Taxonomic review of the Dichotomius mamillatus group (Coleoptera: Scarabaeidae), with a description of a new species, Dichotomius (Dichotomius) gandinii sp. nov., from western Amazonia

We conducted a morphologically based taxonomic revision of the Dichotomius mamillatus species group (Scarabaeinae). A new species, Dichotomius (Dichotomius) gandinii sp. nov., from western Amazonia is described, illustrated and included in this group, along with Dichotomius (Dichotomius) mamillatus (Felsche, 1901), Dichotomius (Dichotomius) robustus (Luederwaldt, 1935) and Dichotomius (Dichotomius) worontzowi (Pereira, 1942). The external and genital morphology of the mamillatus group and its species are defined through an in-depth examination of the surviving type material, as well as hundreds of reference specimens deposited in several natural history collections. An illustrated identification key to the Dichotomius species of the group is provided. Pinotus calcaratus Arrow, 1913 (currently Dichotomius calcaratus) is considered to be a junior subjective synonym of Dichotomius (Dichotomius) mamillatus. This research further resolves the many taxonomic questions that are still hampering the correct identification of Dichotomius species in the Neotropics, providing key diagnostic tools for researchers interested in these scarab beetles. The comprehensive morphological study of the D. mamillatus group highlights the important taxonomic value of the often-overlooked endophallic structures, as well as their central role in informing hypotheses of systematic relationships.