divided by a broad, deep transverse impression before middle; prescutellar foveae separated by a raised line extending from base to middle connecting pre-and postimpression portions and merging into them; anterior margin arcuate; posterior margin bisinuate; sides of pronotum convex before and after transverse constriction; lateral margins narrowly rimmed; posterolateral angles orthogonal; anterolateral angles slightly produced. Hypomeron fi nely microreticulate, widest in middle. Prosternum moderately long in front of procoxae, with anterior margin concave; sides raised around procoxae, forming carinae, not reaching anterior margin, prosternal process long, moderately broad, with posterior margin broadly rounded. Mesoventrite coarse, short and wide, with deep triangular groove for reception of prosternal process; posterior margin around mesocoxae raised. Metaventrite slightly wider than long, shiny with set igerous punctures; disc convex with very shallow to indistinct medial, triangular depression in posterior half; discrimen in basal ⅔, very thin, indistinct; prebasal foveae absent. Elytra (EL: 0.77-0.81 mm, EW: 0.58-0.64 mm) convex, widest in anterior ⅔; sides strongly declivous; surface shiny, with dense tiny punctures; elytral margin narrowly rimmed; humeri protruding from outline; epipleuron tapering posteriorly. Prominent carina on sixth interval reaching ⅘ of elytron. Elytra with ten rows of small, shallowly impressed punctures, separated by a 2-4 times puncture diameter, diminishing toward lateral margins and elytral apex or absent. Scutellum subovate, fl at. Legs moderately long; femora clavate; tibiae longest. Protibiae with anterior cleaning fringe on apical ⅓; mesotibiae with two cleaning fringes -anterior on apical ⅕ and posterior on apical ⅓; metatibiae with posterior cleaning fringe on apical ⅓. Tarsi 5-segmented, fi rst four segments each with one fi ne pale, recumbent seta, fi fth segment slightly shorter than remaining segments combined; claws with a large subbasal and smaller basal teeth.

A
. With 5 ventrites. First ventrite with basal margin broadly rounded, fi fth ventrite longest, apically setose with posterior margin broadly arcuate. Aedeagus (Fig. 9A-B) elongate. Penis without fi bula; corona in apical half; in ventral view parallel-sided, apically extended, apex subtriangular; in lateral view straight, ventrally narrowed from base to apex. Parameres absent. Phallobase slightly longer than penis, parallel-sided, in ventral view straight, slightly curved in lateral view.

Female
Externally similar to male, except fi fth ventrite more elongate and extension of femoral tomentum slightly greater.

Biology
Collected from small shaded stream in forest with cascades, riffl es, substrate with large boulders, stones, gravel and sand (Fig. 11C).

Etymology
The specifi c epithet splendida is from the Latin ʻsplendidusʼ (ʻshinyʼ) referring to the glabrous, shiny elytra.
Other material ECUADOR • 1 ♂; same collection data as for holotype; CCB.

C
. Head, pronotum and elytra black; venter dark brown to black with reddish tinge; coxae, femora and tarsi dark brown with reddish tinge; trochanters and tibiae brown; basal segments of antennae and tarsal claws pale brown.

