A most important fact in this connection should not be overlooked, namely, that in the caterpillar stage the sexual organs are so little developed that it is generally impossible at this time to distinguish between the sexes, unless a microscopic examination is made. This gives us, perhaps, a clew as to the difference between the mature sexual forms. These differences are connected with difference of sex itself. This conclusion also fits in well with the fact that during the period when the sexual organs are at the height of their development the individuals are most brilliantly colored. The primary cause of the brilliant color of many animals concerns us here only secondarily, for, since it is known that many of the lower forms are no less brilliantly and elaborately colored than are the sexes of the higher forms, it is not surprising that the sexes themselves sometimes differ in this respect.
Organs for producing sounds of different sorts are present in some insects, and these organs Darwin includes under the head of secondary sexual organs. In the group of Hemiptera, or bugs, the cicadas are the most familiar species that produce sounds. The noise is made by the males; the females are quite mute.
“With respect to the object of the music, Dr. Hartman, in speaking of the _Cicada septemdecim_ of the United States, says, ‘the drums are now (June 6th and 7th, 1851) heard in all directions. This I believe to be the marital summons from the males. Standing in thick chestnut sprouts about as high as my head, where hundreds were around me, I observed the females coming around the drumming males.’ He adds, ‘this season (August, 1868) a dwarf pear-tree in my garden produced about fifty larvæ of _C. pruinosa_; and I several times noticed the females to alight near a male while he was uttering his clanging notes.’ Fritz Müller writes to me from S. Brazil that he has often listened to a musical contest between two or three males of a species with a particularly loud voice, seated at a considerable distance from each other: as soon as one had finished his song, another immediately began, and then another. As there is so much rivalry between the males, it is probable that the females not only find them by their sounds, but that, like female birds, they are excited or allured by the male with the most attractive voice.”