VERMES, FLY LARVAE, AND ECHINODERMS 189 



is exposed being due to change of intensity caused by the 

 extension and contraction of the anterior end. 



i. Mechanics of orientation. — Holmes (1905, p. 105) 

 says, " If a strong light is thrown upon a larva from one 

 side it may swing the head either towards or away from 

 the light," intimating that it is turned in one direction as 

 often as in the other. I exposed various individuals to sud- 

 den lateral illumination by direct sunlight, or light of nearly 

 equal intensity from the three glowers, at different times 

 and recorded the direction in which the anterior end turned. 

 The results appear in Table V. It will be seen by referring 



TABLE V 

 Number of times anterior end is turned 



to this table that in all there were 177 turns from the light 

 to 165 toward it, i.e., nearly the same number in both 

 directions. Later however I obtained results very different 

 from these. They are recorded in Table VI. The results 

 recorded in Table VI show that when a larva is first exposed 

 to intense unilateral illumination, it turns toward the source 

 of light practically as frequently as from it, and orientation 

 is indirect, but that after being exposed for some time it 

 turns considerably more often from the source of light than 



