98 H. E. EWING. 



Sources of Error. Although there were important changes in 

 humidity during these constant temperature experiments, yet, as 

 has already been stated, Headlee has shown that changes of 

 humidity from 37 per cent, to saturation are without effect upon 

 the metabolism of plant lice. As for light changes, it is probable 

 that metabolism was to some extent affected by these, but ex- 

 perience with other insects has shown that unless these are quite 

 marked (e. g., bright sunlight or total darkness) the effects are 

 not great. There was no change in the food, unless perhaps the 

 changes in temperature themselves changed somewhat the nature 

 of the food derived from the wheat shoots. 



The results obtained showing the effects of the four constant 

 temperatures employed; 60 F., 70 F., 80 F., and 90 F., are 

 given in tabular form in Table XI. 



1. As to Growth. At 60 F. it took the winged forms 14^ days 

 to reach maturity (this being the average for all individuals 

 reared). No wingless forms were produced at this temperature, 

 so that we have no data upon the growth period for this form at 

 this period. At 70 F., the wingless forms reached maturity 

 in 6 days, the winged in 9^2 days. At 80 F., the wingless forms 

 reached maturity in 7 days, the winged in 7^ days. At 90 

 F., no nymphs of the winged forms had reached maturity in 

 12 days. No nymphs of wingless forms were observed at this 

 temperature, and all the individuals used died before reaching 

 maturity. The optimum temperature for growth, therefore, 

 must be different for the two forms. For the wingless forms it 

 probably is a little above 65 F. as before stated, but in the case 

 of the winged forms is somewhere near 80 F. 



2. As to Size. Only two records for the size of adult wingless 

 forms were obtained. The mean for those reared at 70 F. was 

 1.155 mm., and for those at 80 F., 1.246 mm. Both these 

 means are below the mean of the pure line stock from which 

 the mothers were taken, and far below the mean! Can it be that 

 constant temperatures, no matter at what degree, have a dwarfing 

 effect on plant lice? Does the changing of temperature stimulate 

 growth and metabolism in general? These are interesting prob- 

 lems for further investigation. The results obtained in this 

 experiment appear to show that such dwarfing actually does take 



