366 
SEXUAL SELECTION. 
Part II. 
the solitary species, as I hear from the same distin- 
guished entomologist, the sexes often differ in colour. 
The males are generally the brightest, and in Bombus 
as well as in Apathus, much more variable in colour 
than the females. In Anthophora retusa the male i s 
of a rich fulvous-brown, whilst the female is quite 
black: so are the females of several species of Xylocopa, 
the males being bright yellow. In an Australian bee 
( Lest/is bombylans), the female is of an extremely brilliant 
steel-blue, sometimes tinted with vivid green ; the male 
being of a bright brassy colour clothed with rich fulvous 
pubescence. As in this group the females are provided 
with excellent defensive weapons in their stings, it is 
not probable that they have come to differ in colour 
from the males for the sake of protection. 
Mutilla Europtm emits a stridulating noise ; and ac- 
cording to Goureau 57 both sexes have this power. Be 
attributes the sound to the friction of the third and 
preceding abdominal segments; and I find that these 
surfaces are marked with very fine concentric ridges, 
but so is the projecting thoracic collar, on which the 
head articulates ; and this collar, when scratched with 
the point of a needle, emits the proper sound. It is 
rather surprising that both sexes should have the 
power of stridulating, as the male is winged and the 
female wingless. It is notorious that Bees express 
certain emotions, as of anger, by the tone of their 
humming, as do some dipterous insects ; but I have 
not referred to these sounds, as they are not known to 
be in any way connected with the act of courtship. 
Order, Coleoptera (Beetles). — Many beetles are 
coloured so as to resemble the surfaces which they 
57 Quoted by Westwood, ‘Modern Class, of Insects,’ vol. ii. p. 211. 
