E. Evidence from Insects.
In 1899 Oudemans succeeded in finding a method of removing the testes and ovaries from caterpillars, using a dimorphic species, Ocneria dispar, the gipsy moth. The results were negative; none of the secondary sexual characters of the male or female moths or the accessory organs of copulation were in the least affected by the operation. The castrated male copulated as readily with the female as did the normal male, while the spayed females also behaved as normal individuals of that sex behave. Kellogg, in 1904, repeated the same operation in the silkworm moth on a small scale with the same results. Kopec and Meisenheimer, in 1909, repeated in a more detailed way Oudemans’s work. A further important addition was made by Kopec and by Meisenheimer. They transplanted ovaries into a castrated male and testes into a spayed female. Neither gonad produced any effect on the characters of the other sex. It is interesting to note that the testes underwent their normal development in the body of a spayed female, and even in one with the ovaries present, and that the ovary also underwent normal development in the body of the male. In other words, there is no intolerance of the tissue of one sex to the gonad of the other. This result is all the more unexpected, because other observations have shown that the color of the blood, and its chemical properties, is quite different in the male and female moths of certain species.
In the case of moths, therefore, if these cases be regarded as typical, the situation from the point of view of sexual selection is much simpler than in birds in the sense that the secondary sexual characters are directly the product of the genetic constituents of all the cells, and not influenced indirectly by the secretions from the testes or the ovaries. Sexual selection, therefore, if it is an agent in the evolution of the differences between males and females, has acted on the genetic complex to produce these effects on either sex without the result being involved in the condition of the ovary or the testes.
Regen castrated crickets, Gryllus campestris, in the larval stages and found no effects on the adult structures. The castrated males chirped like normal males and mated with the females. Spayed females were like normal females; they bored holes in the ground, but laid no eggs in them, of course, as the ovary had been completely removed.
The only genetic evidence in the group of insects, outside of the vinegar fly, relating to the secondary sexual inheritance of the secondary sexual characters is the following important experiments made by Foot and Strobell:
The male of one of the bugs, Euchistus variolarius, has a black spot on the end of the abdomen—a spot that is not present in the female. Foot and Strobell crossed a female of this species to another bug, E. servus, that lacks the spot in both sexes. The daughters had no spot, the sons a faint spot less developed than in variolarius. These inbred gave (in F₂) 249 females without a spot, 107 males with a spot, and 84 males without a spot. The results are explicable on the view that a single dominant Mendelian factor, not-sex-linked, causes the spot in the males, but the presence of the gene in the female produces no effect. The effect, therefore, is sex-limited, i. e., its expression is determined by the rest of the complex male or female.
The very important breeding experiments carried out by Goldschmidt on varieties of the gipsy moth should be referred to in this connection, but as I have recently reviewed these results in the paper on gynandromorphs written in collaboration with C. B. Bridges,[19] I need only refer to that account here.
[Note added April 21, 1919.]
Shortly after the preceding paper was finished a theses by A. Pézard on the secondary sexual characters of birds reached me. In it the author gives an account of a number of experiments that he has made with poultry and with pheasants. His description of the changes that take place after castration are more exact and more detailed than any other so far recorded; but in general the results obtained by Pézard, through castration, are the same as those that had been obtained by others. Castration of 4 male silver pheasants are reported. No change in the plumage results, although the changes that take place in the comb and wattles are the same in kind as those observed in fowls. The sexual instincts and peculiarities of the voice and their belligerency are also lost. Similarly 4 golden pheasants that were operated on gave the same results.
Three pheasants with mixed plumage (Phasianus colchicus) were examined. Their testes proved, on histological examination, to be imperfectly developed. It is not evident what relation existed between the facts and the mixed plumage. The suggestions made by Pézard seem inadequate to cover the cases.