WALTER E. GARREY 113 



The upper and lower parts of the eye show much more clearly the 

 control of different groups of muscles by different areas of the eye. 

 Experiments illustrating this were described by Axenfeld^^ who states 

 that when the upper parts of the eyes of flies were blackened the 

 head was elevated, and that it was depressed when the lower parts 

 were blinded. Holmes^^-^^ depicts marked postural changes when the 

 eyes of Ranatra are similarly blackened, or illuminated from above or 

 below. The results of our experiments on Proctacanthus are most 

 striking and suggested to the writer the similarity to those produced 

 by defects of the superior and posterior semicircular canals of 

 vertebrates. 



Efect of Blackening Lower Halves of Both Eyes. — The effects of 

 blackening the lower halves of both the eyes of Proctacanthus are 



Fig. 5. Robber fly with the lower halves of both eyes blackened. Body in 

 opisthotonus, head up, front legs extended. 



shown in Fig. 5. The posture is a striking one. The anterior and 

 middle pair of legs are in extreme extension and pushed far forward. 

 The anterior pair may be poised in the air. The anterior end of the 

 insect is lifted far up and back from the surface of the table, and the 

 head is tipped far back against the thorax so that the proboscis pro- 

 jects directly forward. The abdomen is concave dorsally in opistho- 

 tonus, and its terminal segments are pressed against the table. The 

 wings rest on the abdomen or to the side of it. 



When walking, these robber flies progress but slowly, and give the 

 impression of chmbing up into the air; they often poise resting on 

 the depressed wings, tip of the tail, and posterior legs. The tendency 

 to fly is pronounced and at the slightest disturbance these flies soar 



25 Axenfeld, Arch. ital. bid., 1899, xxxi, 370. 



