From John Walters page: http://johnwalters.co.uk/research/oil-beetle-identification.php. Unfortunately this page is now gone, so I dared to save a copy.
Meloe proscarabaeus – (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.
Meloe violaceus – (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.
Males of the M. proscarabaeus and M. violaceus 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.
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