H
. Partly retractable into prothorax, dorsally shiny. Antennae fi liform, 11-segmented; pedicel about twice as long as scape; remaining segments about 4 times as long as scape and pedicel combined; segments 3-10 subrectangular, subequal in length; terminal segment longest, suboval, with pointed apex. Labrum with anterior margin very slightly emarginate medially; anterolateral angles broadly arcuate with numerous golden, recumbent hair-like setae; clypeus shorter and wider than labrum, about 3.5 times as wide as long, anterior margin slightly concave, anterolateral angles rounded; frontoclypeal suture almost straight. Eyes well developed, HW: 0.30-0.31 mm, ID: 0.17-0.18 mm, suboval in lateral view, protruding from head outline in dorsal view, circumocular surface raised. Frons between eyes convex. T . Pronotum widest behind middle, PW: 0.41-0.43 mm, PL: 0 .47-0.49 mm; surface shiny, with narrow reticulation along basal margin and posterolateral angles, with dense tiny punctures; sublateral carinae absent, fi ne raised line indistinct; disc convex, divided by a broad, deep transverse impression before middle; two prescutellar foveae separated by a raised line extending from base to apical discal half, connecting pronotal halves and merging into them; anterior margin arcuate; posterior margin bisinuate; sides of pronotum convex before and after transverse constriction; lateral margins narrowly rimmed; posterolateral angles orthogonal; anterolateral angles slightly produced. Hypomeron fi nely microreticulate, widest in middle. Prosternum moderately long in front of procoxae, with anterior margin concave; sides raised around procoxae, forming carinae, not reaching anterior margin, prosternal process long, moderately broad, with posterior margin broadly rounded. Mesoventrite coarse, short and wide, with deep triangular groove for reception of prosternal process; posterior margin around mesocoxae raised. Metaventrite slightly wider than long, shiny; disc convex with very shallow to indistinct, medial, triangular depression in posterior half; discrimen in basal ⅔, very thin, almost indistinct; with one prebasal fovea on each side of metaventrite (Fig. 5B). Elytra (EL: 0.89-0.92 mm, EW: 0.64-0.68 mm) convex, widest in anterior ⅔; sides strongly declivous; surface shiny, with dense tiny punctures; elytral margin narrowly rimmed; humeri protruding from outline; epipleuron tapering posteriorly; sublateral carina absent; strial punctures absent (in few specimens a partial striation is present). Scutellum subovate, fl at. Legs moderately long; femora clavate; tibiae longest. Protibiae with anterior cleaning fringe on apical ⅓; mesotibiae with two cleaning fringes -anterior on apical ⅕ and posterior on apical ⅓; metatibiae with posterior cleaning fringe on apical ⅓. Tarsi 5-segmented, fi rst four segments each with one fi ne pale, recumbent seta, fi fth segment slightly shorter than remaining segments combined; claws with a large subbasal and smaller basal teeth.

A
. With 5 ventrites. First ventrite with basal margin broadly rounded, fi fth ventrite longest, apically setose, with posterior margin broadly arcuate. Aedeagus (Fig. 9C-D) elongate. Penis elongate, without fi bula; corona in basal half; in ventral view concave with rounded apex; in lateral view evenly narrowed from base to curved apex. Parameres absent. Phallobase almost 2 times as long as penis, parallel-sided, in ventral view straight, curved in lateral view.

Female
Externally similar to male, except metaventrite without prebasal foveae, fi fth ventrite more elongate and extension of femoral tomentum greater.

Biology
Collected from stream ca 7 m wide in deforested area with riffl es and pools, close to the confl uence with a larger stream, substrate composed of boulders, stones and gravel (Fig. 11F).

Etymology
This species is named after daughter of ZČZ and FČ, Lenka Čiamporová. She helped a lot during collecting in Ecuador although she was only fi ve at the time.

C
. Head, pronotum and elytra black; venter dark brown to black with reddish tinge; coxae, femora and tarsi dark brown with reddish tinge; trochanters and tibiae brown; basal segments of antennae and tarsal claws pale brown.

H
. Partly retractable into prothorax, dorsally shiny. Antennae fi liform, 11-segmented; pedicel about twice as long as scape; remaining segments about 4 times as long as scape and pedicel combined; segments 3-10 subrectangular, subequal in length; terminal segment longest, suboval, with pointed apex. Labrum with anterior margin almost straight; anterolateral angles broadly arcuate with numerous golden, recumbent hair-like setae; clypeus shorter and wider than labrum, about 3.5 times as wide as long, anterior margin slightly concave, anterolateral angles rounded; frontoclypeal suture almost straight. Eyes well developed, HW: 0.28-0.30 mm, ID: 0.15-0.16 mm, suboval in lateral view, protruding from head outline in dorsal view, circumocular surface raised. Frons between eyes convex.

