I find these under bark allyear round, and although I quickly learnt they are the unmistakable larvae of one of 3 UK Pyrochroa spp. I never really knew how to split them.
And today, once again on the Beetles of Britain and Ireland FB page some legend (Colin Le boutillier) dragged up a ley from the 40's and posted this diagram...
Then there was a species description from here
'The orange-brown, dorso-ventrally flattened larvae are found beneath bark and can also be identified to species level relatively easily. The smaller S. pecticornis larvae have strongly curved posterior cerci while those of Pyrochroa are short and straight. Pyrochroa serraticornis larvae have a raised bar across the enlarged 8th tergite, near the base, which is absent in P. coccinea.'
And this magnificent couplet (from Marks site) from van Emden, F.I. (1943). Larvae of British beetles. IV. Various small families. Entomologist’s monthly magazine, ?78?(1943), 209 – 223 and 259 – 270.
In Van Emden’s 1943 key The three Pyrochroa larvae are keyed in Volume 79 on pages 261, 263 (November, 1943 issue) and page 265 (December, 1943 issue) with illustrations of tergite 8 + cerci on page 268 (figs 34-36, identified on page 270).
1 (4) Cerci straight (figs. 34 and 35), the apices farther apart than the middle part of the inner surface. Ninth abdominal segment strongly but gradually narrowed from the widest part of the basal expansion to almost base of cerci; shortly before the latter the basal expansion ends posteriorly in a blunt or pointed tooth on ventro-lateral surface. The part below the base of the cerci, on which the pair of pits lies, more or less projecting. (Sbg. Pyrochroa s.str.)
2 (3) Eighth tergite (fig. 34) without a sub-basal raised line, praetergum not defined from tergum. The ventrolateral tooth of the ninth abdominal segment blunt and hardly visible when the segment is seen strictly dorsally; the prominence between the bases of the cerci less projecting and more or less truncate. Antennae slender, the third segment at least as long as the second . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . coccinea L.,
3 (2) Eighth tergite (fig. 35) with a complete transverse raised line near base, which defines the praetergum from the tergum. The ventrolateral tooth of the ninth abdominal segment pointed and very conspicuous even in a strictly dorsal view ; the prominence between the bases of the cerci strongly projecting, sharply rounded. Antennae stout, the third joint in full-grown larvae about two-thirds the length of the second . . . . . . . . . . . .serraticornis Scop.
4 (not reproduced here)
So it is not pecticornis (cerci are straight).