68 THE PHYSIOLOGY OF INSECT SENSES 



The undifferentiated papilla may be sensitive to all strains in the 

 cuticle of the base. 



The beat of the two halteres is synchronized. They are oscillating 

 masses which generate forces at the base of the stalk as the whole fly 

 rotates (Pringle, 1948). They are thus gyroscopes, but it is questionable 

 that they act as direct stabiHzing gyroscopes (Schneider, 1953). It is 

 more likely that their action is indirect, in that the activity of their 

 sense organs signals the nervous system to set up the necessary correc- 

 tions in the flight mechanism (Pringle, 1957). There is convincing 

 experimental evidence that they are instrumental in the control of yaw 

 (Pringle, 1948; Faust, 1952; Schneider, 1953), pitch (Faust, 1952), and 

 roll (Faust, 1952). 



Finally, in the control of flight there are visual stimuli (e.g., Schaller, 

 1960). Movement in the visual field is employed directly by Calliphora 

 in incorrecting yaw (Schneider, 1956) and roll (Faust, 1952 ; Schneider, 

 1956). Vision is also employed indirectly (Antrum and Stocker, 1952; 

 Mittelstaedt, 1950) to correct roll, as the following example of dragon- 

 fly (Anax) behaviour illustrates. 



The head is a broad, heavy object, and rotations of the body produce 

 movement in the neck region. The position of the body relative to the 

 head is signalled by two pairs of prothoracic hair plates. When the head 

 of a flying or stationary dragonfly is twisted there is a compensatory 

 twisting of the wings. In flight this corrects rolling; when the insect is 

 perched it assists in the maintenance of balance. In nature the insect 

 moves its head so that the light intensity is always greatest dorsally. By 

 means of proprioceptive feedback from the hair plates, the body is 

 then rotated until it is ahgned with the head (Mittelstaedt, 1950). 



Swimming 



Insects that swim beneath the surface of water are faced with orienta- 

 tion problems more nearly resembling those encountered in the air 

 than on land. They can move freely in three dimensions in a homo- 

 geneous medium and must possess stability, just as must flying insects. 

 In Notonecta, Naucoris, and Macrotorixa the normal swimming 

 position also represents the stable equiUbrium position. This position 

 is maintained in the imago oi Naucoris and Macrotorixa by the distri- 

 bution of air and the position of the legs; in Notonecta larvae, princi- 

 pally by the location of the centre of gravity of the body mass (Oever- 

 mann, 1936). When these insects are Winded, deprived of the air film 

 on the surface of the body, and artificially weighted so that the position 



