Chasmogenus Sharp, 1882

Giron, Jennifer C. & Short, Andrew Edward Z., 2021, The Acidocerinae (Coleoptera, Hydrophilidae): taxonomy, classification, and catalog of species, ZooKeys 1045, pp. 1-236 : 1

Genus Chasmogenus Sharp, 1882 Figs 2 , 4 , 11H , 24 , 25

Chasmogenus Sharp, 1882: 73; Fernández 1986: 189 [generic status reinstated].

Chasmogenus fragilis Type species: Chasmogenus fragilis Sharp, 1882: 73; by monotypy.

Helochares (Chasmogenus) Sharp; d’Orchymont 1919c: 149 [as subgenus of Helochares ]; Knisch 1924: 195 [catalog].

Dieroxenus Spangler, 1979: 753; Girón and Short 2018: 154 [synonymy].

Dieroxenus cremnobates Type species: Dieroxenus cremnobates Spangler, 1979: 754; by original designation and monotypy.

Gender.

Masculine.

Type species.

Chasmogenus fragilis Sharp, 1882: 73; by monotypy.

Diagnosis.

Body length ranging from 2.5-5.0 mm. Body shape oval in dorsal view, parallel-sided to broader around midlength, dorsoventrally flattened, weakly to moderately convex in lateral view (Fig. 24 ), either evenly convex or flattened along anterior half. Surface of head, pronotum and elytra smooth, with usually shallow ground punctation. Coloration ranging from yellowish orange to dark brown, usually uniform along body, sometimes darker on head or only frons. Shape of head trapezoid (Fig. 11H ). Eyes varying in size, usually subquadrate in dorsal view, only very weakly emarginated anteriorly, and usually projected from outline of head. Clypeus trapezoid, with anterior margin mesally weakly to strongly emarginated; membranous preclypeal area visible when clypeus strongly emarginated (Fig. 11H ). Labrum fully exposed, semioval, anteriorly mesally emarginated. Mentum usually rather smooth, with anterior depression often reaching midlength of mentum, sometimes limited by low transverse carina. Antennae with eight antennomeres, with cupule slightly asymmetric and rounded. Maxillary palps usually slender and slightly longer than width of head, with inner margin slightly and evenly curved, and outer margin curved along apical half. Pronotum evenly convex. Elytra with sutural striae, with outer margins slightly flared; ground punctures usually only shallowly marked, serial punctures absent and at least one median row of systematic punctures clearly visible on each elytron (Fig. 24 ). Surface of prosternum usually flat, only rarely with low medial carina along intercoxal process. Posterior elevation of mesoventrite with an either blunt or sharp longitudinal elevation; anapleural sutures sinuate, separated at anterior margin by distance similar or slightly shorter than anterior margin of mesepisternum. Metaventrite with posteromesal and posterolateral glabrous patches (Fig. 24C ). Protibiae with spines of anterior row semi erect, relatively long, thick and sparse; apical spurs of protibiae moderately long and thick, reaching apex of protarsomere 2. Metafemora with tibial grooves moderately developed, with sharp posterior margin; hydrofuge pubescence covering at least basal 3/4 of anterior surface of metafemora (Fig. 24C, F ). Metatarsomeres 2-4 with two rows of spiniform setae on ventral surface; metatarsomere 5 nearly as long as 3 and 4 combined; metatarsomere 2 shorter to nearly as long as 5. Apex of fifth abdominal ventrite emarginate, with fringe of flat and stout setae. Aedeagus trilobed (Fig. 25 ); basal piece shorter to nearly as long as parameres; outline of apical region of parameres variable; sometimes parameres asymmetrical; median lobe triangular, either simple or bearing additional sclerite, with well-developed lateral basal apodemes and gonopore.

Differential diagnosis.

Chasmogenus most closely resembles Crephelochares , although they do not co-occur in the same biogeographic regions ( Chasmogenus occurs exclusively in the Neotropical region, whereas Crephelochares occurs throughout the Old World). They can be differentiated by the number of antennomeres (eight in Chasmogenus , nine in Crephelochares ) and by the form of the aedeagus (trilobed in most Chasmogenus , Fig. 25 ), divided and further modified in Crephelochares , Fig. 27B-D ). Among New World taxa, Chasmogenus can easily be distinguished by the presence of sutural striae, a character shared only with Primocerus , from which it can be distinguished by the shape of the posterior elevation of the mesoventrite: longitudinally elevated in Chasmogenus , transversally elevated in Primocerus . Although Primocerus is quite rare and has a more restricted range in the Neotropics compared with Chasmogenus , the two genera can co-occur in forested steams in the Guiana Shield region.

Distribution.

Neotropical: Argentina, Brazil ( Amapá, Amazonas, Minas Gerais, Pará, Piauí, Rio de Janeiro, Roraima, São Paulo), Costa Rica, Ecuador, French Guiana, Guatemala, Guyana, Panama, Paraguay, Suriname, Venezuela; Fig. 4 .

Natural history.

