Tropicus Pacheco

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 : 30

Genus Tropicus Pacheco

Tropicus Pacheco 1964: 131 .

Type species. Heterocerus pusillus Say 1823: 200 . (by original designation)

Discussion. The smaller, orange-brown heterocerids familiar to many New World coleopterists belong to the genus Tropicus Pacheco , which currently contains 25 species. This genus is known only from the New World and is most diverse in South America. Three species occur north of Mexico and five species occur on Caribbean Islands, the other 17 occur in Mexico, Central America, and South America. Tropicus was erected relatively recently by Pacheco (1964) and about half of the known species have been described during the last 20 years.

Tropicus pusillus is the most commonly collected species of heterocerid in the eastern U.S. This species occurs in all sorts of habitats and, unlike most other heterocerids, can be collected from areas that seem to be entirely sand, and lacking mud. Outside of this, virtually nothing is known of the specific biology of this group.

Diagnosis. Individuals of the various Tropicus species are small (length = 1.7 - 3.5 mm) and their elytra are either uniformly colored, have a dark, common, central macula, or rarely have longitudinally elongate markings on each elytron. In any case, the elytra are never trifasciate as they are in most species of Heterocerus and Augyles . Nine antennomeres are present, although the small pubescent antennae are usually difficult to examine. Males of most species have dorsolateral processes on the mandibles that curve over the lateral edges of the labrum. All species of Tropicus lack both post-mesothoracic coxal and post-metathoracic coxal lines. The male genitalia are composed of a single dorsoventrally flattened sclerotized tube with various folds, sutures and sculpturing on its shovel-like apical end. The basal third of the genitalia is constricted into a narrow cylindrical structure with a knobbed tip.

Although small size and lack of trifasciate elytra almost always betray the identity of Tropicus , several species of New World Heterocerus are as small as Tropicus , and some Asian Augyles are small or uniformly colored. A common macula on the elytral disc, number of antennomeres, and male genitalia will readily distinguish Tropicus from these species. Micilus Mulsant and Ray is rare and comprises two species from isolated localities in Europe and Asia. Members of Micilus are small, uniformly colored, have nine antennomeres and simple male genitalia. Micilus do have post-mesothoracic coxal lines, however, which will distinguish them from Tropicus .

Molecular data collected during this study strongly supports the placement of Tropicus as sister to the remainder of the family. The high level of sequence variation between Tropicus and the rest of the family suggests that it diverged early in the history of the family and was geographically isolated from the other groups by vicariance events during the Jurassic.