Blatchley Fig. 358 (1). A useful illustration showing that the larvae in this family have only four functional legs.
a, larva;
b, pupa;
c. beetle (adult, i.e., imago);
d, hind leg of larva, showing atrophied joints;
e, same, enlarged.
(After Riley.)

Blatchley does not mention this, but the diagram (e) shows the toothed structure on the reduced hind leg. This is used to stridulate with a corresponding structure on the middle leg. Looking at the termniology, I guess the plectrum (pick) would be on the middle leg and this comb-like structure on the hind leg would be the pars stridens. If anyone knows better, please leave a comment!