Placosternus Hopping, 1937

Placosternus Hopping, 1937 View in CoL View at ENA , reinstated

Placosternus Hopping, 1937: 447 View in CoL .

Type species. Cyllene difficilis Chevrolat, 1862 View in CoL (original designation).

Included species. Placosternus crinicornis (Chevrolat, 1860) , P. difficilis (Chevrolat, 1862) , P. erythropus (Chevrolat, 1835) , and P. guttatus (Chevrolat, 1860) .

Diagnosis. Pedicel with tuft of long and erect setae, with aspect of spine; antennomere III slightly longer than IV; sides of prothorax rounded; elytra with longitudinal costae; prosternal process wider than procoxa, spatulate and forming a flat surface together with mesoventral process; elytral apex with outer spine ( Hopping 1937; Martins & Galileo 2011).

Remarks. Hopping (1937), when describing Placosternus , separated it from Cyllene Newman, 1840 (now Megacyllene ) especially by the absence of spines on the basal antennomeres.

Martins & Galileo (2011) reported the uselessness of this feature in to separate the South American species of Megacyllene from Placosternus . According to them, the presence and absence of spines on the basal antennomeres can be useful to separate North American species from species of Megacyllene but most of the South American Megacyllene species lack spines on the basal antennomeres. Therefore, the best feature to separate them is the prosternal process wider than the procoxa, spatulate and forming a flat surface together with the mesoventral process ( Fig. 1E View FIGURE 1 ): clearly observed in Placosternus ; and prosternal process narrower than procoxa, not forming a flat surface together with the mesoventral process ( Fig. 1F View FIGURE 1 ), observed in Megacyllene ( Martins & Galileo 2011) .

Zamoroka (2021) realized the phylogenetic analysis of Clytini using molecular data and considered Placosternus as a junior synonym of Megacyllene . He used only four species of Megacyllene [ Megacyllene caryae (Gahan, 1908) , Megacyllene proxima (Laporte & Gory, 1841) , Megacyllene robiniae (Forster, 1771) , and Megacyllene sp. ], and one species of Placosternus [ Placosternus crinicornis (Chevrolat, 1960) ]. Therefore, he did not include the type species of both genera, Placosternus difficilis (Chevrolat, 1862) and Megacyllene antennata (White, 1855) .

Thus, the proposed synonymy based on a phylogenetic analysis with molecular data did not include all the diversity of each genus ( Placosternus 1 of 4 species and Megacyllene 4 of 59). Furthermore, it ignored structural characters easily observed in the studied species of the genera (see Martins & Galileo 2011). Finally, a new phylogeny with all species of both genera will be necessary to understand the relationship between them. However, we believe that the shape of the prosternal and mesoventral processes is enough to separate Placosternus from Megacyllene .