236 SOME ASPECTS OF ECOLOGY 



to evaluate owing to its influence upon movement. A number of 

 observers have recorded an increase in CO2 production under its 

 influence as compared with darkness, the increment in most cases 

 being attributable to the stimulating effect of light upon move- 

 ment. In the case of the larvae of Drosophila-this same explanation 

 may hold good and excess of movement thus caused may result 

 in retarded development. It would seem desirable that critical 

 experiments, upon the influence of light, be carried out with 

 reference to eggs and pupae in which conditions of standard 

 metabolism prevail. 



It is very questionable whether light, even when of high 

 intensity, is ever the cause of death in insects. Closely connected 

 is the problem of radiant heat, and there is good evidence that the 

 death of insects under direct sunlight is due to this cause, or to 

 its effect on water loss from the body. 



The effects of the daily duration of light upon the production 

 of different types of reproductive forms in aphides have been 

 investigated by Marcovitch (1924), and by Davidson (1929). 

 They have shown that the incidence of the sexuales generations 

 appears to be influenced by length of day under the conditions of 

 their experiments. By reducing the normal amount of daylight 

 per diem the production of sexual forms was induced at times of 

 the year when they do not normally occur. Conversely, by 

 extending the duration of daylight, asexual viviparous 

 reproduction results during times of the year when only sexual 

 reproduction normally prevails. The influence of light in such 

 experiments appears to be important, but it is not a clearly 

 defined result, owing to its effect on the host plants upon which 

 the aphides were feeding. Also, the factor of temperature 

 cannot be entirely excluded. 



Very similar results have been obtained more recently by 

 Wadley (1931), who found that in the aphid Toxoptera grarninum 

 sexual forms can be induced to appear under defined experimental 

 conditions. These conditions involved the shortening of the days 

 to less than twelve hours and temperatures averaging below 22° C. 

 No other conditions affecting their appearance were noted. 



Early experiments, on the differential effects of limited parts of 



