14 



INSECT PHYSIOLOGY 



osmotic pressure which accompanies muscular contraction, especial- 

 ly if the insect is meanwhile exposed to air with a reduced content of 

 oxygen. This has the effect of improving the supply of oxygen locally 

 to those tissues whose need is greatest. But in some insects the trache- 

 oles seem always to contain air right to their extremities; perhaps in 

 these cases the wetting properties of the tracheole membrane are 

 different. 



fig. 4. — Tracheoles running to a muscle fibre : semischematic 



A, muscle at rest; terminal parts of tracheoles (shown dotted) contain fluid; B, muscle 

 fatigued; air extends far into tracheoles. (Modified after Wigglesworth) 



These movements of fluid in the endings of the tracheoles are pure- 

 ly passive responses to simple physical forces around them. But the 

 tracheoles are living cells and are responsible for maintaining the 

 normal permeability of their lining membrane : as soon as the cells 

 die fluid rapidly seeps into their endings. And when the insect first 

 hatches from the egg, or at the time of moulting, the tracheole cells, 

 and indeed the cells bounding the entire tracheal system, actively 

 absorb the fluid contents. 



The diffusion theory of insect respiration 



The next problem in the physiology of the tracheal system is how 

 oxygen is conveyed to the tissues. Although the tracheae are perme- 

 able to oxygen throughout their length, the abundant supply of 

 tracheal capillaries in and round the most active tissues leaves no 

 doubt that most of the oxygen enters the organs through these end- 

 ings ; and the problem resolves itself into how oxygen is conveyed 



