64 THE PHYSIOLOGY OF INSECT SENSES 



amputation of the tarsi of flies abolishes the reflex (Fraenkel, 1932; 

 Friedman, 1959). 



In Periplaneta another flight-initiating reflex has been described by 

 Diakonoff'(1936). This is a change in the relative positions of the pro- 

 and mesothorax, the so-called 'fall-reflex'. The sense organs concerned 

 are small trichoid sensilla functioning in a manner similar to that of 

 hair plates. A similar mechanism can cause flight to commence in 

 giant water bugs (Dingle, 1961). Giant water bugs are also stimulated 

 to fly by action of wind on sensilla trichodea located on the head 

 between the eyes. 



Although some insects {Drosophild) may continue to fly to ex- 

 haustion without further mechanical stimulation (Chadwick, 1939; 

 Wigglesworth, 1949), others (e.g., Muscina and Schistocerca) require 

 continuous stimulation. In the case oi Muscina stabulans a flow of air 

 against the antennae, causing the arista-bearing segment to move with 

 respect to the second joint, results in the legs being flexed in the flying 

 position. Air flow against the antennae also seems to be necessary for 

 sustained flight (Rollick, 1940). 



In Schistocerca ten groups of sensilla trichodea bilaterally arranged 

 on the leading surfaces of the head are sensitive to wind. When they are 

 stimulated the forelegs are flexed in flight attitude and continuous 

 flight occurs (Weis-Fogh, 1949, 1956). They assist in stabihzation in a 

 horizontal plane (yaw), as is shown by the fact that the locust turns if a 

 jet of air is directed on the hairs from the side rather than from the 

 front (Weis-Fogh, 1949). Whether they are static or phasic receptors 

 or both has not been satisfactorily ascertained. It has been observed 

 that they do not vibrate in the wind; they are damped. The hair plates 

 on the legs oi Periplaneta, which these resemble, are sensitive both to 

 continuous deformation and the vibrations of a loudspeaker (Pringle, 

 1938 c). According to Weis-Fogh (1956), *wind on the wings' is also an 

 adequate stimulus for the maintenance of flight in Schistocerca. 



Maintenance and alterations of flight velocity depend in con- 

 siderable measure upon information supplied by the Johnston's organ . 

 The detailed experiments with Calliphora (Burkhardt and Schneider, 

 1957), Aedes (Bassler, 1957, 1958), and Apis (Reran, 1957, 1959) have 

 revealed its hitherto unsuspected role. When insects are flying, air 

 currents impinging on the antennae cause the Johnston's organ to be 

 stimulated. As a result of the sensory information received, flight 

 velocity is reduced and the antennae are actively brought forward. 

 Forward alignment reduces the angle of attack of the air current, with 

 consequent reduction in the intensity of stimulation. Reran suggested 