T
. Pronotum widest behind middle, PW: 0.38-0.40 mm, PL: 0.46-0.48 mm; surface shiny, with narrow reticulation along basal margin and posterolateral angles, with dense tiny punctures; sublateral carinae never well-developed, but a short, very fi ne raised line in basal ⅛ present; disc convex, divided by broad, deep transverse impression before middle; two prescutellar foveae separated by a raised line extending from base to discal half, connecting pronotal halves and merging into them; anterior margin arcuate; posterior margin bisinuate; sides of pronotum convex before and after transverse constriction; lateral margins narrowly rimmed; posterolateral angles orthogonal; anterolateral angles slightly produced. Hypomeron fi nely microreticulate, widest in middle. Prosternum moderately long in front of procoxae, with anterior margin concave; sides raised around procoxae, forming carinae, not reaching anterior margin, prosternal process long, moderately broad, with posterior margin broadly rounded. Mesoventrite coarse, short and wide, with deep triangular groove for reception of prosternal process; posterior margin around mesocoxae raised. Metaventrite slightly wider than long, shiny with setigerous punctures; disc convex with moderately deep, medial, triangular depression in posterior half; discrimen in basal ⅔, very thin, almost indistinct; with prebasal fovea on each side (Fig. 5A, C). Elytra (EL: 0.77-0.81 mm, EW: 0.57-0.61 mm) convex, widest in anterior ⅔; sides strongly declivous; surface shiny, with tiny punctures; elytral margin narrowly rimmed; humeri protruding from outline; epipleuron tapering posteriorly. Prominent carina on sixth interval reaching ⅘ of elytron; strial punctures absent (in a few specimens a partial striation is present). Scutellum subovate, fl at. Legs moderately long; femora clavate; tibiae longest. Protibiae with anterior cleaning fringe on apical ⅓; mesotibiae with two cleaning fringes -anterior on apical ⅕ and posterior on apical ⅓; metatibiae with posterior cleaning fringe on apical ⅓. Tarsi 5-segmented, fi rst four segments each with one fi ne pale, recumbent seta, fi fth segment slightly shorter than remaining segments combined; claws with a large subbasal and smaller basal teeth.

A
. With 5 ventrites. First ventrite with basal margin broadly rounded; fi fth ventrite longest, apically setose, with posterior margin broadly arcuate. Aedeagus (Fig. 10A-B) elongate. Penis without fi bula; corona in apical half; in ventral view subparallel with apex rounded; in lateral view curved, evenly narrowed from base to apex. Parameres absent. Phallobase slightly longer than penis, parallelsided, in ventral view straight, curved in lateral view.

Female
Externally similar to male, except metaventrite without distinct medial, triangular depression in posterior half and without two prebasal foveae, fi fth ventrite more elongate.

Biology
Collected from small stream directly below waterfall surrounded by remnants of primary forest, with water fl owing between larger stones (Fig. 11B).

Distribution
Known only from the type locality in Napo Province (Figs 11B,.

Etymology
This species is named for being the smallest sized of all known Onychelmis species.
Other material ECUADOR • 1 ♀; same collection data as for holotype; CCB.

H
. Partly retractable into prothorax, dorsally shiny. Antennae fi liform, 11-segmented; pedicel about twice as long as scape; remaining segments about 4 times as long as scape and pedicel combined; segments 3-10 subrectangular, subequal in length; terminal segment longest, suboval, with pointed apex. Labrum with anterior margin almost straight; anterolateral angles broadly arcuate with numerous golden, recumbent hair-like setae; clypeus shorter and wider than labrum, about 3.5 times as wide as long, anterior margin slightly concave, anterolateral angles rounded; frontoclypeal suture almost straight. Eyes well developed, HW: 0.25-0.27 mm, ID: 0.15-0.16 mm, suboval in lateral view, protruding from head outline in dorsal view, circumocular surface raised. Frons between eyes convex.

