LIGHT 235 



ment and barometric pressure. Shelford, in America, has devoted 

 a good deal of attention to this subject and has adopted the 

 method of measuring evaporation from an exposed surface of 

 water by means of a porous cup atmometer. Several types of 

 the latter instrument have been devised and are described in 

 Shelford's recent work on ecology (1929). By use of an atmometer 

 he conducted experiments on the reactions of insects to air of 

 different evaporating powers. It was found that certain species 

 reacted to air of similar evaporating capacity irrespective of 

 whether humidity, air currents or temperature was the chief 

 controlling factor influencing the rate of evaporation. The 

 atmometer is probably the most convenient instrument so far 

 devised for giving a single measurement of the combination of 

 factors that are involved, but, at the same time, it provides little 

 or no information as regards any individual effects. When 

 critical analyses of the factors involved are desired it is necessary 

 to take measurements of wind, temperature and humidity at the 

 same time. 



Light 



Almost all experimental work dealing with the effects of light 

 on insects is in relation to their sensory behaviour and not to its 

 influence upon general metabolism and growth. Northrop (1926) 

 is one of the few experimenters who have investigated the effect of 

 light intensity in relation to growth ; he tested this factor with 

 reference to the duration of the larval and imaginal instars of 

 Drosophila, using cultures which had been previously kept in the 

 dark for 200 generations. The source of light used was a con- 

 centrated filament Mazda bulb immersed in a vessel of running 

 water, and its intensity was measured with a contrast photometer 

 against a standard Hefner amyl acetate lamp. At intensities 

 around 2,500 metre candles the duration of th- larval life was 

 slightly reduced, but it became increasingly lengthened at higher 

 intensities until they were killed at 7,000 to 10,000 metre candles , 

 while the pupae were killed at 5,000 metre candles. Above 1,000 

 metre candles the duration of the imaginal life was rapidly 

 shortened. The actual effect of the action of the light is difficult 



