Crucisternum Giron & Short, 2018

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 Crucisternum Giron & Short, 2018 Figs 1Q , 2 , 5 , 14C , 29 , 30A-E

Crucisternum Girón & Short, 2018: 116.

Gender.

Masculine.

Type species.

Crucisternum ouboteri Girón & Short, 2018: 121; by original designation.

Diagnosis.

Small beetles, body length 2.0-2.5 mm. Body shape elongated oval in dorsal view; moderately convex in lateral view (Fig. 29 ). Color orange brown to dark brown. Head trapezoid. Eyes moderate to small, projected from outline of head. Clypeus trapezoid, with anterior margin broadly and roundly emarginate. Labrum fully exposed. Mentum with lateral oblique ridges; anterior median depression marked by transverse carina (Fig. 29C ). Antennae with nine antennomeres, with cupule only slightly asymmetrical and rounded. Maxillary palps moderately long, slightly longer than width of head (Fig. 29A ). Elytra without sutural striae, with outer margins of elytra slightly flared; serial punctures, ground punctures and systematic punctures similar in size and degree of impression, either shallow or rather sharply marked; all punctures seemingly arranged in rows (Fig. 29A ). Prosternum with well-developed median, longitudinal, laminar carina (Fig. 29C ). Posterior elevation of mesoventrite with a strongly produced, anteriorly pointed transverse ridge, longitudinally carinate (Fig. 14C ); anapleural sutures sinuate, separated by distance nearly 0.6 × width of anterior margin of mesepisternum. Metaventrite densely pubescent, except for median and postero-lateral glabrous patches (Fig. 29C ). Protibiae with spines of anterior row long and thick; apical spurs of protibiae short and stout, almost reaching apex of protarsomere 2. Metafemora covered by hydrofuge pubescence along basal 4/5 (Fig. 29C ). Metatarsomeres 2-4 gradually slightly decreasing in size; metatarsomere 5 slightly longer than 2; ventral coverage of tarsomeres composed of fine and spiniform setae. Fifth abdominal ventrite apically rounded, truncate, or slightly emarginate, without stout setae. Aedeagus trilobate (Fig. 30A-E ); basal piece 0.2-0.25 × the length of parameres; median lobe with well-developed lateral basal apodemes, and acute to narrowly rounded apex; parameres nearly as long as median lobe, with outer margins usually sinuate; gonopore situated distad of midlength of median lobe.

Differential diagnosis.

Although Crucisternum is generally unremarkable dorsally from other small-bodied Neotropical acidocerines, several sternal features are strikingly unique and easily separate the genus from all others. The strongly developed prosternal carina found in the genus, combined with the cruciform shape of the posterior elevation of the mesoventrite (formed by the fusion of both transverse and longitudinal ridges), is unique for this genus in the subfamily. Crucisternum is most likely to be confused in samples as a very small Chasmogenus but can also easily be distinguished from that genus by the lack of sutural striae.

Distribution.

Neotropical: Brazil (Minas Gerais, Pará), French Guiana, Guyana, Suriname, Venezuela; Fig. 5 .

Natural history.

All species of the genus are associated with forested streams, usually along margins that contain ample detritus. A single specimen of C. ouboteri was collected at a black light trap.

Larvae.

Immature stages are not known for the genus.

Taxonomic history.

The genus was only recently described.

Remarks.

There are seven species currently known.

Species examined.

Holotypes and paratypes of all the known species were examined for this study.

Selected references.

Girón and Short 2018: original description of the genus and all its known species; Short et al. 2021: phylogenetic placement.

Figures

Figure 1. Variation across Acidocerinae, dorsal and lateral views A Colossochares ellipticus B Peltochares sp. C Peltochares conspicuus D Aulonochares tubulus E Helochares sp. F Helochares tristis G Novochares sp. H Helopeltarium ferrugineum I Batochares sp. J Helobata larvalis K Radicitus sp. L Nanosaphes tricolor M Agraphydrus cf. attenuatus N Tobochares luteomargo O Tobochares sulcatus P Quadriops similaris Q Crucisternum ouboteri R Primocerus neutrum S Agraphydrus coomani T Agraphydrus sp. U Globulosis flavus V Crephelochares nitescens.

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 5. Known distribution of genera of Acidocerinae: Crucisternum, Ephydrolithus, Globulosis, Helobata, Helochares, Helopeltarium, Katasophistes, and Nanosaphes.

Figure 14. Scanning electron micrographs of thorax in ventral view A, B prosternum: A Tobochares striatus with white arrow pointing to anterior projection B Quadriops reticulatus with white arrow pointing to anterior projection C-G mesoventrite and metaventrite: C Crucisternum ouboteri with white arrows pointing to anteriorly pointed transverse ridge and longitudinal carina of mesoventrite, metaventrite with median glabrous patch D Nanosaphes tricolor with black arrow pointing to longitudinal carina along mesoventrite and white arrows pointing to median and postero-lateral glabrous patches of metaventrite E Quadriops reticulatus with black arrow pointing to transverse carina across mesoventrite and metaventrite uniformly pubescent F Tobochares communis with black arrow pointing to longitudinal carina along mesoventrite and white arrow pointing to narrow postero-medial glabrous patch on metaventrite G Tobochares kasikasima with black arrow pointing to transverse elevation across mesoventrite and white arrow pointing to broad postero-medial glabrous patch on metaventrite. Scale bars: 100 μm.

Figure 29. Habitus of Crucisternum ouboteri A dorsal habitus B lateral habitus C ventral habitus. Scale bar: 1 mm.

Figure 30. Aedeagi of Crucisternum, Ephydrolithus, and Globulosis spp. A C. ouboteri B C. toboganensis C C. sinuatus D C. vanessae E C. queneyi F E. teli G E. spiculatus H E. ogmos I E. minor J G. flavus. Scale bars: 0.25 mm.