INSECTS 



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- 

 mum 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 is 

 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 rise 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. 



All 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- 

 creasing 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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