RESPIRATION 13 



trunks a certain amount of chitin is present outside this; but in the 

 small branches chitin is absent. The nature of the membrane lining 

 the fine terminations is not known; it may perhaps consist of un- 

 tanned lipoprotein. 



When the tracheal branches have become reduced to about 2 |i in 

 diameter they break up abruptly to give rise to much finer tubes 0-6- 

 0-8 [i in thickness. These are termed tracheoles. As viewed with the 

 light microscope they appear to have smooth walls; but examination 

 with the electron microscope has revealed that their lining membrane 

 is thrown into spiral or annular folds just like the tracheae. The 

 tracheoles run a course of some 200-400 y. gradually tapering to end 

 in a blind rounded extremity at about 0-2-0-5 n diameter. Each 

 tracheole is really a single, greatly elongated cell with its nucleus 

 lying about one-third of the way along it. Sometimes the tracheoles 

 may be branched. It is possible that they may occasionally anasto- 

 mose. Often they arise from the side of quite large tracheae. 



When the insect moults, new and layer tracheae are laid down by 

 the tracheal epidermis, and the cuticular lining of all the tracheae is 

 shed and drawn out through the spiracles. But the linings of the 

 tracheoles are not shed: the old tracheoles are secured to the new 

 tracheae by rings of a special cementing substance. 



The permeability of the tracheal system shows a gradation from 

 the spiracle to the tracheole ending. To some extent it is permeable 

 to gases throughout, though naturally it is much more so where the 

 walls become so delicate as they are in the tracheoles. Like the epi- 

 dermal cuticle, it is probably impervious to water, except in the 

 tracheoles, which are freely permeable. Indeed, the terminal part of 

 the tracheoles in many insects, both terrestrial and aquatic, normally 

 contains a variable amount of fluid. There seems to be an equilibrium 

 between the capillarity of the tracheoles (which are readily wetted by 

 water) on the one hand and the 'swelling pressure' or 'colloid os- 

 motic pressure' of the cytoplasm outside the tracheoles on the other. 

 If the tracheoles are exposed artificially to slightly hypertonic fluids 

 (1-0 per cent, sodium chloride), this will increase the 'swelling pres- 

 sure' of the cells, fluid is sucked out of the tracheole endings and the 

 column of air extends farther along the tracheoles towards the 

 tissues. These same changes can be brought about by the increase in 



