Evolution and Adaptation

by Thomas Hunt Morgan

“Whilst reflecting on the beauty of many butterflies, it occurred to me that some caterpillars were splendidly colored; and as sexual selection could not possibly have here acted, it appeared rash to attribute the beauty of the mature insect to this agency, unless the bright colors of their larvæ could be somehow explained. In the first place, it may be observed that the colors of caterpillars do not stand in any close correlation with those of the mature insect. Secondly, their bright colors do not serve in any ordinary manner as a protection. Mr. Bates informs me, as an instance of this, that the most conspicuous caterpillar which he ever beheld (that of a Sphinx) lived on the large green leaves of a tree on the open llanos of South America; it was about four inches in length, transversely banded with black and yellow, and with its head, legs, and tail of a bright red. Hence it caught the eye of any one who passed by, even at the distance of many yards, and no doubt that of every passing bird.”

Darwin applied to Wallace for a solution of this difficulty, and received the reply that he “thought it probable that conspicuously colored caterpillars were protected by having a nauseous taste; but as their skin is extremely tender, and as their intestines readily protrude from a wound, a slight peck from the beak of a bird would be as fatal to them as if they had been devoured. Hence, as Mr. Wallace remarks, ‘distastefulness alone would be insufficient to protect a caterpillar unless some outward sign indicated to its would-be destroyer that its prey was a disgusting morsel.’ Under these circumstances it would be highly advantageous to a caterpillar to be instantaneously and certainly recognized as unpalatable by all birds and other animals. Thus the most gaudy colors would be serviceable, and might have been gained by variation and the survival of the most easily recognized individuals.”

It need scarcely be pointed out that an occasional peck can scarcely be supposed to have led to the splendid development of color shown by some caterpillars, and Darwin confesses that at first sight this hypothesis appears bold, but nevertheless he believes that it will be found to be true. He adds, “We cannot, however, at present thus explain the elegant diversity in the colors of many caterpillars.”

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.”

In the flies the following cases are given by Darwin:—

“That the males of some Diptera fight together is certain; for Professor Westwood has several times seen this with the Tipulæ. The males of other Diptera apparently try to win the females by their music: H. Müller watched for some time two males of an Eristalis courting a female; they hovered above her, and flew from side to side, making a high humming noise at the same time. Gnats and mosquitoes (Culicidæ) also seem to attract each other by humming; and Professor Mayer has recently ascertained that the hairs on the antennæ of the male vibrate in unison with the notes of a tuning-fork, within the range of the sounds emitted by the female.”

In the crickets, grasshoppers, and locusts, the males “are remarkable for their musical powers”; and it is generally assumed that the sounds serve to call or to excite the female. In these forms the noise is made by rubbing the wings over each other or the legs against the wing-covers.

In some of these forms both sexes have stridulating organs, and in one case they differ to a certain extent from each other. “Hence we cannot suppose that they have been transferred from the male to the female, as appears to have been the case with the secondary sexual characters of many other animals. They must have been independently developed in the two sexes, which no doubt mutually call to each other during the season of love.”