Redescription of Derallus strigipennis Orchymont, 1940 (Coleoptera: Hydrophilidae), new records and a key to species of Derallus Sharp 1882

: The type of Derallus strigipennis Orchymont, 1940 is redescribed and figured. New records are given for three other species of the genus: D.ambitus Orchymont, 1940 from Ecuador, D.argutus Orchymont, 1940 from Argentina and Bolivia


INTRODUCTION
The Hydrophilidae belonging to the tribe Berosini are characterized by the presence of swimming hairs on the middle and hind tibiae and by the long, narrow scutellum (Hansen, 1991). Berosini are represented by three genera in the Neotropical Region. The genus Derallus Sharp 1882, established for D. angustus Sharp, 1882 from Guatemala, is characterized by the very convex, laterally compressed body with shining black dorsal parts, protibiae broadened towards the apex and weakly prominent eyes (Sharp, 1882). The genus Derallus also differs from Hemiosus Sharp 1887 and Berosus Leach 1817 by the effaced humeral humps, the absence of intercalar striae and the short maxillary palpi (not longer than ½ of the head width) (Oliva, 1989). Both Sharp (1882) and Hansen (1991) have overlooked the absence of an intercalar stria, because the sculpture of the elytral disk is often obsolete in individuals of D. angustus. The species of Derallus have 7-segmented antennae and five apparent urosternites, differing in this from the genera Regimbartia Zaitzev, 1908 and Allocotocoerus Kraatz, 1883, not found in America (Hansen, 1991). The male genitalia are depressed, with basal piece symmetrical at base and simple distal pieces. The genus comprises 15 species of Neotropical distribution (Hansen, 1999), all of them small (2-5 mm in length), very convex beetles with deeply melanic, glossy dorsum. There is no sexual dimorfism. The elytra bear ten rows of punctures which at least in some species have a complex structure (Oliva 1992, figs. 66, 72, 77). Makhan (2005) described three new species of Derallus from Suriname. However, the descriptions and illustrations do not allow recognition of the species, which must therefore be treated as species dubiae. On the subject of Makhan's incursions into the taxonomy of Coleoptera, see Coleopterologische Rundschau 77 (2007)

MATERIAL AND METHODS
This paper is the result of two visits made by the author in 2009 to the Institut Royal des Sciences Naturelles de Belgique (Brussels, Belgium) and to the Division of Entomology of the University of Kansas (Lawrence, USA).
The material in every case consisted of pinned beetles. Morphological characters were studied under microscope. When needed, the specimens were relaxed in hot water. Both optical and electron microphotographies of the type specimen of D. strigipennis were taken.
Holotype and only known specimen in the collection of the Institut Royal des Sciences Naturelles de Belgique (Brussels, Belgium). The type, a male, was glued to a card in dorsal aspect, with the abdomen and male genitalia dissected and glued to the card. I detached the insect and re-glued it with Arabic gum in lateral aspect to allow observation of sternal characters. However, I did not dare relax the unique specimen to reposition the legs. Therefore, the mesosternal process is not easy to observe. Labels: A d'Orchymont det/ Derallus/ strigipennis m.// Brasilien Parana/ S. E. Lagoa VIII 1 (male).
Dorsal sculpture thick, shallow. Head and pronotum finely punctate. The two outer series of punctures sunken into striae on the posterior half of the elytron, the interstriae between them strongly convex (Fig. 1). Elsewhere all interstriae flat and striae marked only by larger punctures.
Punctures on outer interstriae about 2/3 of strial punctures. The SEM images show a single sensillum-bearing puncture, slightly excentrical to the large sunken area (Fig. 2). In D. altus each sensillum is placed in a small deep puncture with two equidistant accessory punctures; in D. ambitus there is a single accessory puncture; in D. angustus there are three, placed in a triangle with the setigerous puncture in the centre (Oliva, 1992).
Mesosternal process thickly laminar, short, prominent, with the ventral edge produced, rounded, more prominent than the small anterior tooth. First urosternite with a thick, strongly raised carina which reaches the posterior edge.
This species ressembles D. subglobosus Oliva 1984, but it is larger (2.9 mm as opposed to 2.0-2.5 mm), with the two outer rows of elytral punctures sunken into grooves at least in part of their length and with the interstriae between them convex. The mesosternal process is rounded ventrally, whereas it is truncate in D. subglobosus. Note: in Oliva 1984 the numbers on plate II are mistaken. Nº 5 should read 6 (D. anicatus) and nº 6 should read 5 (D. subglobosus).

Distribution of the species of Derallus
Of the 15 species described, only D. rudis Sharp, 1887, appears to be absent from South America. Two species (D. altus and D. angustus) extend from southern North America to tropical and subtropical South America; the latter has been found as far south as Sierra de la Ventana, Buenos Aires Province, Argentina. The remaining species are distinctly South-American in distribution.
Six species are recorded for Argentina; of these, D. angustus, D. argentinensis and D. paranensis have been collected from many localities along the Paraná basin, while D. altus, D. ambitus and D. argutus present few records in the north of the country. Brazil has the greatest diversity, with 10 species described from Brazilian localities. Some of these species have an Amazonian distribution, and they have been collected in countries adjacent to Brazil, but always upon the Amazonas basin.