Mechano- and Equilihrhun-Reception 



525 



the haltere oscillates about axis AA, the line BB will tend to move into the 

 position yy, due to a torcjue which develops as a result of the displaced 

 center of gravity. This torque is discontinuous, fluctuating at twice the 

 frequency of the haltere oscillation, but the damping of the fluid contents 

 averages out these oscillations. If the haltere oscillations end abruptly the 

 backward movement of the haltere can be clearly seen: it has a delay of 

 about Vz sec. Thus, during vertical oscillation of the haltere, forces exist 

 which move the haltere forward. It is this movement of the haltere that is 

 significant in orientation, for it is affected by movements of the organism 

 about its transverse and longitudinal axes (pitch and roll) and its vertical 

 axis (yaw). 





Fig. 194. Tracings from flash photographs of a haltereless fly, Eristalis tenax, in free 

 flight. The interval between exposures is Vk sec. Note the instabihty in the horizontal 

 or yawing plane. From Pringle." 



The electrical recording of action potentials in the nerve innervating the 

 haltere sense organs, under conditions of normal flight, pitching, rolling, and 

 yawing, indicates considerable differences. Oscillations of the halteres with 

 normal attitude result in the appearance of a burst of impulses for each 

 half cycle of oscillation, presumably occurring at the maximum and min- 

 imum of each excursion. There is a period of inactivity between each 

 burst of impulses. During rotation in the rolling plane the nerve activity 

 is augmented, but not to the extent that it is by movement in the yawing 

 plane. Presumably the altered nerve discharge pattern produces reflex 



