Subgenus Mesolethrus Nikolajev, 2003

Type species: Lethrus microbuccis Ballion, 1871 by original designation

Diagnosis. Dorsal surface of each mandible in both sexes with oblique keel; keel on the left mandible nearly longitudinal, subparallel to lateral margin of mandible; keel on the right mandible nearly transverse, subparallel to anterior margin of mandible, can be obtuse and less distinct; keels do not reach the lateral margin of mandibles (Figs. 1–8, 45). Mandibles of male lacking mandibular processes (shape of mandibles same as in females), only sometimes in L. dyonini A. Shapovalov & Bagaturov, new species, left mandible with small obtuse protrusion approximately at middle of ventral margin (Fig. 38). Labrum slightly asymmetrical, right lobe more weakly developed than left one (Figs. 1–8). Postorbital denticles very small or indistinct (Figs. 1–8, 32–37). Anterior angles of pronotum rounded (Figs. 1–8). Elytral apices truncate or slightly prominent in males, more or less angularly prominent in females (Figs. 40–41). Epipleural keel not reaching elytral apex. Subapical visible ventrite with tuft of long erect setae (Figs. 40–43), or (in L. inermis) ventrites 1–5 usually with prominent denticle distally (Fig. 39). Femora entirely unarmed.

Notes. According to Nikolajev (2003), the subgenus is divided into two species groups based on the shape of the mandibles and male genitalia. These groups were not formally named in his work, therefore we use the senior species-names for these groups: the L. inermis -species group and the L. microbuccis -species group.

The record on distribution of the subgenus Mesolethrus in southern Kazakhstan (Bagaturov & Hillert 2023) is erroneous.