THE PHOTIC REACTIONS OF SARCOPHAGID FLIES 



187 



The white thread could be plainly seen on the black background. 

 The individual larvae were placed in the central square E6 as 

 gently as possible by overturning the small pastboard box in which 

 each was kept. The intensity calculations were made for the cen- 

 ter of this square EQ. By noting the square occupied b}^ the larva 

 at the end of a given period (e.g. thirty seconds), its movement 

 with reference to the light could be easily ascertained and recorded. 

 As illustration, a case may be cited as follows. The larva was 

 placed in the square EQ and the dark box was quickly closed ; after 

 thirty seconds by the stop watch the box was opened and at a 



Fig. 1 Plan of low intensity apparatus. A, chamber containing 7.2 cp. incan- 

 descent lamp (L) and a vertical white screen (CD); B, a second dark chamber 

 containing a glass stage upon which rests the reading frame E; F, diaphragm 

 through which the light from L passes into chamber B after reflection from the 

 screen CD. 



glance it was seen that the larva was in square G4. This shows 

 that the larva had travelled away from the light in a slightly diag- 

 onal direction. On the other hand, had the larva been found in 

 E9 it is evident that a direction toward the light would have been 

 taken. 



Another piece of apparatus designated as a light grader by Mast 

 ('06, pp. 363-366.) and also described in detail by him was con- 

 structed and applied with the results as set forth later in this 

 paper. A single Nernst filament was used on a voltage of 220, 

 which gave a light of 70.69 CM. at the principal focus of the 



