106 H. E. EWING. 



in plant lice temperature as well as nutrition is the cause of 

 fluctuations in body length. The effect of temperature on the 

 variations in body length is well shown in Fig. 17. The large 

 frequency polygon represented by the heavy broken line at the 

 top can be analyzed into three components representing re- 

 spectively the frequency polygons for the first score, second 

 score, and last four generations of isolation II. The broken 

 line describes the frequency polygon for the first score of genera- 

 tions where no control was made over temperature conditions. 

 The light unbroken line describes the frequency polygon for the 

 second score of generations where the temperature was so 

 regulated about the optimum for the development of wingless 

 forms that only wingless forms appeared . The dotted line toward 

 the bottom represents the frequency polygon for the last four 

 generations where the individuals were reared on much older 

 wheat plants. 



In studying the curves for the first and second scores of genera- 

 tions we note no change of the mean for body length which in 

 both cases is a little less than 1.65 mm., but by observing the 

 bases of these two polygons we find the extremes of variation in 

 body length to be much greater in the case of the one for the first 

 score of generations when the extremes of temperature were also 

 much greater. 



When a change was made in the nature of the food (temperature 

 being left the same) we find a shifting of the mean (see lower 

 polygon in Fig. 17) for body length. It no longer is near 1.65 

 mm., but is between 1.35. and 1.45 mm. 



But fluctuating variations were not the only kinds observed 

 in the pure line of Aphis averuz. Some abrupt variations were 

 also observed. These abrupt, or discontinuous, variations were, 

 however, very rare in the pure line of the grain aphis. I found 

 only four thai were quite noticeable. One of these was a drop- 

 ping out of the branch of the sector of the third discoidal vein 

 in the anterior wings of the winged form, another the dropping 

 out of the sector itself; the third, an addition of one or more 

 cross veins to the bifurcating branches of the sector of the third 

 discoidal vein; the fourth, the complete arrestment of develop- 

 ment of the individual during the third or fourth nymphal stages. 