T
. Pronotum widest behind middle, PW: 0.34-0.35 mm, PL: 0.37-0.39 mm, surface shiny, with narrow reticulation along basal margin and posterolateral angles, with dense tiny punctures; sublateral carinae never well-developed, fi ne raised line indistinct; disc convex, divided by broad, deep transverse impression on apical side of middle; two prescutellar foveae separated by raised line extending from base to apical discal half, connecting pronotal halves and merging into them; anterior margin arcuate; posterior margin bisinuate; sides of pronotum convex before and after transverse constriction; lateral margins narrowly rimmed; posterolateral angles orthogonal; anterolateral angles slightly produced. Hypomeron fi nely microreticulate, widest in middle. Prosternum moderately long in front of procoxae, with anterior margin concave; sides raised around procoxae, forming carinae, not reaching anterior margin, prosternal process long, moderately broad and with posterior margin broadly rounded. Mesoventrite coarse, short and wide, with deep triangular groove for reception of prosternal process; posterior margin around mesocoxae raised. Metaventrite slightly wider than long, shiny with setigerous punctures; disc convex with very shallow to indistinct, medial, triangular depression in posterior half; discrimen in basal ⅔, very thin, almost indistinct; prebasal foveae absent. Elytra (EL: 0.64-0.69 mm, EW: 0.52-0.54 mm) convex, widest in about middle; disc convex, sides strongly declivous; surface shiny, with tiny punctures; elytral margin narrowly rimmed; humeri feebly developed; epipleuron tapering posteriorly. Prominent carina on sixth interval reaching ⅘ of elytron; strial punctures absent. Scutellum subovate, fl at. Legs moderately long; femora clavate; tibiae longest. Protibiae with anterior cleaning fringe on apical ⅓; mesotibiae with two cleaning fringes -anterior on apical ⅕ and posterior on apical ⅓; metatibiae with posterior cleaning fringe on apical ⅓. Tarsi 5-segmented, fi rst four segments each with one fi ne pale, recumbent seta, fi fth segment slightly shorter than remaining segments combined; claws with a large subbasal and smaller basal teeth. . With 5 ventrites. First ventrite with basal margin broadly rounded; fi fth ventrite longest, apically setose, with posterior margin broadly arcuate. Aedeagus (Fig. 10C-D) elongate. Penis without fi bula; corona in apical half; in ventral view concave, constricted before rounded apex with medial projection; in lateral view evenly narrowed from base to slightly curved apex. Parameres absent. Phallobase slightly longer than penis, parallel-sided, in ventral view straight, curved in lateral view.

Female
Externally similar to male, except fi fth ventrite more elongate.

Biology
Collected from slowly fl owing stream ca 7 m wide with submerged vegetation, sparse larger stones and deep layer of fi ne gravel and sand (Fig. 11E).

Distribution
Known only from the type locality in Pastaza Province (Figs 11E, 12A-B).

