Crucisternum , Giron, Jennifer C. & Short, Andrew Edward Z., 2018
Giron, Jennifer C. & Short, Andrew Edward Z., 2018, Three new genera of acidocerine water scavenger beetles from tropical South America (Coleoptera, Hydrophilidae, Acidocerinae), ZooKeys 768, pp. 113-158: 114-116
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Crucisternum gen. n. Figs 1, 2, 3, 4, 5, 6, 7, 8
Small beetles, total body length 2.0-2.5 mm, width 1.1-1.4 mm. Color orange brown to dark brown. Body shape elongated oval in dorsal view; moderately convex in lateral view (see Figs 1-3). Antennae with nine antennomeres (e.g., Fig. 2C). Maxillary palps curved inward, moderately long (e.g., Fig. 2D). Elytra without sutural striae; 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; outer margins of elytra slightly flared. Prosternum with a well-developed median longitudinal carina (e.g., Fig. 3B). Posterior elevation of mesoventrite with a strongly produced, anteriorly pointed transverse ridge, longitudinally carinate (Figs 4A, 5A). Posterior femora glabrous at most along apical fifth. Fifth abdominal ventrite apically rounded, truncate or slightly emarginate, without stout setae (e.g., Figs 1G, 4C, 5B).
Although Crucisternum is generally unremarkable dorsally from other small-bodied acidocerines, several sternal features easily separate the genus from all others. Within the Acidocerinae , the strongly-developed prosternal carina found in the genus is extremely rare, occurring only in the Afrotropical genus Acidocerus . Additionally, the strongly produced postero-medial projection of the mesoventrite, formed by the fusion of both transverse and longitudinal ridges (Figs 4A, 5A), is unique in the subfamily. It is most likely to be confused in samples as a small Chasmogenus , but can also easily be distinguished from that genus by the lack of a sutural stria.
Body elongate oval, moderately convex, orange brown to dark brown in color, sometimes slightly paler on ventral surface and appendages. Head. Frons and clypeus (e.g., Fig. 2D) with moderately marked ground punctures, irregularly and rather densely distributed over the surface, accompanied by scattered seta-bearing systematic punctures, particularly noticeable on anterior area of frons; surface between punctures smooth and shiny; anterior corners of clypeus roundly angulated; anterior margin of clypeus widely roundly emarginate. Eyes subquadrate in dorsal view. Labrum wide, fully exposed, nearly half as long, and collinear to perpendicular to clypeus (e.g., Fig. 2H); dorsal surface slightly convex, with scattered fine punctures; anterior margin roundly bent inwards, mesally emarginate and with tiny denticles along emargination; lateral and anterior margins fringed by rather long setae. Temporae densely covered by very short and fine setae (hydrofuge pubescence). Mentum parallel sided, with lateral oblique longitudinal ridges; anterior margin with wide, deep, concave median impression marked by a u- or bell-shaped transverse carina. Submentum sunken and pubescent at base, glabrous, shiny and ascending at apex; ocular ridge well-developed (e.g., Fig. 1G). Maxilla (see Fig. 2C) with ventral surface of cardo and stipes smooth and shiny, with scattered and shallow punctures; outer dorsal margin of palpifer with a row of stiff, decumbent, curved, spiniform setae; limit between cardo and stipes oblique; maxillary palps brown to orange, longer than antennae, with palpomere I usually darker at base, gradually paler towards apex; apex of palpomere 3 bearing sensilla. Mandibles with apex bifid (examined in C. ouboteri and C. sinuatus ). Labial palps yellowish to brown, usually shorter than mentum, at most nearly as long, dorsoventrally flattened; palpomere 2 with outer margin convex apicad of midpoint, sometimes with one (preapical) or two (one median, one preapical) setae on outer surface; palpomere 3 digitiform, with a long subapical seta on outer corner. Antennae (see Fig. 2C) with 9 antennomeres, either similar or paler than general coloration of head; antennomere 1 anteriorly projected near base, at most reaching midpoint of ventral surface of eye (not reaching cardo-stipes joint), nearly 1.7-2.5-times longer than antennomere 2; antennomere 2 nearly as long as antennomeres 3-5 combined; antennomere 6 forming a well differentiated, symmetric cupule; antennomeres 7-9 slightly flattened, forming a loosely articulated, pubescent club (antennomere 8 shortest, 9 longest); apex of antennomere 9 with longer setae than general pubescence of club. Thorax. Pronotum widest at base, narrowed anteriorly, surface evenly convex; ground punctation moderate, uniformly dense, with surface between punctures smooth and shiny; seta bearing systematic punctures forming paired anterolateral semicircles. Scutellar shield of moderate size, triangular, nearly as long as wide, with punctation as in pronotum. Prosternum (e.g., Fig. 2C) nearly as long as 2/3 the length of a procoxa, with a strong median longitudinal carina; anterior margin of prosternum mesally projected as a wide triangle; surface of prosternum usually convex and with scarce crenulations, with scattered, rather long, fine setae; intercoxal process projected from posterior margin of procoxal cavities, triangularly shaped in outline, mesally longitudinally carinate. Mesoventrite (Figs 4A, 5A) not fused to mesepisterna, with anterior margin nearly 0.3-times as wide as anterior margin of mesepisternum; anterior rib of mesoventrite bearing a medial teardrop-shaped, pearlescent macula; posterior elevation of mesoventrite with a strongly produced, anteriorly pointed transverse ridge, longitudinally carinate, bearing scarce, rather long