The vast majority of Chasmogenus are known from forested habitats, including the margins of streams and forest pools. A few species are known from open marsh habitats (e.g., Chasmogenus australis García and Chasmogenus sapucay Fernández). They can be found among the vegetation and submerged leaf litter. They are also attracted to lights, though usually not in large numbers. Only one species [ Chasmogenus cremobates (Spangler)] has been collected in seepages. See Smith and Short (2020) for more detail on habitat information.

Larvae.

The larvae of Chasmogenus remain unknown. The only descriptions of immature stages were made for Chasmogenus nitescens Fauvel (from Australia), which is now assigned to Crephelochares .

Taxonomic history.

Chasmogenus was originally described by Sharp (1882) as a genus to accommodate one Neotropical species from Guatemala and Panama. d’Orchymont (1919c: 149) synonymized Chasmogenus with Crephelochares (from the Old World) and placed it as a subgenus of Helochares . The generic rank of Chasmogenus was re-established by Fernández (1986: 189), with Crephelochares maintained as a junior synonym. Some authors continued to treat Crephelochares as a valid subgenus (e.g., Hebauer 1992, 1995) while others did not recognize any distinction between the two names ( Hansen 1991, 1999). The monotypic genus Dieroxenus was synonymized with Chasmogenus by Girón and Short (2018). The recent phylogeny by Short et al. (2021) offered support considering Chasmogenus and Crephelochares as separate genera and affirmed Dieroxenus as a derived lineage within Chasmogenus .

Remarks.

There are 33 described species of Chasmogenus to date, and we are aware of many yet undescribed species in South America. Chasmogenus is a fairly commonly found group of beetles with very little variation in external morphology. Recent collecting efforts and taxonomic study in the genus have revealed a hidden diversity and interesting biogeographic patterns in South America ( Smith and Short 2020).

Species examined.

Chasmogenus australis García *, C. amplius Smith & Short*, C. bariorum García *, C. barrae Short*, C. cremnobates (Spangler), C. lineatus Smith & Short*, C. Chasmogenus lorenzo Short*, C. ruidus Short*, C. schmits Smith & Short*. Paratypes of the species marked with an asterisk were available for this study.

Selected references.

Sharp 1882: 73: genus description; Spangler 1979: 753: description of Dieroxenus ; Fernández 1986: notes on the genus and one new species; Hebauer 1992: notes, recognition of two subgenera, emphasis on Crephelochares ; García 2000: four new species from Venezuela; Short 2005: new species from Costa Rica; Short and Fikáček 2013: inclusion of Chasmogenus species in molecular phylogeny; Clarkson and Ferreira-Jr 2014b: four new species from Brazil; Girón and Short 2018: synonymization of Dieroxenus ; Alves et al. 2020: description of a new species from Brazil; Smith and Short 2020: description of 18 new species from northeastern South America; Short et al. 2021: phylogenetic placement.

Figures

Figure 2. Phylogeny of the Acidocerinae simplified from Short et al. (2021), indicating the distribution, preferred habitat, and currently described number of species for each genus. For habitat, filled black circles indicate that at least some species of the genus are commonly found in this habitat; light grey circles indicate the genus has been found in this habitat, but is rare or not typical for the group; white circles indicate no species have been recorded for the genus in this habitat.

Figure 4. Known distribution of genera of Acidocerinae: Acidocerus, Agraphydrus, Aulonochares, Batochares, Chasmogenus, Colossochares, and Crephelochares.

Figure 11. Head of miscellaneous Acidocerinae A-D anterolateral view: A Tobochares luteomargo with white arrow pointing to straight anterior margin of eye B Tobochares emarginatus with white arrow pointing to Tobochares canthus emarginating anterior margin of eye C Quadriops politus with white arrow pointing to Tobochares canthus fully dividing the eye in dorsal and ventral faces D Batochares sp. black arrow pointing to transverse carina on labrum E-L dorsal view of head: E Batochares sp. F Helochares tristis G Crephelochares nitescens, H Chasmogenus australis with black arrow pointing to preclypeal membrane I Colossochares ellipticus J Aulonochares tubulus K Peltochares conspicuus L Helobata larvalis.

Figure 24. Habitus of Chasmogenus spp. A-C C. ruidus: A dorsal habitus B lateral habitus C ventral habitus D-F C. cremnobates: D dorsal habitus E lateral habitus F ventral habitus G C. lineatus H C. amplius I C. itatiaia J C. fluminensis. G, H from Smith and Short 2020; I, J from Clarkson and Ferreira Jr 2014. Scale bars: 1 mm.

Figure 25. Aedeagi of Chasmogenus spp. A C. acuminatus B C. schmits C C. lineatus D, E C. tafelbergensis: D dorsal view E lateral view. Images from Smith and Short 2020. Scale bars: 0.5 mm.

Figure 27. Aedeagi of Colossochares and Crephelochares spp. A Colossochares ellipticus B Crephelochares szeli C Crephelochares sp. (Australia) D Crephelochares abnormalis (Thailand). Scale bars: 0.5 mm.