Another little tale from my enigma bucket. For a number of years I have been casually trying to work out the chemicals produced by some of the local melyrid beetles. Many species have eversible abdominal and/or thoracic pouches, vivid aposomatic colors and some tropical species are apparently are the source (or a source) of defensive compounds for birds and frogs. I can easily show that they are distasteful by offering a predator like a crab spider an unprotected prey item like the cricket in this video , which the spider grabs and eats and then compare that to what happens when a melyrid is grabbed by a crab spider . The crab spider clearly rejects the beetle and the beetle walks away unharmed. The beetle and crab spider in the video were collected together on flowers of a native Prunis in southern California and so this sort of encounter must happen regularly in nature.
Even stranger, when the flightless, intertidal melyrid, Endeodes was offered to a jumping spider that lives in the same habitat with it, the spider grabbed the beetle, carried it around for several minute without biting it and then released it. The beetle was a little roughed up, but not obviously harmed. What is that about?!
As for the chemicals that I presume must be on the eversible sacs, initial analyses didn’t show anything remarkable (looked at by me and also by the late Thomas Eisner). Various assays, including rinses and extracts of the beetles applied to crickets, had no deterrent effect. I haven’t done the deep dive on this and everything is very preliminary, So like the mystery of the bursal glands in my last post, this remains an puzzle for another day or another person for figure out.