114 INSECT PHYSIOLOGY 



ganglia are fewer; in higher Diptera the thoracic ganglia become 

 united into one; in Hemiptera all the thoracic and abdominal ganglia 

 have coalesced to form a single mass. When a given movement of 

 the body demands the co-operation of a number of body segments, 

 there exists in one of the ganglia a higher centre which co-ordinates 

 the activities of the others. In those insects in which most movements 

 involve only the thorax, as in the winged forms, the ganglia have 

 become enlarged and concentrated, and the isolated thorax is then 

 capable of performing many of the motor functions - walking, clasp- 

 ing, vibration of the wings - of which the whole animal is capable. 



The brain 



There is also a concentration of ganglia in the head : the suboeso- 

 phageal ganglion and the supraoesophageal ganglion or 'brain'. Of 

 these, the suboesophageal seems to be the equivalent of the motor 

 ganglia that we have been discussing; it controls the movements of 

 the mouth appendages, and may contain co-ordinating centres - as 

 in caterpillars, where it controls the forward walking movements, 

 and in the adult of Aeschna, where it inhibits the tenacious clasping 

 reflex of the legs. The brain, on the other hand, seems never to con- 

 tain motor centres of this kind. It seems rather to be the central 

 receptor for the abundant sense organs located in the head; and as 

 such it orders the movements of the whole insect in accordance with 

 the stimuli which these receive. The reflex movements of the isolated 

 body segments are themselves purposive in a limited degree; but 

 when co-ordinated by the brain, all the body movements serve the 

 purpose of the animal as a whole. The 'mushroom bodies' in the 

 brain are collections of association neurones which are regarded as 

 the most important centres regulating behaviour; they are highly 

 developed in ants and bees. 



If the brain is removed, the reflex reactions of the body become 

 exaggereated ; it reacts to stimuli which in the normal insect would 

 have no effect ; and movements, such as the cleaning movements in 

 the bee, may continue without cessation for hours at a stretch. Thus 

 one function of the brain, one method by which it disciplines the 

 organism, is to inhibit the too rigid execution of reflexes by the lower 

 centres. If one half of the brain is destroyed, the insect tends to move 



