A small family of saproxylic beetles all of which feed on wood-decay fungi, including the Oyster Mushroom Pleurotus ostreatus and allies as well as bracket fungi. There are four species of Triplax, two of Dacne plus Tritoma bipustulata. Until quite recently they were best identified using Joy (1932). But now the best key to the British species is by Richard Wright in Beetle News Volume 3, Issue 3, October 2011 (PDF, 252Kb). It’s a simple but effective key with photos of all but the two rarer Triplax species (lacordairii (see below) and scutellaris (see here)).
Richard’s key also includes the family Tetratomidae. Although distantly related (Erotylidae is in the superfamily Cucujoidae whereas Tetratomidae is in the Tenebrionoidea), it is quite a good idea to key them out together. Both are found on or in wood-decay fungi, and the commonest species in both families (Triplax aenea (Erotylidae) and Tetratoma fungorum (Tetratomidae)) are superficially alike.
Joy, N.H. (1932). A practical handbook of British beetles. 2 volumes. H.F. & G. Witherby.