rough parts of their bodies, which happened to be in contact; and that
from the noise thus produced being in some way useful, the rough
surfaces were gradually developed into regular stridulating organs.
Some beetles as they move, now produce, either intentionally or
unintentionally, a shuffling noise, without possessing any proper
organs for the purpose. Mr. Wallace informs me that the Euchirus
longimanus (a Lamellicorn, with the anterior legs wonderfully elongated
in the male) makes, whilst moving, a low hissing sound by the
protrusion and contraction of the abdomen; and when seized it produces
a grating sound by rubbing its hind-legs against the edges of the
elytra. The hissing sound is clearly due to a narrow rasp running
along the sutural margin of each elytron; and I could likewise make the
grating sound by rubbing the shagreened surface of the femur against
the granulated margin of the corresponding elytron; but I could not
here detect any proper rasp; nor is it likely that I could have
overlooked it in so large an insect. After examining Cychrus, and
reading what Westring has written about this beetle, it seems very
doubtful whether it possesses any true rasp, though it has the power of
emitting a sound.
From the analogy of the Orthoptera and Homoptera, I expected to find
the stridulating organs in the Coleoptera differing according to sex;
but Landois, who has carefully examined several species, observed no
such difference; nor did Westring; nor did Mr. G.R. Crotch in preparing
the many specimens which he had the kindness to send me. Any difference
in these organs, if slight, would, however, be difficult to detect, on
account of their great variability. Thus, in the first pair of
specimens of Necrophorus humator and of Pelobius which I examined, the
rasp was considerably larger in the male than in the female; but not so
with succeeding specimens. In Geotrupes stercorarius the rasp appeared
to me thicker, opaquer, and more prominent in three males than in the
same number of females; in order, therefore, to discover whether the
sexes differed in their power of stridulating, my son, Mr. F. Darwin,
collected fifty-seven living specimens, which he separated into two
lots, according as they made a greater or lesser noise, when held in
the same manner. He then examined all these specimens, and found that
the males were very nearly in the same proportion to the females in
both the lots. Mr. F. Smith has kept alive numerous specimens of
Monoynchus pseudacori (Curculionidae), and is convinced that both sexes
stridulate, and apparently in an equal degree.
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