PHASES OF LOCUSTS 303 



that of the background of the cages in which they are reared. 

 Hertz and Imms (1937) have explored this subject further and 

 analysed the relation between this phenomenon and the spectral 

 and other qualities of the light rays that are involved. It was 

 also shown by Faure that green nymphs are produced as the result 

 of humid conditions and an abundance of moist green food 

 irrespective of the nature of the background. In the production 

 of the green nymphs, Hertz and Imms came to the conclusion 

 that high humidity alone is involved and that the threshold 

 of humidity in this connection appears to differ for different 

 individuals. Below this threshold green forms are not produced 

 and the insects assume a coloration in conformity with the nature 

 of the background. A green background, however, produces 

 yellowish forms and never green. It would appear that this 

 faculty of colour adaptation may serve a protective function of 

 rendering the developing insects less conspicuous in their environ- 

 ment. The green nymphs in a state of nature, it may be added, 

 occur among an abundance of green herbage when the humidity 

 would presumably be relatively high. Physiological differences 

 between the two extreme phases have not so far been explored 

 in any detail. Butler and Innes (1936), however, have shown that 

 the oxygen uptake of individuals in the gregarious phase is 

 markedly greater than those in the solitary condition. Strel'nikov 

 (1936) states that careful body-temperature readings indicate that 

 the black and orange hoppers of the gregaria phase absorb more 

 radiant heat than individuals (green) in the solitaria phase. In 

 the shade the body temperature of hoppers of both phases was 

 more or less alike and slightly above that of the surrounding 

 air. After six minutes in the sun the temperature of gregaria 

 rose by 15° C. and that of solitaria by only 10-2° C. On being 

 shaded, gregaria individuals lost heat more rapidly than solitaria. 

 Under crowded conditions the nymphs become extremely 

 active and this naturally affects the rate of metabolism of the 

 organism. Faure believes that more excretory products are 

 produced than are liberated by the Malpighian tubes and that 

 these, or their derivatives, become deposited in integument, thus 

 imparting the black and orange coloration that is an invaluable 



