Tropicus nigrellus, King & Lago, 2012

King, Jonas G. & Lago, Paul K., 2012, The variegated mud-loving beetles (Coleoptera: Heteroceridae) of Mississippi and Alabama, with discussion and keys to the species occurring in the southeastern United States, Insecta Mundi 2012 (275), pp. 1-53 : 31-32

19. Tropicus nigrellus n.sp.

( Fig. 29 , 52 , 72 )

Type material. Holotype male, labeled: USA, Mississippi, Hinds County, 19 August 1964, VH Owen . Allotype female, same data as holotype. Paratypes (two males, three females). One male, two females, labeled: Mississippi, Hinds County, 19 August 1964, VH Owens. USA. One male, labeled: South Carolina, Florence, 16 August 1955, V.M. Kirk. USA. One female, labeled: Florida, Old Town, May 1967, Dr. Lenczy. All specimens are deposited in the California Academy of Sciences, San Francisco.

Description. Holotype male - Length 2.7 mm, width across elytra 1.0 mm, width of pronotum 0.9 mm. Body uniformly black; covered in recumbent setiform pubescence ( Fig. 29 ), pubescence especially dense on the head and labrum. Head finely punctate, punctures separated by approximately one diameter, almost entirely obscured by dense pubescence. Apical margin of clypeus slightly concave. Labroclypeus triangular. Margins of labrum rounded with an oblong projection at the apex. Labial palpi with surface slightly roughened. Males with labrum relatively elongate, mandibles long and slender, extending past apex of labrum for 25% of their length, lateral mandibular process (as in Fig. 27 ) developed from a dorsal ridge at base of mandible, wrapping around the lateral edge of the labrum and obscuring about 1/4 of its surface. Prostheca almost clear, with 10 - 15 alternating small and large teeth. Pronotum about twice as wide as long, slightly tapered anteriorly, all angles rounded. Pronotum of male as wide, or slightly wider, than the base of the elytra. Pronotal edges entire. Pronotal pubescence concentrated near the anterior margin, several long setae project laterally near the anterior angles.

Scutellum triangular, glabrous, its margins obscured by fine pubescence along its borders. Elytra oblong, disc elevated in middle half of conjoined surface, tapering posteriorly over the apical third after reaching a width slightly greater than their basal width; apicies rounded, edges entire, pubescence more sparse than on pronotum, fine punctures on the elytra visible. Punctures irregularly spaced, separated by about 1-3 diameters. Setae longer on apical fourth of elytra and generally recumbent posteriorly.

Ventral surface black with areas of dark reddish brown near the abdominal sutures, pubescence relatively sparse; fine punctures visible, separated by 1 - 2 diameters, setae concentrated around edges of sternites and coxae; no metathoracic coxal, mesothoracic coxal, or epipleural lines present; stridulatory ridge present on the first sternite, curving to the anterior edge of the second sternite directly behind the metacoxae.

Legs black, setose, prothoracic legs more robust that metathoracic legs, with a row of 7-10 spines along the posterior margin of the prothoracic tibiae, and with similar rows of smaller spines along the anterior margin of the meso- and metathoracic tibiae. Femora approximately equal in length to tibiae on all legs. Tarsi 4-4-4, inserted between first two large tibial spines, glabrous; basal tarsomere twice as long as second or third tarsomeres; apical tarsomere approximately as long as first three combined.

Male genitalia dark brown, appearing black near sutures, semi-transparent in some spots near apex; the basal 1/3 constricted to a linear structure that curves asymmetrically near the basal tip ( Fig. 51 ), distal 2/3 expanded into a spoon shaped structure five times wider than the basal third at its widest point; dorsal surface smooth and relatively transparent, with rounded edges, composed of one continuous plate, structures on the ventral side are visible through dorsal surface; ventral surface with a depression comprising about 1/2 of the entire surface of the phallobase ( Fig. 51 ), depression with a dark, y-shaped suture which is clearly visible from either side of the genitalia.

Allotype female - Similar to male in most respects. Length 2.8 mm, width of elytra 1.0 mm, width of pronotum 0.8 mm. Body uniformly black, clothed in dark setae. Mandibles not extending past the tip of the labrum, which is relatively short and rounded, and lacking lateral processes.

Paratypes vary in length from 2.0 - 3.0 mm but otherwise are very similar to the holotype and allotype in most respects, including sexually dimorphic mandibles. Pronotum as wide as, or slightly narrower than, the base of the elytra.

Diagnosis. Tropicus nigrellus is similar in overall morphology and in genitalic structure to T. pusillus . Relying on color alone, however, it can be easily distinguished from all other described species of Tropicus from Central and North America, none of which are entirely black. The ventral side of the male genitalia has a depression with a dark, y-shaped suture, which also separates T. nigrellus from T. pusillus (this suture is not present in the latter species).

Distribution. This species has been collected from three scattered locations in the Southeast ( Fig. 70 ), each located in a different region of the southeastern Coastal Plain: the East Gulf Coastal Plain (NC), the Lower Coastal Plain (MS) and the Floridian Coastal Plain (FL).

Etymology. The specific epithet, nigrellus , refers to the unusually dark coloration of this beetle.

Specimens examined. 3 males (2 previously disarticulated), 6 females.

Figures

Figures 26-29. Habiti of small southeastern species. 26) Tropicus pusillus. This level of color variation can be seen in either sex. 27) Head of Tropicus pusillus. 28) Heterocerus selanderi. 29) Tropicus nigrellus, holotype male and allotype female. Scale line = 1mm.

Figures 45-53. Male genitalia of southeastern species. The ventral view is shown unless otherwise stated. 45-50) Heterocerus species in the undulatus group. 45) H. undatus. 46) H. collaris. 47) H. texanus; left - ventral view, right - lateral view. 48) H. schwarzi; left - ventral view, right - lateral view. 49) H. tenuis. 50) H. selanderi. 51-53) Augyles and Tropicus species. Figures 52-53 are at roughly 2x relative scale of prior genitalia. 51) Augyles auromicans. 52) Tropicus nigrellus; left - dorsal view, center - ventral view, right - lateral view. 53) Tropicus pusillus; left - dorsal view, right - ventral view.

Figure 66-72. Distribution by county in the United States. 66) Heterocerus undatus. 67) Heterocerus texanus. 68) Heterocerus schwarzi. 69) Heterocerus tenuis. 70) Augyles auromicans. 71) Tropicus pusillus. 72) Tropicus nigrellus.