232 BRADLEY M. PATTEN 



light were stronger. The trails are nearly perfect mirror images 

 of each other. Although this deflection is first to the observer's 

 left and then to his right, it is in both cases toward the larva's 

 left, for the direction of its crawling was reversed between trails. 

 Such a response indicates an asymmetry in the neuro-muscular 

 mechanism of the larva. The right side must be either more 

 sensitive, or muscularly more active than the left for it gives a 

 greater response to the same amount of stimulation. This 

 asymmetry is by no means as uncommon as might be expected, in 

 fact a certain degree of asjanmetry is far more common than even 

 an approximately perfect balance of sensitiveness. Whether the 

 asymmetry is anatomical or physiological is a question which will 

 be taken up later. The point we wish to bring out here is the 

 method of eliminating the effect of such asjrmmetry, for it might 

 easily be a source of serious error. Many markedly asymmetrical 

 larvae were thrown out by the preliminary test (fig. 3, a). But 

 many of those giving a perfectly symmetrical response in the 

 test-trails showed marked as>Tnmetry when run in the balanced 

 lateral beams. The reason for asymmetry appearing in the 

 later records, when it was not shown in the test trails, I believe 

 to be this. In the case of the test trails, the larva is always 

 orienting to a single light, and when orientation is attained, the 

 anterior end of the larya is in the shadow of the posterior end. 

 If the direction of light is changed, the larva keeps turning till it 

 again crawls with its sensitive anterior end within its own shadow. 

 If the 'head' swings out of the shadow, it is strongly stimulated 

 and swings back again into orientation. Even if there were 

 not a perfect balance of sensitiveness on the two sides, the lack of 

 balance, unless it amounted to almost total insensitiveness of 

 one side, would not appear under these conditions, for if, as the 

 larva throws its anterior end from side to side in crawling, it 

 passes beyond the boundary of the shadow, the change of in- 

 tensity is sufficiently abrupt to produce a response on the less 

 sensitive as well as on the more sensitive side, thus holding the 

 animal within a course sharply limited by its own shadow. 



When on the other hand the larva is made to crawl in a field of 

 balance light, both lateral surfaces are illuminated, no matter 



