594 BRADLEY M. PATTEN 



maturer individuals, larvae two days old gave extremely vacillat- 

 ing trails (fig. 2, B). 'Wig-wag' movements of the anterior end 

 were frequent and very pronounced. The larvae at this age 

 were also noticeably more sensitive than fully grown larvae in 

 their reaction to moisture. The tendency to follow moist trails 

 was not infrequently strong enough to throw them out of 

 orientation to the light. In making the records of their light 

 reactions, precautions were taken that this disturbing factor 

 should be eliminated. 



On the third day the trails were made with less hesitation than 

 on the second day, though the larvae still made noticeably more 

 ' wig-wag' movements and more changes of course then when fully 

 grown (fig. 2, C). 



By the fourth day the maggots had for the most part attained 

 their full growth. Their reactions were more rapid and much 

 more decisive. To put it anthropomorphically, 'Hhey seemed 

 to have very definite ideas as to the direction in which they meant 

 to travel" (fig. 2, D). 



After the fourth day the wig-wag movements were still further 

 decreased in frequency and extent, as is shown by the smooth- 

 ness of the trails photographed in figure 2. There was also a 

 noticeable tendency to move more deliberately and to respond 

 less quickly to the change in light direction. The curves of figures 

 3 and 4 indicate quantitatively the decline in photoreactivity 

 taking place between the fourth or fifth day and pupation. 



Just how much of the rise in the curve of reaction (fig. 3) dur- 

 ing the first four days is due to increased facility of crawling or 

 better coordination between the photoreceptors and muscular 

 system, and how much is due to actual increase of sensitivity, it 

 is impossible to say with certainty. I believe, however, that 

 after the first day the muscular movements are discharged with 

 enough apparent facility to justify the interpretation that the 

 sensitivity of the larvae to light is really increased between the 

 second and fourth day. There seems to be still more reason to 

 believe that the drop in reactivity after the fourth day is due to 

 decrease in sensitivity. The reactivity falls off during the so- 

 called migratory period, when the larvae leave food and wander 



