226 BRADLEY M. PATTEN 



change the reactions of the larvae in no way. The number of 

 the box in which the larva was kept, in combination with the date 

 of the experiment,^ made an identification number for all the 

 trails of an individual larva. Thus the whole series of records 

 was made permanent and can be referred to at any time. 



The trails of the standard test were made in the following 

 manner. A maggot was placed in front of the orienting light 

 (fig. 1, a), which forced it to crawl toward the center of the stage 

 in a line perpendicular to the ray direction of the mirror beams. 

 When it was well on to the stage, the orienting light was turned 

 out and at the same time the side light (fig. l,/0) thrown on, 

 thus subjecting the larva to strong illumination from the left. 

 The lateral illumination caused the larva to turn to its own right. 

 After this trail had been completed, the side light was turned off, 

 and the larva at once placed at the further end of the observation 

 plate, where it was again started across the path of the mirror 

 beams, this time under the influence of the opposite orienting 

 light (fig. 1, b). The same side light (fig. l,/0) was then turned 

 on, but as the larva was crawling in the opposite direction, the 

 light now operated on the right photo-sensitive area, producing a 

 turning again toward the observer's right, but toward the larva's 

 left. 



Figures 2 and 3 are photographs of actual trails made in this 

 way, except that the methylen blue trail has been blackened with 

 India ink to show better in the photographs. The sharpness of 

 the bend in such a pair of trails is an index of the light sensitive- 

 ness of the larva, the symmetry of the curves, an index of the 

 photo-sensitive balance. Only those larvae orienting to the new 

 direction of light so accurately that both trails come to lie ap- 

 proximately parallel to the direction of the raj^s, were used in 

 making the records compiled in tables 2 to 5. The amount of 

 variation within the limits of accurate orientation, according 

 to this criterion, is shown in trails a, b and c of figure 2, all of 

 which meet the standard requirement of sensitiveness. Though 

 there is a clearly marked response to the new direction of light 



' The method of writing dates employed in these records is the day, month, 

 year, notation; thus 17/6/' 13 is the 17th day of June, 1913. 



