382 
SEXUAL SELECTION. 
Part II. 
sound by rubbing the sbagreened 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 in his two papers 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 that the stridulating organs in the 
Coleoptera differed according to sex ; but Landois, who 
has carefully examined several species, observed no 
such difference ; nor did Westring ; nor did Mr. G. It. 
Crotch in preparing the numerous specimens which 
he had the kindness to send me for examination. Any 
slight sexual difference, however, would be difficult to 
detect, on account of the great variability of these organs. 
Thus in the first pair of the Necrojphorus Immcdor and of 
the Pelohius which I examined, the rasp was consider- 
ably 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 ; consequently my son, Mr. F. Darwin, in order 
to discover whether the sexes differed in their power of 
stridulating, collected 57 living specimens, which he 
separated into two lots, according as they made, when 
held in the same manner, a greater or lesser noise. He 
then examined their sexes, but found that the males 
were very nearly in the same proportion to the females 
in both lots. Mr. F. Smith has kept alive numerous 
specimens of Mononychus pseudacori (Curculionidse), and 
is satisfied that both sexes stridulate, and apparently in 
an equal degree. 
Nevertheless the power of stridulating is certainly a 
