THE PHOTIC REACTIONS OF SARCOPHAGID FLIES 179 



accepted as conclusive evidence for chemotaxis with regard to food 

 conditions (fish). 



A reaction by half grown larvae of Lucilia caesar was noted and 

 described by Pouchet '('72, pp. 133-134) which is undoubtedly 

 analogous to the positive reactions under consideration. The 

 reaction is referred to as a "Perversion provoquee de la sensation 

 lumineuse." 



On the beach the entire mass of larvae can be caused to collect 

 at a point on the fish nearest the light from an acetylene lantern 

 but usually in touch with the fish through other individuals. 

 The behavior of the larvae on the beach depends on their age, 

 as was also observed in the laboratory, i.e., older individuals react 

 more slowly and sooner or later always return to the flesh, while 

 the younger may remain away from the flesh indefinitely. It 

 should be noted here that the feeding animals react in the manner 

 described throughout the feeding period, but not to the same de- 

 gree. Very young larvae, from hatching to several hours there- 

 after, do not react thus in an appreciable manner; the ages 

 between twelve and forty-eight hours are evidently influenced 

 most, for after two days the positive response becomes feebler until 

 migration, when the larvae are decidedly negative to any kind of 

 light. To strong diffuse daylight and sunlight they are always 

 negative. 



Experiment 3. July 28, 1906, 9:00 p.m. Larvae between 

 fifty-five and sixty hours old, still feeding within the body of a 

 fish on the beach, were drawn toward the light in an aggregated 

 condition. The body of the fish had been cut open along the abdo- 

 men and through this opening the larvae crawled out upon the 

 sand and along the edge of the fish toward the point of strongest 

 illumination, accumulating in that region. Moving the lamp 

 either toward the head or toward the tail of the fish caused the 

 mass to follow. The response was quite rapid, not more than five 

 to six minutes being required to shift the position of the entire 

 mass over a space of ten to twelve centimetres. The lamp was 

 left in place twenty-five minutes ; on returning at the end of that 

 time, it was found that the animals had all retreated to the origi- 

 nal position within the fish. 



