I04 Journal of Comparative Neurology and Psychology. 



Dr. LoEB does not discuss just how the rays of light orient 

 the body of the fly larvae, so we are left to suppose that he 

 would adopt the same theory of orientation in this case that he 

 has applied to other forms, although from his account of the 

 movements of these larvae it would almost seem that he had in 

 mind the method of orientation that we have found to be follow- 

 ed by the angleworm and leech. Observations which I have 

 made upon the phototaxis of blow-fly larvae with the problem 

 of orientation especially in mind soon convinced me that the 

 movements of these forms are directed by light through follow- 

 ing up those random movements which bring them away from 

 the stimulus. In normal locomotion the fly larva raises the an- 

 terior third of its body, extends it and places it down upon the 

 surface to which it adheres; then the posterior end of the body 

 is pulled up, and the anterior end extended again, and so on. 

 At each contraction the ventral part of the posterior portion of 

 the body is brought forward more than the dorsal and is held in 

 place by means of small projections on the surface while the an- 

 terior end is being extended, thus giving the larva a forward im- 

 petus. Successive waves of contraction pass from the posteri- 

 or end and as they reach the anterior part of the body they ex- 

 tend it at the same time that it is raised. The locomotion is a 

 sort of combination of ordinary vermicular movements and the 

 looping movement which is found in the crawling of the larvae 

 of the geometrid moths. As the larva progresses the anterior 

 part of the body is often swayed from side to side. When the 

 head is put down, say to the right, the rest of the body is pull- 

 ed along in the same direction. The source of progressive lo- 

 comotion is mainly in the posterior half of the body, the anter- 

 ior portion being used mainly to set the direction of movement. 



When strong light is thrown on a fly larva from in front, the 

 anterior end of the creature is drawn back, turned towards one 

 side, and extended again. Often the head is moved back and 

 forth several times before it is set down. Then it may set the 

 head down when it is turned away from the light and pull the 

 body around. If the head in moving to and fro comes into 

 strong light it is often retracted and then extended again in some 



