Stag beetle larvae when disturbed behave in a very characteristic way. That is they stay in the same place, wriggling and stridulating with their legs. Also, as a defencive reaction they defaecate and produce liquids in their mouths as well. And these videos illustrate their behaviour.
Mature stag beetle Lucanus cervus larva. Video shot on 11 May 2008 by Maria Fremlin, Colchester.