230 BRADLEY M. PATTEN 



lights were shut off ; the mirror beams by shdes, and the opposite 

 orienting Hght by a switch. The larva, started straight by the 

 groove, was forced to move on to the paper in the direction of the 

 orienting hght, and so directly across the field of light from the 

 mirrors. When it was well on the paper, the slides were pulled 

 simultaneously from the two mirror beams, and at the same time 

 the orienting light turned off. This subjected the larva to equal 

 bilateral stimulation by the two beams of light, for the orienting 

 light brought it into the field with its median plane at right angles 

 to the line connecting the sources of light. Under these condi- 

 tions there should be no deflection toward either light, and 

 in fact there was none, for the larva continued in the direction in 

 which it was started by the orienting light (fig. 4, 6). At the 

 end of this trail the maggot was allowed to crawl into its box, 

 held edge down to the paper, then without being touched in any 

 other way, it was rolled into the groove in front of the opposite 

 orienting light (fig. 1, 6) with its anterior end toward the center 

 of the stage, and driven back across the field of light from the 

 mirrors. The lateral beams were then thrown on the larva again 

 and the second trail completed in the same way as the first. 

 After a rest, a second pair of trails (fig. 4, c) was run in the same 

 way, except that the order of running was reversed and the trail 

 from the farther orienting light run first. These four trails, 

 together with the test sheet, form the complete record of an 

 individual and the unit for compiling the tables. 



6. Elimination of asymmetry 



But the trails do not all conform so closely to the theoretical 

 response as those of figure 4. For example, those shown in figure 

 5 have a marked deflection under the same conditions of equal 

 bilateral illumination. A careful study of this set will show the 

 necessity of running the trails in pairs from opposite directions. 

 In the lower trail (fig. 5, 6), the larva deflected quite markedly 

 to the left when the side lights were turned on, exactly as if the 

 light from the right were stronger. But when its direction of 

 crawling was reversed, it deflected to the right as if the left hand 



