Philonthus cognatus is a rove beetle. Adults are 8 to 10 mm long. They are mainly black in colour, although the elytra

Elytron

An elytron is a modified, hardened forewing of certain insect orders, notably beetles and a few of the true bugs ; in most true bugs, the forewings are instead called hemelytra, as only the basal half is thickened while the apex is membranous...

 have a metallic sheen, and are profusely pitted. A distinctive feature of this species is that the underside of the first antennal

Antenna (biology)

Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....

 segment is yellow, contrasting with the black upperside. It is a common species in Britain

Great Britain

Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

.