QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 253 



The blowfly larvae move from place to place entirely by means of 

 muscular contractions. They proceed somewhat as follows : the anterior 

 end is raised, thrust forward toward one side, fastened to the sub- 

 stratum, and then the posterior end is pulled forward, after which the 

 anterior end is again raised and thrust forward, now toward the oppo- 

 site side, fastened, and the posterior end again drawn up. (I may add 

 here that the posterior end is not merely pulled forward, but is also 

 shoved ahead by the setting of the spines of the creeping pads.) The 

 anterior end is thus turned alternately toward the right and left quite 

 regularly during the process of locomotion. The extent of this lateral 

 movement 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. 



If the larva is subjected to lateral illumination, as it crawls 

 along in this manner, it may orient in one or two ways: it may 

 stop crawling, raise the anterior end more than in the usual ad- 

 vancing motion, and wave it from side to side, finally fixing it 

 away from the source of light and rapidly swinging the body into 

 alignment; or it may proceed without any interruption of loco- 

 motion and without the conspicuous waving of the anterior end,, 

 simply swing a little more widely away from the light than 

 towards it, as it extends the head first to one side and then to 

 the other in the normal crawling movements. The most char- 

 acteristic feature of the first method of orientation is the swaying 

 of the raised anterior end from side to side. The term 'wig- 

 wagging,' which Walter ('07) applied to a similar process in Pla- 

 naria may be used to characterize this form of reaction. The 

 swinging may be first toward the light and then away, or vice 

 versa; it may be a single movement, followed immediately by 

 locomotion in the direction in which the anterior end is fixed, 

 or it may be repeated several times before locomotion is resumed. 

 Whatever the details, and there are very many different individual 

 modifications of the behavior, the result is a sharp turning of the 

 anterior end away from the light, followed by the orientation 

 of the rest of the body. 



The function of the wigwag movement in orienting the larva 

 is not altogether clear. Holmes believed that the direction in 

 which the swinging first occurred bore no relation to the direction 

 from which the stimulus came, but that the swinging was rather 



THE JOUBN-A.L OF EXPERlME>fTAL ZOOLOGY, VOL. 17, NO. 2 



