114 HELIOTROPIC MECHANISM 



upward and backward striking the cover of a confining glass dish or 

 looping the loop backward. Upon alighting they fall on their backs 

 and right themselves only by turning a backward somersault. If the 

 lower parts of the eyes are unequally blackened, there is a combina- 

 tion of these effects with the tendency to circle toward the eye with 

 more surface exposed to the light. 



Effect of Blackening Upper Halves of the Eyes. — The resting attitude 

 assumed by Proctacanthus with the upper halves of both eyes black- 

 ened is exactly the opposite of that just described, as may be noted in 

 Fig. 6. The body of the insect is in emprosthotonus. The abdomen 

 is concave ventrally and there is a considerable angle between the 

 wings and the dorsum of the abdominal segments, due to the ventral 



Fig. 6. Robber fly with the upper halves of both eyes blackened. Body in 

 emprosthotonus, head down, anterior legs flexed. 



flexion of the thorax. The head too is bent down with the crown for- 

 ward and the frontal aspect pressed toward the table. The anterior 

 legs are flexed, while the posterior legs by a combination of flexion 

 and extension of dift'erent segments, tilt the thorax upward and forward 

 so that in some cases the robber fly literally stands on its head. Such 

 an animal when it creeps forward strikes the head against the slight- 

 est obstacle and is precipitated in a forward somersault. In attempted 

 flight it is not able to hft the body and crowds forward and down 

 against the surface of the table, while if thrown into the air, it is 

 precipitated downward in a forward somersaulting flight to the floor, 

 where it lands on its back. In this position the fly is almost helpless, 

 being able to right itself only if its struggles make it favorable to turn 

 sidew^avs; otherwise its futile eft'orts result in a ventral flexion of the 



