Size
Any oil beetles over 2.5 cm long with kinked anteannae will be either Black or Violet Oil Beetles though there is great size variation in all species – see photos. Newly emerged oil beetles have very short abdomens, they usually feed up quickly and the abdomen swells enormously. Females generally have very large bloated looking abdomens but the males can have large abdomens as well.
Colour
Black Meloe proscarabaeus and Violet Oil Beetles Meloe violaceus vary greatly in colour. Some Violet Oil Beetles are all black and some Black Oil Beetles can be violet-blue. Despite their names colour is not always the best way to separate them.
Antennae
The males of the Black and Violet Oil Beetles have very kinked antennae (the antennae of the females is slightly kinked). The Rugged, Mediterranean and Short-necked Oil Beetles have straight antennae in both sexes.
Main identification features
Violet Oil Beetle – square shaped thorax, indented lower edge of thorax with depressed area at base, large tooth at base of thorax, fine punctures on thorax, up to 30 mm long.
Black Oil Beetle – square shaped thorax, almost straight lower edge of thorax which is flat (not with depressed area at base), small tooth at base of thorax, large coarse punctures on thorax, up to 30 mm long.
Rugged Oil Beetle Meloe rugosus and Mediterranean Oil Beetle Meloe mediterraneus – are very similar in appearance, both have a rectangular shaped thorax which is wider than long, rough matt black upperparts, long straight antennae, up to 22 mm long (rugosus) and 36mm (mediterraneus). The Rugged Oil Beetle has a sulcus (narrow slit) running down the middle of the thorax this is lacking in the Mediterranean Oil Beetle. The thorax is more densely punctured in the Rugged Oil Beetle.
Short-necked Oil Beetle Meloe brevicollis – rectangular shaped thorax which is wider than long, shiny blue- black upperparts, short straight antennae which thicken towards the tip, up to 24 mm long.
Sexing
It is fairly easy to tell the sexes of the_Black Meloe proscarabaeus and Violet Oil Beetles Meloe violaceus apart as the males have a distinct kink in their antennae and the antennal segments are very broad at the kink. The females have slightly kinked antennae with all segments about the same width.
Separating the sexes of the Short-necked Meloe brevicollis, Rugged Meloe rugosus and Mediterranean Oil Beetles Meloe mediterraneus requires examining the underside of the last segment of the abdomen. This is distinctly notched in males and rounded in females.