366 SEXUAL SELECTION". Part II. 



the solitary species, as I hear frorn 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 ' is 

 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 

 (Lestis 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 Europsea emits a stridulating noise; and ac- 

 cording to Groureau 57 both sexes have this power. He 

 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 thev 



57 Quoted by Westwood, 'Modern Class, of Insects,' vol. ii. p. 214. 



