476 ALFRED O. GROSS 



glowers for the yellow, and three glowers for the green and for 

 the blue lights. The finer adjustments of the intensity were 

 accomplished by regulating the size of the diaphragms until each 

 light gave the same reading on the radiomicrometer, that is, the 

 lights were made to contain the same amount of radiant energy. 

 When the lights from each generator were thus equalized any 

 consistent difference in the responses of the animals to the dif- 

 ferent colors was believed to be due not to the quantity of the 

 light but to its quality. 



4. OBSERVATIONS 



A. Calliphora erythrocephala Meigen (larva) 



Because of the reversal of their phototropism during develop- 

 ment, certain species of flies such as the blow-flies are very inter- 

 esting from the standpoint of their reactions to light. When 

 the blow-fly larva first emerges from the egg, it is either indiffer- 

 ent, only slightly negative, or, as Herms ('11, p. 177) has shown 

 for aggregate larvae, even positively phototrophic to hght. As 

 it grows, it becomes more and more j:"esponsive to directive light 

 and by the time the feeding period is ended, it is very strongly 

 negative in its response to light. When the adult fly emerges, 

 it is no longer negative but strongly positive. This complete 

 reversal of its phototrophic behavior is closely correlated with 

 the habit of the animal. 



1. Material. The blow-fly, Calliphora erythrocephala Meigen 

 is easily reared in the laboratory throughout the year and hence 

 it is excellent material for experimental purposes. For the inves- 

 tigations about to be described the larvae were kept in culture 

 jars which were constructed by placing large lamp chimneys on 

 earthenware plates of suitable size, and filling them with about 

 two. or three inches of moist sand. This sand served to absorb 

 the excess of liquid from the food. Codfish heads were found 

 to be a conyenient and desirable food for the larvae. The heads 

 were renewed every few days in order to keep the cultures in 

 the best condition. After the adult flies had deposited their 

 eggs on a piece of fish, the latter was placed in a culture jar 



