Onthophagus Latreille

The genus Onthophagus embraces about 2300 species distributed world-wide, of which about 70% occur in Africa and Asia. As one of the most species-rich genera of animals, it is increasingly the subject of taxonomic and phylogenetic scrutiny (Breeschoten et al. 2016, and references therein). The United States is home to about 30 species last reviewed by Howden and Cartwright (1963). The Big Bend fauna includes four widespread species, three of which are putative associates of wood rats. All four species are small (2.5–7.5 mm long), and as tunneling (rather than ball-rolling) dung beetles, they are not likely to be observed on the surface. The nesting behavior of these beetles consists of packing the blind end of a tunnel with a “sausage” of food into which is laid a single egg, a process repeated several to many times in a single nest below the food source (Halffter and Edmonds 1982). Of interest is the fact that the three inquilinous Big Bend species are collected in pitfall traps baited with human feces. But while adults are not obligatory feeders on wood rat dung, the question remains whether or not they would use it to provision their nests.

The four smaller-size, dark colored Big Bend species can be similar to the naked eye, but each is easy to identify with the following key:

1. Pronotum evenly covered with small, shiny tubercles (Fig. 48).......................... 2 — Pronotum punctate, lacking distinct granules (Fig. 40, 53)............................. 3

2(1) Male (Fig. 44–45) — head with pair of erect, slightly inclined horns set between eyes; pronotum convex except for shallow anterior concavity receiving appressed head horns. Female (Fig. 46–47) — pronotum convex, almost always lacking any indication of anterior prominence...................................... Onthophagus velutinus Horn

— Male (Fig. 41–42) — head lacking horns, with 2 transverse carinae, posterior one slightly raised medially; pronotum with large, flat, apically bifurcate prominence extending well over head. Female (Fig. 43) — pronotum with conspicuous low, transverse ridge above anterior margin........................... Onthophagus browni Howden and Cartwright

3(1) Pronotum densely covered with flat, umbilical, setose punctures (Fig. 53). Small, length ≤ 5 mm. Male — pronotum with small, triangular projection above anterior margin (Fig. 49), otherwise evenly convex; protibia narrow, elongate, apical third strongly curved inward (Fig. 52A). Female — pronotum more or less evenly convex (Fig. 50–51), lacking distinct anterior feature; protibia (Fig. 52B) proportionately shorter, wider and less curved than in male............................................. Onthophagus knausi Brown

— Pronotum evenly covered with simple, impressed punctures (Fig. 40). Larger, length over 5 mm. Protibia similar in both sexes (Fig. 39). Male (Fig. 36–37) — pronotum strongly humped, anterior surface concave on either side of prominent midline. Female (Fig. 38) — pronotum humped anteriorly, anterior surface evenly concave, proportionately lower than in male............................................ Onthophagus brevifrons Horn