Page:Insects - Their Ways and Means of Living.djvu/202



Finally, it has recently been shown experimentally by Dr. A. Franklin Shull that winged and wingless condi- tions in the potato aphis may be produced artificially by a variation in the relative amount of alternating light and darkness the aphids receive during each twenty-four hours. Shortening the illumination period to twelve hours or less results in a marked increase in the number of winged forms born of wingless parents. Continuous darkness, however, produces few winged offspring. Maxi- mure results perhaps are obtained with eight hours of light. The effect of decreased light appears from Doctor Shull's experiments to be directly operative on the young from thirty-four to sixteen hours before birth, and it lS not to be attributed to any physiological effect on the plant on which the insects are feeding. It is evident, therefore, that various unfavorable local conditions may give fise to winged individuals in a colony of wingless aphids, thus enabling representatives of the colony to migrate in the chance of finding a more suit- able place for the continuance of their line. The regular production of Spring and Fall migrants is brought about possibly by the shorter periods of daylight in the earlier and later parts of the season. The final chapter of the aphid story opens in the fall and, like all last chapters done according to the rules, it contains the sequel to the plot and brings everything out right in the end. AIl through the spring and summer the aphid colonies have consisted exclusively of virgin females, winged and wingless, that give birth to virgin females in ever-in- creasmg numbers. A prosperous, self-supporting femi- nist dominion appears to be established. When summer's warmth, however, gives way to the chills of autumn, when the food supply begins to fail, the birth rate slack- ens and falls off steadily, until extermination seems to threaten. By the end of September conditions have reached a desperate state. October arrives, and the

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