350 
SEXUAL SELECTION. 
Part II. 
of these differences is quite unknown, they may be here 
passed over. The females are generally larger and more 
robust than the males. With British, and, as far as 
Mr. Douglas knows, with exotic species, the sexes do 
not commonly differ much in colour ; but in about six 
British species the male is considerably darker than the 
female, and in about four other species the female is 
darker than the male. Both sexes of some species are 
beautifully marked with yermilion and black. It is 
doubtful whether these colours serve as a protection. 
If in any species the males had differed from the females 
in an analogous manner, we might have been justified 
in attributing such conspicuous colours to sexual selec- 
tion with transference to both sexes. 
Some species of Reduvidse make a stridulating noise ; 
and, in the case of Pirates stridulus , this is said 21 to 
be effected by the movement of the neck within the 
pro-thoracic cavity. According to Westring, Beduvius 
personatus also stridulates. But I have not been able 
to learn any particulars about these insects ; nor have I 
any reason to suppose that they differ sexually in this 
respect. 
Order, Ilomoptera , — Every one who has wandered in 
a tropical forest must have been astonished at the din 
made by the male Cicadse. The females are mute ; 
as the Grecian poet Xenarchus says, “ Happy the 
“ Cicadas live, since they all have voiceless wives.” 
The noise thus made could be plainly heard on board 
the “ Beagle,” when anchored at a quarter of a mile 
from the shore of Brazil ; and Captain Hancock says 
it can be heard at the distance of a mile. The Greeks 
formerly kept, and the Chinese now keep, these insects 
21 Westwood, ‘Modern Class, of Insects,’ vol. ii. p. 473. 
