WALTER E. GARREY 



107 



possible movement is one in which the flexed legs pull, and the ex- 

 tended legs push the animal toward the illuminated eye. The legs 

 on the side of the illuminated eye cannot be widely separated nor can 

 those of the other side be easily approximated, thus tending to produce 

 a wider arc of progression on the side of the blackened eye. Coupled 

 with the asymmetrical position of the legs there is a tilting of the 

 whole body toward the side of the unblackened eye, so that the legs 

 on that side may be pressed to the table (Fig. 2). That the body 

 muscles are also involved in this tilting is probable, for in some indi- 

 viduals, especially with strong illumination, the muscles of the 



Fig. 2. 



Fig. 3. 



Fig. 2. Robber fly with right eye blackened, seen from above as in Fig. 1. 

 Body tilts to the left, head rotated still farther to left. Left legs flexed, right legs 

 extended so that, in moving, the fly circles to the left only. 



Fig. 3. Robber fly with right eye blackened, viewed from the front. Posture 

 as in Fig. 2, 



abdominal segments are contracted on the side of the unblackened 

 eye, thus producing a sHght bending of the abdomen with its concavity 

 toward that side. The head is also twisted, with the crown to the 

 left, farther than can be accounted for by the tilting of the body 

 (Fig. 3). This means that the head is actually rotated on the long 

 axis of the body; at the same time the head is also turned with the 

 blackened eye advanced, thus opening the angle with the front of the 

 thorax on that side. Measurement of the relative tilt of the body 



