Small Carrion Beetles: A Bunch of SBBs

A fairly typical small carrion beetle, Catops tristis, copyright Trevor and Dilys Pendleton.

Anyone who takes on the task of beetle identification will soon discover that (to agree with Haldane) their sheer diversity can be overwhelming. Bird-watchers often complain about the challenges of identifying what they refer to as LBJs, Little Brown Jobs, but entomologists may have as much if not more to complain about when faced with the prospect of SBBs: Small Brown Beetles. The features marking a particular SBB as one family or another are often (at least to a novice) difficult to distinguish; members of unrelated families may look remarkably similar, whereas close allies may look surprisingly different.

The Leiodidae are, for the most part, firmly in the ranks of SBBs. This taxonomically small but morphologically diverse family is hard to come up with a coherent description for: though modern coleopterists have little doubt that they form a coherent clade, certain subgroups have become notably divergent. At least one leiodid, the beaver parasite Platypsyllus castoris, barely even looks like a beetle at all and was classified for a brief period in the 1800s as a distinct order of insects. Nevertheless, most leiodids are recognised by the structure of their antennae: the five-segmented club at its end has a distinct constriction as the eight antennal segment is smaller than the seventh and ninth segments on either side. Many leiodids are scavengers of plant or animal matter, but some are fungivores and a few (as already indicated) are parasites of mammals.

Small carrion beetles of the genus Sciodrepoides feeding on a deer carcass; copyright Stephen Cresswell.

Among the various subgroups of the Leiodidae are the Cholevini, commonly known as small carrion beetles. As their name indicates, these mostly feed on the remains of dead animals, though at least some are not above scavenging on other decaying matter. Some species are found in subterranean habitats, such as caves or the burrows of rodents, feeding on guano or other refuse. The Cholevini are one of the tribes in the leiodid subfamily Cholevinae, which has sometimes been treated in the past as a separate family Cholevidae or Catopidae. The Cholevinae differ from most other leiodids in the presence of an occipital carina or crest on the back of the head; such a carina is also present in the parasitic Leptininae, which Peck (1990) speculated to be derived from the cholevines. Members of the tribe Cholevini differ from other Cholevinae in having the setae on the elytra irregularly arranged (vs arranged in rows), giving the elytra a granular rather than a striate appearance.

Members of the Cholevini are mostly found in the Holarctic region, with only a few species in the Oriental region and none further south (Peck & Cook 2002). The greatest diversity in the group is found in Eurasia; only four of the 24 genera are found in North America, and only one of these (the monotypic Catoptrichus frankenhauseri) is unique to that continent. For the most part, cholevins do not vary much in appearance, and species are difficult to distinguish without examining the genitalia (these are true SBBs). Catoptrichus frankenhauseri has distinctive antennae, with lateral projections on either side of each segment(C. frankenhauseri is also noteworthy for the manner of its initial discovery, with the type specimen being collected from a human cadaver [Peck & Cook 2002]). Some of the subterranean species of cholevins have reduced eyes or wings, and a handful of species are entirely flightless.

REFERENCES

Peck, S. B. 1990. Insecta: Coleoptera Silphidae and the associated families Agyrtidae and Leiodidae. In: Dindal, D. L. (ed.) Soil Biology Guide pp. 1113–1136. John Wiley & Sons.

Peck, S. B., & J. Cook. 2002. Systematics, distributions, and bionomics of the small carrion beetles (Coleoptera: Leiodidae: Cholevinae: Cholevini) of North America. Canadian Entomologist 134: 723–787.