CHAPTER 2 



Respiration 



Terrestrial life is a perpetual conflict between the need for oxygen 

 and the need for water; for those conditions which favour the entry 

 of oxygen into the body favour also the escape of water. This con- 

 ception, applied to the insects, illuminates the whole subject of their 

 respiration. The majority of insects, as we have seen, have become 

 encased in a waterproof covering; a covering which is relatively im- 

 permeable to oxygen also. They breathe, as was discovered by Mal- 

 pighi (1669), through tracheal tubes, which open along the sides of 

 the body through a series of spiracles, and convey air directly to the 

 tissues (Fig. 5, A). The tracheal trunks anastomose one with another, 

 and then branch and rebranch, becoming finer and finer, until their 

 ultimate ramifications either penetrate the individual fibres of the 

 muscles or the cells of other tissues, or form a network around them 

 which is more or less rich according to their oxygen requirements. 



The tracheae and tracheoles 



Morphologically, the tracheae are invaginations of the ectoderm. 

 Consequently, they consist of a cellular matrix and a lining cuticle. 

 As a result, no doubt, of physical stresses developing within this 

 cuticle during its formation, it is commonly thrown into spiral folds, 

 and sometimes, probably because the cells have continued to secrete 

 the cuticular substance after the folds have developed, the final result 

 appears as though the tracheae were made up of a uniform mem- 

 brane, thickened with spiral filaments or taenidia. This spiral forma- 

 tion enables the tracheal tubes to be bent and pressed upon without 

 collapsing. 



The chemistry of the tracheal cuticle resembles that of the general 

 surface of the insect. The innermost layer is composed of the non- 

 chitinous 'cuticulin', perhaps associated with wax; in the larger 



