THE NERVOUS SYSTEM 115 



in a circle towards the sound side. This reaction is due partly to the 

 increased propulsive activity in the limbs on the operated side fol- 

 lowing the removal of inhibition; partly to an effect of the brain in 

 increasing the muscular tone of the flexor muscles on its own side. 

 But neither of these mechanisms will provide a complete explanation 

 of the circus movement; for after mutilation of the limbs on either 

 side, the performance of the muscles is so modified that the same 

 movement still occurs. The cause of this behaviour therefore lies 

 deeper: each half of the brain, when acting alone, tends to bring 

 about movement towards its own side - movement that requires the 

 co-operation of muscles in all parts of the body. 



Reflex responses in the normal insect 



Comparatively simple reflexes undoubtedly play a large part in the 

 responses of the normal insect; and our main problem is to describe 

 the whole of insect behaviour in terms of such reflexes. Pressure upon 

 the tarsus causes a reflex withdrawal of the leg; removal of the feet 

 from contact with the ground evokes a turning over reflex or initiates 

 the vibration of the wings; contamination of the antenna induces 

 cleaning movements. The mechanical character of such reflexes is 

 well seen in the cockroach: if the antenna is stimulated, and the in- 

 sect is offered a bristle, it will clean the bristle and neglect the an- 

 tenna; and many insects will continue the cleaning movements after 

 their antennae have been amputated. But though mechanical, these 

 reflexes are purposive and plastic : the water beetle Dytiscus deprived 

 of one hind-leg will swim with the middle leg instead ; and if the fore- 

 legs of the cockroach are removed, the stimulated antenna will be 

 held by the middle pair. 



Even the simplest of these reflexes involves many parts of the body, 

 and some movements of the whole insect are also clearly reflex. This 

 is notably the case with certain inhibitions. We have seen how loss of 

 contact between the feet and the ground will initiate flight; con- 

 versely, flight is inhibited when contact is regained. And contact 

 with other parts of the body may similarly inhibit movement: the 

 apparent preference for dark crevices in such insects as the moth. 

 Amphipyra, the earwig Forficula, and the bed-bug Cimex, depends in 

 fact upon a reflex inhibition of their movements by contact ('thigmo- 



