356 



ROBERT S. MCEWEN. 



fly was to crawl. Each tube measured 16 mm. in internal 

 diameter, and the length from the end away from the light to 

 the point where the trial terminated was 172 mm., this being 

 identical with the distance employed by Cole. Beyond this 

 point the tube continued for 70 mm. to the end, which was 



14 16 IS 00 



Time in Seconds. 



GRAPH I. The solid line shows the record, corrected for use of wings, of 95 

 wild flies, 45 males and 50 females, in an unlined vertical tube. The broken linet 

 shows the record, corrected for slipping, of 92 vestigial flies, 45 males and 47 females 

 also in an unlined vertical tube. In each case the time is plotted against the number 

 of insects. 



sealed with a piece of cover slip. A 75 watt gas filled lamp on 

 a no-volt alternating current was used as the source of illu- 

 mination, and the distance from the end of the tube furthest 

 from the light to the center of the concentrated filament of the 

 lamp was approximately 317 mm. At the point from which the 

 fly started, therefore, it was subject to an intensity of about 

 1,238 candle meters, an intensity slightly less than the maximum 

 of 1,500 candle meters used by Cole. At the beginning of a test 

 an insect was transferred from its vial to the testing tube by 

 making use of its light or gravity reaction, or when necessary 



