Many biologists are familiar with the Bombardier Beetles in the ground beetle tribe Brachinini, as their defensive tactic of aiming an explosive spray has been studied extensively. The Brachinini are even celebrated by creationists as animals that couldn’t possibly evolve.
As it turns out, though, bombardiers have evolved at least twice. The second, less known radiation comprises the subfamily Paussinae, also in the Carabidae, and as we’d expect from an independent origin the spray dispersing mechanism is different, using a flange instead of a nozzle. Metrius contractus is a North American species, and when handled roughly its chemical discharge stains the skin:
Myrmecologists may be familiar with the paussines for another reason: many species are obligate inhabitants of ant nests. For information about these beetles, I recommend visiting Wendy Moore’s expert pages on the paussines.
photo details (both photos): Canon MP-E 65mm 1-5x macro lens on a Canon EOS 20D
ISO 100, f/13, 1/250 sec, flash diffused through tracing paper