176 LIGHT AND THE BEHAVIOR OF ORGANISMS 



source of light is changed, the larvae turn immediately and 

 directly from the light in its new position until both sides 

 are again struck by the rays at the same angle. 



Holmes' observations of the orienting reactions of fly 

 larvae do not support^ Loeb's theory. He found among 

 other things (1905, p. 105), " If a strong light is thrown 

 upon a larva from one side it may swing the head either 

 towards or away from the light ... In the animals here 

 described there is, so far as I can discover, no forced orien- 

 tation brought about by the unequal stimulation of the 

 two sides of the body, but an orientation is produced 

 indirectly by following up those chance movements which 

 bring respite from the stimulus. I do not deny that there 

 may be an orienting tendency of the usual kind, but if 

 there is it plays only a subordinate role in directing the 

 movements of the animal. The orientation of these forms 

 is essentially a selection of favorable chance variations of 

 action and following them up." 



b. Locomotion. — The blowfly larvae move from place 

 to place entirely by means of muscular contraction. They 

 proceed somewhat as follows: the anterior end is raised, 

 thrust forward toward one side, fastened to the substratum, 

 and then the posterior end is pulled forward, after which 

 the anterior end is again raised and thrust forward, now 

 toward the opposite side, fastened, and the posterior end 

 again drawn up. The anterior end is thus turned alter- 

 nately toward the right and left quite regularly during the 

 process of locomotion. The extent of this lateral move- 

 ment varies much, but it is usually great enough so that 

 the extremity of the anterior end is nearly at right angles 

 to the direction of locomotion (see Fig. 31). 



In drawing forward the posterior end the whole body 

 contracts, but the contraction is greater on the ventral than 

 on the dorsal surface, forming an arch, in which the extreme 

 anterior end is nearly vertical and the sensitive tip (Fig. 30) 

 well drawn under so as to be hidden from view. Thus the 

 tip of the anterior end becomes alternately thrust out and 



