ioned
by C. Darwin as probable examples of the action of sexual selection;
they are, however, frequently present in both sexes, and in some
families also in the larvae. An account of the principal types of
stridulators that have been described has been published by C. J. Gahan
(1900). The file may be on the head--either upper or lower surface--and
the scraper formed by the front edge of the prothorax, as in various
wood-boring beetles (_Anobium_ and _Scolytus_). Or ridged areas on the
sides of the prothorax may be scraped by "files" on the front thighs, as
in some ground-beetles. Among the longhorn beetles, the prothorax
scrapes over a median file on the mid-dorsal aspect of the mesothorax.
In a large number of beetles of different families, stridulating areas
occur on various segments of the abdomen, and are scraped by the elytra.
It is remarkable that these organs are found in similar positions in
genera belonging to widely divergent families, while two genera of the
same family may have them in different positions. It follows, therefore,
that they have been independently acquired in the course of the
evolution of the Coleoptera.

Stridulating organs among beetle-larvae have been noted, especially in
the wood-feeding grub of the stag-beetles (_Lucanidae_) and their allies
the _Passalidae_, and in the dung-eating grubs of the dor-beetles
(_Geotrupes_), which belong to the chafer family (_Scarabaeidae_). These
organs are described by J. C. Schiodte and D. Sharp; in the stag-beetle
larva a series of short tubercles on the hind-leg is drawn across the
serrate edge of a plate on the haunch of the intermediate legs, while in
the Passalid grub the modified tip of the hind-leg acts as a scraper,
being so shortened that it is useless for locomotion, but highly
specialized for producing sound. Whatever may be the true explanation of
stridulating organs in adult beetles, sexual selection can have had
nothing to do with the presence of these highly-developed larval
structures. It has been suggested that the power of stridulation would
be advantageous to wood-boring grubs, the sound warning each of the
position of its neighbour, so that adjacent burrowers may not get in
each other's way. The root-feeding larvae of the cockchafer and allied
members of the _Scarabaeidae_ have a ridged area on the mandible, which
is scraped by teeth on the maxillae, apparently forming a stridulating
organ.

_Luminous Organs._--The function of the s