It will be seen that these variations are not so great as those among plants or among snails.
There is, nevertheless, one very striking and curious kind of permanent modification possible among insects. I refer to the artificial production of royal couples among ants, and also to the production of queens among bees, both brought about by differences in the food supply. It would appear that the factor which determines whether a young female bee becomes a queen or a worker is whether she is, or is not, fed upon the royal food. In the case of ants this is not so clear, for the results may depend largely upon original differences in the eggs. Even if these transformations are brought about solely by environmental alterations they need not have much bearing on the present discussion, since this unusual possibility is of adaptive value to the species and may have been especially evolved and maintained. This is utilized by the insects themselves, "If an old queen dies, or if by accident all the prospective queens have been lost."[2]
Experiments which bring about changes in the life-cycle, and also those which produce parthenogenesis in insects which are normally sexual, can not be considered surprising. As to changes in the lifecycle, the case of the rose aphid is a good illustration. This insect produces parthenogenetically female offspring during the summer months, and normally on the appearance of winter begins to produce both males and females. This winter condition may be indefinitely postponed if the animals are kept in a suitable environment with plenty of food and water. Here we are not witnessing a profound modification, but only the failure of a certain normal change to take place in the failure of its normal stimulus.
Quite separate and different from this is the question of producing segmentation and growth in an unfertilized egg by some artificial chemical or physical means. This is called artificial parthenogenesis, and to have any special bearing on the present discussion must be produced in eggs that normally require fertilization by spermatozoa. The silkworm happened to be one of the first of the invertebrates to lend itself to this form of modification. "In 1886 Tichomiroff published the fact that the unfertilized eggs of the silkworm Bombyx mori, can be caused to develop by rubbing them gently with a brush, or by putting them for a short time into concentrated sulphuric acid."[3]
