According to Robinet, the white race, called Sina, by careful selection during the last seventy-five years, "est arrivee a un tel etat de purete, qu'on ne voit pas un seul cocon jaune dans des millions de cocons blancs." (8/75. Robinet ibid pages 306-317.) Cocoons are sometimes formed, as is well known, entirely destitute of silk, which yet produce moths; unfortunately Mrs. Whitby was prevented by an accident from ascertaining whether this character would prove hereditary.
ADULT STAGE.
I can find no account of any constant difference in the moths of the most distinct races. Mrs. Whitby assured me that there was none in the several kinds bred by her; and I have received a similar statement from the eminent naturalist, M. de Quatrefages. Captain Hutton also says (8/76. 'Transact. Ent. Soc.' ut supra page 317.) that the moths of all kinds vary much in colour, but in nearly the same inconstant manner. Considering how much the cocoons in the several races differ, this fact is of interest, and may probably be accounted for on the same principle as the fluctuating variability of colour in the caterpillar, namely, that there has been no motive for selecting and perpetuating any particular variation.
The males of the wild Bombycidae "fly swiftly in the day-time and evening, but the females are usually very sluggish and inactive." (8/77. Stephen's Illustrations, 'Haustellata' volume 2 page 35. See also Capt. Hutton 'Transact. Ent. Soc.' ibid page 152.) In several moths of this family the females have abortive wings, but no instance is known of the males being incapable of flight, for in this case the species could hardly have been perpetuated. In the silk-moth both sexes have imperfect, crumpled wings, and are incapable of flight; but still there is a trace of the characteristic difference in the two sexes; for though, on comparing a number of males and females, I could detect no difference in the development of their wings, yet I was assured by Mrs. Whitby that the males of the moths bred by her used their wings more than the females, and could flutter downwards, though never upwards. She also states that, when the females first emerge from the cocoon, their wings are less expanded than those of the male. The degree of imperfection, however, in the wings varies much in different races and under different circumstances. M. Quatrefages (8/78. 'Etudes sur les Maladies du Ver a Soie' 1859 pages 304, 209.) says that he has seen a number of moths with their wings reduced to a third, fourth, or tenth part of their normal dimensions, and even to mere short straight stumps: "il me semble qu'il y a la un veritable arret de developpement partiel." On the other hand, he describes the female moths of the Andre Jean breed as having "leurs ailes larges et etalees. Un seul presente quelques courbures irregulieres et des plis anormaux." As moths and butterflies of all kinds reared from wild caterpillars under confinement often have crippled wings, the same cause, whatever it may be, has probably acted on silk-moths, but the disuse of their wings during so many generations has, it may be suspected, likewise come into play.
The moths of many breeds fail to glue their eggs to the surface on which they are laid (8/79. Quatrefages 'Etudes' etc. page 214.) but this proceeds, according to Capt. Hutton (8/80. 'Transact. Ent. Soc.' ut supra page 151.), merely from the glands of the ovipositor being weakened.
As with other long-domesticated animals, the instincts of the silk-moth have suffered. The caterpillars, when placed on a mulberry-tree, often commit the strange mistake of devouring the base of the leaf on which they are feeding, and consequently fall down; but they are capable, according to M. Robinet (8/81. 'Manuel de l'Educateur' etc. page 26.) of again crawling up the trunk. Even this capacity sometimes fails, for M. Martins (8/82. Godron 'De l'Espece' page 462.) placed some caterpillars on a tree, and those which fell were not able to remount and perished of hunger; they were even incapable of passing from leaf to leaf.