202 



WILLIAM BRODBECK HERMS 



The larvae were started in directive light from a drop of tap 

 water (slightly colored with Methylene blue), which afforded the 

 necessary moisture for crawling upon a sheet of paper. The indi- 

 vidual was given two to three centimetres in which to gain proper 

 orientation. This brought its extreme posterior end on the start- 

 ing line, from which the larva was timed by means of a stop-watch 

 over a distance of 10 cm. The course of these animals when once 

 properly oriented is nearly straight, and the time is based on con- 

 tinuous movement; larvae which paused in the course were 

 removed and not used again till after they had rested. This 

 measure was verj^ seldom necessary. The light intensity was cal- 

 culated for the middle point of the course, since the results in rate 

 will be more nearly in accord with the given intensity as an approx- 

 imate average. 



The tabulated results are based on the movements of ten 

 larvae, each given a single trial (except as above mentioned), 

 with a rest of from one half to one hour before exposure to the next 

 intensitj^ This method was pursued in order to guard against 

 excessive mechanical stimulation or exhaustion. No heat screen 

 was used, since no difference was found in average between the 

 rates of larvae in high intensities with and without a receptacle 

 of water interposed to absorb the heat. 



TABLE 6 



The relation between the intensity of directive light and the rate of movement of sarco- 

 phagid larvae, based on the time in seconds required for migrated individuals 

 to travel ten centimetres. The intensity of the light is calculated for the middle 

 point in the course. 



