INSECTS
summer, the green apple aphis is found principally on young shoots of the apple twigs, and on water sprouts growing in the orchard.
During the early part of the summer, the rate of production rapidly increases in the aphid colonies, and individuals of the summer generations sometimes give birth to young a week after they themselves were born. In the fall, however, the period of growth again is lengthened, and the families drop off in size; until the last females of the season produce each a scant half dozen young, though they may live to a much greater age than do the summer individuals.
The young summer aphids born as active insects are inclosed at birth in a tight-fitting, seamless, sleeveless, and legless tunic, as are those hatched from the winter eggs. Thus swathed, each emerges, rear end first, from the body of the mother, but is finally held fast by the face when it is nearly free. In this position, the embryonic bag splits over the head and contracts over the body of the young aphid to the tip of the abdomen, where it remains as a cap of shriveled membrane until it finally drops off or is pushed away by the feet. The infant, now vigorously kicking, is still held in the maternal grasp, and eventually liberates itself only after some rather violent struggling; but soon after if is free it walks away to find a feeding place among its companions on the leaf. The mother is but little concerned with the birth of her child, and she usually continues to feed during its delivery, though she may be somewhat annoyed by its kicking. The average summer female gives birth to two or three young aphids every day.
The succession of forms in the families is one of the most interesting phases of aphid life. Investigations have shown that the winged individuals are produced principally by wingless forms, and experiments have demonstrated that the occurrence of the winged forms is correlated with changes in the temperature, the food
[164]
