The beetle was the big clue in the ID. I did wonder about the "stuff" on the head of one in your photo and found out what it is.
From a research article here; ZooKeys 157: 95-130 (21 Dec 2011)doi: 10.3897/zookeys.157.1472
"In Blepharida, Diamphidia, Podontia, and Polyclada, larvae retain their feces directly on the dorsum. This coating acts as a deterrent to attacking enemies such as ants (Vencl and Morton 1998, 1999). The fecal coat may also serve to moderate body temperature or to reduce water loss but the functions have not been tested. Fecal retention and the dorsally-positioned anus represent complex characters supporting the monophyly of the Blepharida-group (Paterson 1943)"
