Watch where you point those things

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.