Discussion
At fi rst glance, the genus Onychelmis consists of small species with a uniform morphology. This could be part of the reason why only three species have been described so far, the most recent one almost 30 years ago . Although representatives of the genus are reported in several recent studies from various regions (e.g., Amanzo et al. 2003;González-Córdoba et al. 2016;Reymundo & Araujo 2016), they are mostly determined only to the genus or assigned to one of the three (most often O. leleupi) known species. Above all, this situation is most likely due to a lack of extensive material. The situation was quite diff erent concerning this revision. The available material, although not from a large geographical area, includes dozens of individuals from several locations. Besides, we had access to the type material of two of the three known species, as well as DNA barcoding data. The results of the revision of such material were diametrically opposed to the established concept of the genus' diversity.
Primarily, the data from DNA barcoding analysis revealed the surprising diversity of the genus. The discovered genetic variability was subsequently confi rmed by morphological characters, allowing us to describe fi ve new species. DNA-based methods thus proved to be particularly useful in providing a background for a better understanding of the species diversity. The results gained within the study also suggest that the real diversity of the genus is much higher. Signifi cant genetic distances found between the species of Onychelmis support the likelihood of hidden diversity in the genus (as discussed in Tavares et al. 2011;Milá et al. 2012;Blagoev et al. 2013), which awaits discovery and description.
According to the maximum likelihood (ML) analysis (Fig. 1)  The structure of the male genitalia, together with the ML analysis, points to the presence of separate lineages in the genus; however, we found no other stable characters to support this division, and therefore refrain from proposing species groups or subgenera. With the use of DNA barcodes, we also identifi ed larvae of few species of Onychelmis, and their external morphology varies strongly, which could be used to support the existence of these groups in the future. A detailed description of the determined larvae is the subject of another paper.
Externally, all species of Onychelmis are hardly distinguishable. We recorded diff erences in the body size, impression of elytral punctures, shape of humeri, presence of elytral carinae and extension of plastron on femora, but these characters can vary, and reliable determination often requires examination of the male genitalia. Femoral tomentum appears to be useful for the identifi cation of similarly sized and otherwise externally uniform species. Although it is mostly species-specifi c, it diff ers between sexes and can sometimes overlap in some species. This character is usually clearly visible, but sometimes plastron scales can be brushed away, making this feature inapplicable for determination. Females tend to have a greater extension of plastron on legs than males. Although a small extension of plastron can be found on metafemora we focused on pro-and mesofemora only, which have proved to be more illustrative. Sexual dimorphism can also be seen on the length of the fi fth ventrite, on metaventrite in presence of medial, triangular depression in posterior half, and in presence of the two prebasal foveae often accompanied by a tuft of setae. The characters on the metaventrite are best observable in lateral view.
Species distribution, as shown here, must be taken with reasonable caution. Almost all previously reported specimens have been assigned to only two species, which corresponded with the presumed low diversity of the genus. However, as can be seen from the results presented in this revision, the species diversity is greatly underestimated. We recorded six species (fi ve new to science) in Ecuador alone, and similar fi ndings from other countries would not be surprising. This phenomenon of underestimated diversity can be seen in many Neotropical genera, such as Hexanchorus Sharp, 1882(Maier 2013Linský et al. 2019).
The most-reported species, O. leleupi, is distributed from Colombia to southern Peru based on published records. However, the southernmost specimens were females (Shepard, pers. com.), and the possibility that specimens from Peru represent a diff erent species cannot be excluded. The type species of the genus, O. longicollis, has a more northern distribution, following Magdalena Valley in Colombia, and its occurrence in Central America is likely more recent. It is necessary to note that the male genitalia of the holotype of O. longicollis, depicted here for the fi rst time (Fig. 7A-B), diff er from those in Spangler & Santiago (1991). Moreover, O. longicollis does not have elytral striae with punctures "very coarse, deep", and it is likely that Spangler & Santiago (1991) studied another species. Although its size was not specifi ed in Hinton (1941), the type species is smaller than most of the remaining congeners, which can be useful for its determination. The largest species of the genus, O. whiteheadi, is so far known only from the type locality and may represent local allopatric speciation, as may the new species described herein.
The proximity between Onychelmis and Notelmis Hinton, 1941 has been observed in the past (e.g., Jäch et al. 2016). Even Sharp (1882) noted the close similarity of the two species which are now the type species of the two genera. The genus Onychelmis diff ers in possessing subbasal teeth on the claws and an elytral carina is only found on the 6 th interval or it is completely reduced (vs elytral carinae on 6 th and 8 th intervals in Notelmis). Moreover, Onychelmis lacks parameres or has them integrated into the penis (sensu Delève 1968). Generally, at least in Phytophaga, the parameres remain outside the female body during copulation. They serve as a sensory organ for contacting the female or as a grasping organ for stabilization. Sensillae on the penis are responsible for fi nding the female genital opening and it is assumed that males without parameres require a penis with advanced sensillary equipment (Düngelhoef & Schmitt 2010). An absence of parameres is a rare trait among Neotropical genera of riffl e beetles. Out of 45 genera that have published illustrations of male genitalia, only four, Ictelmis , Epodelmis Hinton, 1973, Neolimnius Hinton, 1939 and Onychelmis, lack them. Although a reduction or complete loss of parameres could be a shared evolutionary feature, as it is in the tribe Macronychini (Jäch & Boukal 1995), it does not seem to be true for the four mentioned genera. For example, the genus Epodelmis is undoubtedly more closely related to Holcelmis Hinton, 1973, or even to Hexacylloepus Hinton, 1940, than to any of the three mentioned genera. The loss of parameres could be a moderately rapid event, which could subsequently lead to the formation of a reproductive barrier and thus accelerate speciation.
According to the ML analysis (Fig. 1), the closest genus to Onychelmis is Ictelmis. Both genera share the presence of subbasal teeth on the claws, have similar pronota (Ictelmis with well-developed sublateral carinae), and lack parameres. The genus Ictelmis diff ers in having the pronotum with microreticulation (shiny in Onychelmis), rugose venter, and a unique form of elytron that has only seven elytral striae, likely due to the partial fusion of intervals.
Our study contributes to the knowledge of the Neotropical riffl e beetles. We revised the genus, added fi ve new species to the world Elmidae fauna, and showed very clearly the importance of using molecular data (DNA barcoding) in the taxonomy and systematics of the riffl e beetles. Our results indicate that the Elmidae fauna of the Neotropics is still largely undescribed, and thus intensive and more detailed future research is required to understand its real diversity.