Insects and their world 



Abd.viii 



Fig. 17. The system of tracheacj or breathing- tubes, of a flea; only the left 

 half of the system is shown. Thoracic and abdominal spiracles are indi- 

 cated by numbers. From Wigglesworth, 1953 



tracheae is discussed in Chapter III, along with the function of respiration. 

 Fig. 17 shows a typical arrangement of the tracheal system in an insect. 



The nervous system^, too, is comparatively simple, and consists of a 

 series of nerve centres, or ganglia, one in each segment of the abdomen 

 and thorax, linked together in a chain by double nerve-cords running 

 along the body. In the head these nerve-cords loop round the pharynx, 

 and merge into a bigger ganglion which we call the ' brain '. This ganglion 

 does serve the functions of a brain as far as these go, but it is very much 

 less complex than the brain of even a small mammal. 



In our nervous system, the brain is so very much more elaborate than 

 any other part of the nervous system, that it clearly dominates the entire 

 body. The insect's brain is only the biggest of a chain of gangha, and 

 it seems reasonable to expect that much of an insect's behaviour should 

 be made up of automatic, reflex activity, controlled by a local ganglion, 

 with Uttle reference to the brain. The behaviour of insects is briefly 

 considered in Chapter VI. 



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