I was fortunate this summer to take a short series of Neobisnius specimens while black-lighting in Kentucky. Some species are quite colorful and have a very similar color pattern to the toxic Paederus, and may well be mimics. Luckily there is a thorough monograph of this genus by JH Frank (1980). Males are fairly easy to key out using external characters, although examination of the aedeagus is probably advisable. The tricky part is that males and females aren’t terribly different externally. The front tarsi are somewhat dilated in both sexes! The keys do not work for females, and there are reproductions of Frank’s key that fail to mention it is based on males (*cough*Downie&Arnett*cough*).
I recommend a quick dissection before mounting this group to determine sex. If a male, the aedeagus is usually easily evertable and left attached to the beetle, or it can be glued to a point below the specimen.There are five species of Neobisnius in the midwest that have this Paederus-like color pattern (referred to as “parti-colored” by Horn): N. jocosus, N. jucundus, N. occidentoides, N. paederoides, and N. terminalis. An additional similarly colored species, N. ludicrus, occurs in the east, but appears restricted to the coastal states of the Southeast. A quick key to males of the 5 ‘parti-colored’ species is as follows, but genitalia should always be checked when possible:
- Head with a fovea on head between antennae. 2
Head without such a fovea. 3 Eyes large, almost 1/2 length of head … N. occidentoides Frank
Eyes smaller, only about 1/3 length of head … N. jucundus (Horn) Pale apical elytral band about 1/4 length of elytra … N. terminalis (LeConte)
Pale apical elytral band very narrow. 4 Less than 5 mm, sides of head rounded; base of abdominal tergum 7 reddish … N. paederoides (LeConte)
Larger than 5 mm, head more parallel-sided; abdominal tergum 7 entirely black … N. jocosus Horn
In the series mentioned earlier, out of 13 specimens, 6 were female and 7 were male. Of the males, 2 were Neobisnius paederoides (LeConte) and 5 were Neobisnius occidentoides Frank. The females remain unidentified to species.
Frank, J.H. 1981. A revision of the new world species of the genus Neobisnius Ganglbauer (Coleoptera: Staphylinidae: Staphylininae). Occ. Papers of the Florida State Collection of Arthropods, 1: vi + 60 pp.