VERMES, FLY LARVAE, AND ECHINODERMS 185 



soaked In water and the plate so arranged that the edges 

 of the filter paper which projected over the edge of the 

 glass plate extended into water kept in a vessel below. In 

 this way a smooth surface containing a constant amount 

 of moisture was produced. It was found that such condi- 

 tions are very essential in quantitative work with the larvae, 

 especially that of constant moisture, since their rate of loco- 

 motion depends much upon the amount of moisture in the 

 surface upon which they crawl; either too much or too little 

 causes marked retardation. Two fine white threads were 

 placed parallel with each other on the paper 15 cm.^ apart, 

 so as to form a definitely limited course upon which to try 

 the speed of the larvae under different light conditions. A 

 horizontal beam of light 4 cm. wide was projected from a 

 Nernst glower upon the glass plate perpendicular to the 

 threads. The light in this beam halfway between the 

 threads was 7 ca. m. in intensity. The time it required a 

 given larva taken from the culture jar kept in total dark- 

 ness to travel the distance between the threads was accu- 

 rately ascertained by means of a stop watch. At the end 

 of the course the larva was allowed to crawl onto a piece 

 of black paper supported on a section lifter and then trans- 

 ferred to the starting point without changing its orientation, 

 and allowed to continue on its course w^ith the least disturb- 

 ance possible. After having ascertained the time required 

 to crawl 15 cm. in the beam of 7 ca. m. intensity, the plate 

 was turned through an angle of 90° so as to expose the 

 larva in a similar beam of light but one of a much higher 

 intensity. The intensity of this second beam of light in 

 the middle of the course was 3888 ca. m. It was produced 

 by a group of three Nernst glowers so arranged that a cross 

 section formed a small triangle. When the current was on, 

 the three glowers appeared much like a highly illuminated 



^ Through some oversight I failed to record the distance between the 

 threads. I am not quite positive whether it was 15 cm. or 10 cm. This 

 however does not invalidate the results recorded in the following table since 

 they are comparative in every case. 



