THE PHOTIC REACTIONS OF SARCOPHAGID FLIES 189 



To secure tracings of the paths of larvae, several methods were 

 tried, of which the most satisfactory proved to be that of starting 

 the larva off from a drop of very weak aqueous solution (| to 

 J per cent) of methylene blue, whereby a trail was made on paper 

 by the creeping larva. By comparison with trails made with the 

 use of tap water, it could be seen, as might have been expected 

 from the weakness of the solution, that the methylene blue had 

 no disburbing effect. The larvae were started in a drop of the fluid 

 enough adhering to them to leave a distinct track on the white 

 paper, portraying nearh^ every movement made, except when 

 the head was raised high. White paper did not give results 

 different from those obtained with black paper, except when used 

 in the light grader with light from above. In that case reflection 

 was eliminated by means of a diaphragm, which was removed 

 when the larva entered upon the edge of the field. 



3. Reactiveness and age 



Pouchet (72, p. 312) suggests that the larva avoid the hght im- 

 mediately upon hatching, but that they are not able to react to 

 its direction. This function develops progressively as the indiv- 

 dual grows older and does not attain full force until the larva has 

 completed its growth. This seemed to be an important matter 

 to decide definitely, since all conclusions must in the end be based 

 on the reactiveness of the individual at a given period in its life 

 history. 



To test this it was decided to use the low-intensity apparatus 

 with an intensity of 0.56 CM. In this way reflection can be largely 

 eliminated, and by means of a low intensity of light are avoided 

 such matters as heat and overstimulation both of which produce 

 an "excited" state of the organism. The feeding larvae were per- 

 mitted to crawl on a glass plate well smeared with flsh and re- 

 moistened frequently in order to guard against the possibility 

 that one individual might follow the trail of another. By examina- 

 tion of the data it was evident that successive trails were seldom 

 alike, and my observations satisfied all doubts in the matter. The 

 larvae must either be moist themselves or creep on a moist sur^ 



