PHYSIOLOGY OF WING DEVELOPMENT IN APHIDS. Ill 



that a great number of alate forms would appear under a cer- 

 tain optimum temperature in the form he had been experi- 

 menting with, namely, Aphis avence. I have already shown 

 that the temperature had nothing to do with causing the ap- 

 pearance of the winged forms under a well-controlled environ- 

 ment. Ewing's result, like that of Woodworth on the effect of 

 dryness, was obtained under natural conditions, with perhaps 

 little attention paid to the chemical constituents of the host 

 plant. It is strongly doubted whether any winged forms would 

 have appeared, had they been reared under his optimum tem- 

 perature but on the host charged with any one of the non-wing- 

 developing substances. In fact, I believe that the appearance 

 of winged forms under his "optimum temperature" is due, not 

 so much to the direct action of heat as such, but indirectly to 

 either an increase in the solubility of the wing-developing sub- 

 stances or to a decrease of the non-wing-developing substances. 



Tannreuther (1907), who has made extensive observations on 

 the life history as well as the genetic cycle of Pterocomma salicis 

 and P. salicicola, says: "The prevalent idea that the development 

 of aphis is unstable and controlled directly by external conditions 

 is certainly very misleading especially in the idea that un- 

 favorable conditions or lack of food is a direct cause for the ap- 

 pearance of the winged and sexual forms. We find in the species 

 studied that just the reverse is true, and that the greatest number 

 of winged forms are found in the second parthenogenetic genera- 

 tion; here in some instances ninety-five per cent, may become 

 winged, especially those found on the rose in good condition 

 which furnished an abundance of food." Pterocomma bicolor 

 and P. rufurum found in the vicinity of the University of Cali- 

 fornia campus, however, reacted to the chemicals, but they were 

 peculiar in that they did not produce sexual forms at the sixth 

 generation. 



Now I come to the consideration of the question which was set 

 forth by Tannreuther (1907), to wit, "Why some of the hy- 

 podermic cells of the thorax begin to divide, emarginate and form 

 the adult wings in a few days." According to Anderson (1893) 

 and others there are two groups of salts, one that excites the 

 muscles and one that has little effect upon them. Our so-called 



