Ground beetles in the genus Pterostichus, subgenus Euferonia, are common woodland beetles in the eastern U.S. Almost all have reduced wings, so are unable to fly.
I’ve collected a number of specimens that fit into this species pair, but have struggled with separating them with existing keys. Lindroth noted that “the infraspecific variation of external characters is considerable” and has resulted in many unnecessary names, especially by Casey.
The characters that separate P. stygicus from P. coracinus are the presence of a “tubercle” in the basal fovea of the pronotum and the elytra usually being somewhat iridescent. The iridescence isn’t hard to see, but the “tubercle” is very subjective. It appears to be referring to a low ridge or convexity within the fovae, hardly tuberculate in my opinion, and quite variable. Fortunately, I recently collected a pair of males and decided to consult Lindroth, who illustrates the genitalia of the species. The shape of the penis is very distinct between the two species: symmetrical in P. stygicus and asymmetical in P. coracinus. This led to the reevaluation my previously collected specimens, which turned out to be all P. stygicus.
Now I just need to find P. coracinus!
A recent post on BugGuide brought to my attention to a group of fungi known as Laboulbeniales that infect insects. I knew I had seen these before, but I had never really paid them much attention. Sorting through some Carabidae I had collected this summer, I noticed one of the strange growths on the elytron of a Clivina americana. After posting an image of this beetle to BugGuide, I continued sorting the sample. To my surprise, I found the fungal hyphae on a large number of specimens. Once I had seen the first, they became fairly easy to spot (despite all the moth scales floating around in the alcohol).
I contacted the author of the Laboulbeniales post, Danny Haelewaters, now a doctoral student at the Farlow Herbarium, Harvard University. He is studying Laboulbeniales fungi, so I’ll be sending the specimens to him for his research.
The Carabidae that appeared infected included Clivina americana, Perigona nigriceps, Elaphropus xanthopus, Acupalpus partiarius, Acupalpus indistinctus, Bradycellus rupestris, Stenolophus lecontei, and Stenolophus ochropezus. There were also infected Homaeotarsus cf. bicolor (Staphylinidae) and Melanophthalma sp. (Latridiidae).
Below are images of some infected beetles (Homaeotarsus, Melanophthalma, Elaphropus xanthopus, and Stenolophus ochropezus):