66 THE PHYSIOLOGY OF INSECT SENSES 



Other monitoring of the movements of the wings occurs as a result 

 of stimulation of wing receptors. As many investigators (e.g., Vogel, 

 1911, 1912; Lehr, 1914; Erhardt, 1916; Eggers, 1928; Hertwick, 

 1931; Zacwilichowski, 1936) have shown, the wings of insects are 

 elaborately equipped with sense organs (Fig. 44). These are sensilla 

 trichodea, sensilla campaniformia, and chordotonal sensilla. They 

 are strategically situated to respond to wind on the surfaces of wings or 

 to forces acting on the veins. With the exception of one chordotonal 

 organ, all sensilla are located distal to the hinge; consequently, they 

 will be subjected to strains and distortions set up by aerodynamic and 

 inertial torques, but not elastic torques (Pringle, 1937). Several 

 investigators have shown by electrophysiological monitoring that 

 considerable information is fed back to the central nervous system by 

 these sense organs (Sotavalta, 1954; Wolbarsht and Dethier, 1958; 

 Wilson, 1961). 



The sensilla trichodea are clearly tactile. Although Pringle (1957) 

 did not exclude the possibility that they react to air flow, he suggested 

 that this is improbable, because the small fibre diameter precludes 

 rapid enough impulse conduction to meet the demands of quick flight 

 reflexes. On the other hand, it is known that these hairs mPhormia fire 

 at great speed and adapt very rapidly (Wolbarsht and Dethier, 1958), 

 hence might respond to turbulence. The loss of control during the few 

 wing-beats necessary to allow time for nervous conduction might be 

 inconsequential considering the high rate of wing-beat in most insects. 



The sensilla campaniformia are distributed singly and in groups. 

 The former are usually circular, hence not markedly directional in 

 their sensitivity. Grouped sensilla are usually oval. The direction of the 

 long axis is uniform within a group but differs from one group to 

 another. As Pringle (1938 b) has demonstrated, each group constitutes 

 a unitary organ selectively sensitive to strains whose compression axis 

 is oriented parallel to the direction of the long axis of the sensillum 

 dome. 



The location of each group is such that the sensilla are maximally 

 sensitive to a particular direction of torque in the vein wherein they He. 

 Pringle (1937) has discussed at length the probable mode of action 

 of these sensilla in relation to the torques acting on the wing during 

 flight. Probable modes of action of the chordotonal organs have been 

 deduced with less certainty (Pringle, 1937). 



Any flying machine must possess stability, that is, a tendency to 

 return to a characteristic attitude when displaced if it is to be con- 

 trollable in the air (Pringle, 1950). In this respect insects are able to 



