Nevertheless, the power of stridulating is certainly a sexual character
in some few Coleoptera. Mr. Crotch discovered that the males alone of
two species of Heliopathes (Tenebrionidae) possess stridulating organs.
I examined five males of H. gibbus, and in all these there was a
well-developed rasp, partially divided into two, on the dorsal surface
of the terminal abdominal segment; whilst in the same number of females
there was not even a rudiment of the rasp, the membrane of this segment
being transparent, and much thinner than in the male. In H.
cribratostriatus the male has a similar rasp, excepting that it is not
partially divided into two portions, and the female is completely
destitute of this organ; the male in addition has on the apical margins
of the elytra, on each side of the suture, three or four short
longitudinal ridges, which are crossed by extremely fine ribs, parallel
to and resembling those on the abdominal rasp; whether these ridges
serve as an independent rasp, or as a scraper for the abdominal rasp, I
could not decide: the female exhibits no trace
of this latter structure.
Again, in three species of the Lamellicorn genus Oryctes, we have a
nearly parallel case. In the females of O. gryphus and nasicornis the
ribs on the rasp of the pro-pygidium are less continuous and less
distinct than in the males; but the chief difference is that the whole
upper surface of this segment, when held in the proper light, is seen
to be clothed with hairs, which are absent or are represented by
excessively fine down in the males. It should be noticed that in all
Coleoptera the effective part of the rasp is destitute of hairs. In O.
senegalensis the difference between the sexes is more strongly marked,
and this is best seen when the proper abdominal segment is cleaned and
viewed as a transparent object. In the female the whole surface is
covered with little separate crests, bearing spines; whilst in the male
these crests in proceeding towards the apex, become more and more
confluent, regular, and naked; so that three-fourths of the segment is
covered with extremely fine parallel ribs, which are quite absent in
the female. In the females, however, of all three species of Oryctes, a
slight grating or stridulating sound is produced, when the abdomen of a
softened specimen is pushed backwards and forwards.
In the case of the Heliopathes and Oryctes there can hardly be a doubt
334