62 THE PHYSIOLOGY OF INSECT SENSES 



Steady air-flow, however, the antennae appear to tremble. Under these 

 circumstances the phasic receptors would register on- and off-waves 

 and thus could also register a mean deflexion (Burkhardt, 1960). In 

 short, Johnston's organ can provide the central nervous system with 

 rather accurate information about the strength and time course of 

 antennal deflexion. 



THE ROLE OF MECHANORECEPTION 



IN LOCOMOTION 

 Walking 



Locomotion requires the co-ordination of moving parts. Although it 

 is possible that movements associated with locomotion can arise from 

 an innate central pattern, especially in the case of flying, as Wilson 

 (1961) has shown for the desert locust {Schistocerca gregaria Forskal), 

 a steady stream of information about the position of parts relative to 

 one another and about the magnitude of forces acting upon them is 

 undeniably necessary. In so far as flight is concerned, Weis-Fogh 

 (1956) and Pringle (1957) favour a complete reflex explanation of 

 movements. The consensus now is that walking, too, is reflexly con- 

 trolled (Pringle, 1940; Hughes, 1957, 1958). 



In addition to feedback loops from the moving parts, locomotion 

 also requires information about the orientation of the insect as a whole 

 in the gravitational field. The source of all of this information is pre- 

 dominantly the mechanoreceptors. All types of mechanoreceptors 

 provide the service, and one of the marvels of the central nervous sys- 

 tem is its ability to integrate all of the incoming signals so that the 

 necessary adjustments in the performance of the effectors can be made 

 quickly and accurately. 



In walking, for example, the reflex action of the leg muscles is evoked 

 by stimulation of mechanoreceptors. Most of the muscles of the insect 

 leg are innervated by two nerve fibres only, a *quick' and a 'slow' 

 (Hoyle, 1957). Stimulation of the quick fibre is followed by a large 

 electrical change and a rapid twitch, whereas stimulation of the slow 

 fibre is followed by a smaller electrical change and a tonic contraction. 

 In the cockroach Periplaneta americana L., as Pringle (1940) has 

 shown, the depressor reflex is evoked by stimulation of campaniform 

 sensilla on the trochanter; the levator response, by touch on the upper 

 side of the leg or on the tibial spines. Most of the campaniform sensilla 

 are located and oriented so as to be stimulated by forces produced in 

 the leg when the insect is standing normally. The situation may actually 

 be much more comphcated than this because of the possibihty of the 