setae; surface of mesoventrite reticulated for the most part, with an anteromedial depression, and posterolateral smooth and glabrous areas; mesepisternum obliquely widely concave, with reticulated surface; mesepimeron trapezoid, with reticulate and pubescent surface. Mesofurca (ex amined in C. ouboteri ) with short arms, 0.8-times length of mesocoxae; apical half of arms free, irregularly explanate at apex. Metaventrite mesally elevated, with elevation smooth and glabrous (Fig. 5A), rather narrow anteriorly, wide and flat posteriorly; surface of metaventrite densely pubescent, except for median and postero-lateral glabrous areas. Metepisterna approximately 3-times longer than wide, narrowing only at posterior end. Metepimeron triangular and acute posteriorly. Metafurca (examined in C. ouboteri , Fig. 4B) 1.45-times wider than long, with furcal arms (fa) slightly longer than stalk (s); stalk triangular (wider near the crux (c), gradually narrowing ventrally), with paired longitudinal keels extending along basal third of posterior face, fusing together towards crux, with a well-developed median keel on anterior face extending to anterior margin of dorsal sheets (ds); outer margins of stalk diverging from basal third towards distal third of furcal arms; furcal arms somewhat trapezoid, with apex (hemiductus (h)) only slightly explanate, with apex pointing obliquely; anterior tendons (at) inserted at mid length of dorsal edge of furcal arms; dorsal sheaths well-developed, wider than widest point of lateral sheaths (ls). Elytra. Surface even (without elevations or depressions), without sutural striae; 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; serial punctures not impressed into striae; seta bearing systematic punctures rather scarce; elytral margins slightly flared. Epipleura well-developed, surface slightly oblique, with sparse setae and irregular sculpture, anteriorly wide, gradually narrowing posteriorly, extending up to line of posterior margin of first abdominal ventrite; inner margin of epipleura with small indentation articulating anterior outer corner of metepisternum; pseudepipleura well-developed, perpendicularly positioned, nearly half as wide as anterior portion of epipleura, extending along entire outer margin of elytra. Hind wings well-developed. Legs. All femora with dense pubescence, except on at most apical fifth, in which surface is glabrous, shiny and slightly reticulated; all femora antero-posteriorly flattened; metafemora with rather weak tibial grooves. Tibiae slender, rather cylindrical; well-developed spines along pro- and mesotibiae, smaller and sparser in metatibiae; protibiae with a median longitudinal row of spathulate setae along anterior surface. All tarsi with five tarsomeres, bearing long apical hair-like setae on dorsal face, and two lateral rows of spines and/or hair-like spines on ventral face of tarsomeres 2-4; tarsomeres 1-4 similar in size and shape; tarsomere 5 approximately as long as tarsomeres 3-4 combined, without spines on ventral face; claws rather large, curved; empodium well-developed, bearing a pair of long, curved apical setae. Abdomen. Abdomen with five ventrites, medially convex; all ventrites with uniform, dense, fine pubescence; posterior margin of fifth ventrite uniformly rounded, truncate or slightly emarginate, without thick, flat spine-like setae (Figs 4C, 5B). Aedeagus (Fig. 6) with basal piece between 0.2 and 0.25-times the length of parameres; apical half of median lobe wider to narrower than a paramere; 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, usually with setae at apex; gonopore situated distad of mid length of median lobe.
The immature stages are unknown.
Named from the Latin crucis, meaning cross, combined with the word sternum, in reference to the conspicuous cruciform elevation of the mesoventrite in species of the genus. To be treated as masculine.
Brazil, Guyana, French Guiana, Suriname, Venezuela. See Fig. 7.
Without exception, 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.
Characters of taxonomic importance for Crucisternum .
The external morphology of Crucisternum is highly homogeneous across species, with characters of the aedeagus being the most reliable for species identification, in particular for those with sharply marked elytral punctures. Unassociated female specimens may not be able to be authoritatively identified in some cases.
Punctation. Two groups of species can be recognized by the degree of impression of the elytral ground punctures: Crucisternum toboganensis , C. ouboteri , C. queneyi , and C. vanessae exhibit rather sharply marked punctures, whereas the ground punctures of C. escalera , C. sinuatus , and C. xingu are more shallowly impressed (compare Fig. 1A to Fig. 1E).
Coloration. The dorsal coloration of the body can be either uniform as in C. escalera (Fig. 1A), and C. xingu , or the elytra can be darker than the head and pronotum, as in C. ouboteri (Fig. 2A), C. queneyi , and C. vanessae . Specimens of C. sinuatus collected in 1986 in northern Brazil were preserved in 70% alcohol until the time of this revision, and are darker in coloration, whereas the freshly collected specimens from Minas Gerais, are paler (orange brown). It is possible that the alcohol affected the coloration of the specimens over time causing them to darken. General coloration (e.g., light vs. dark brown) should not be used exclusively as diagnostic.
Aedeagus. The basal piece is strongly reduced in Crucisternum (see Fig. 6). Characters of the median lobe and the parameres are diagnostic at the species level.
Key to the species of Crucisternum
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