PHOTOPERIODIC CONTROL OF DIAPAUSE 607 



Light Intensity (Experiment 4) 



The duration of the effective photoperiod experienced by Metri- 

 ocnemiis knahi in nature depends, of course, upon the minimum hght 

 intensity to which the larvae are sensitive. Experiment 4 was conducted 

 to test the effectiveness of intensities from 0.64 to 0.01 ft-c (see sec- 

 tion on Methods above). Four groups of 30 larvae each, at stations 

 A, B, C, and D, were given 12 hr of light at an intensity of about 40 

 ft-c followed by 1 Vi hr at the lower experimental intensities. A fifth 

 group, E, received the basic 12 hr of light (40 ft-c) plus Wi hr of the 

 very low intensity of light which scattered from the walls and ceiling 

 of the room where the experimental intensities were being adminis- 

 tered. This group was placed in a small uncovered box at station B, 

 but did not receive light directly from the mirror at that station. A 

 short-day control group in the same room received only the basic 

 photoperiod of 12 hr at 40 ft-c. Results of this experiment (Table II) 



Table II. Effect of Low Light Intensities:" 

 Experiment 4, Results after 40 Days 



<" 13H-hr photoperiod given: 12 hr at 40 ft-c plus IJ^ hr at experimental intensities. 



show that all experimental intensities were equally effective and that 

 pupations were obtained even in the scattered-light controls. Similar 

 experiments, data for which are not given, show intensities from 35 

 to 0.0025 ft-c to be equally effective. It is remarkable that some 

 pupations were obtained in the scattered-Hght controls, E, in Experi- 

 ment 4, the very low intensity of which we were unable to measure. 

 Apparently the intensity threshold below which no response is ob- 

 tained is extremely low and was approximated to some degree in the 

 scattered light received by group E. 



