I call this the Madonna Beetle:
*update– over on facebook the Coleopterists chime in with relevant info. Henry Hespenheide notes:
They’re secondary sexual characters, possessed only by males. The species you’ve illustrated is a centrine barid – I’m not sure of the genus (possibly Geraeus) – but Bill Eberhard of the Smithsonian Tropical Research Institute has published a paper on the related genus Parisoschoenus in which the males use the spines in male-male combat.
Mike Ivie continues:
There is a pit between the spines that the oponents spine goes into during the joust. Henry showed me the first one I ever saw in Bridger Canyon in about 1987.
Thus, as for Madonna, the spines are used in a display of sexual aggression.
For more info, see a recent paper by Davis & Engel.
Canon MP-E 65mm 1-5x macro lens on a Canon EOS 7D
ISO200, f/13, 1/125 sec
Lit with 2 off-camera strobes