124 RESPIRATORY MECHANISMS 



while the effect of osmotic changes in the haemolymph are 

 slight and secondary. 



On the point which chiefly concerns us there is complete 

 agreement with Wigglesworth: lack of oxygen is the main 

 factor to cause retraction of fluid and an improved access 

 of oxygen. 



As the permeability of chitin for oxygen is quite low, there 

 is reason to believe that oxygen is mainly absorbed through the 

 walls of the tracheoles, while significant amounts of CO2 can 

 be eliminated all along the tracheae and to some extent through 

 the integument. One consequence of this is that lack of 

 oxygen in any part of the tracheal system does not bring about 

 any appreciable accumulation of CO2 in the tracheae involved. 



Mechanical ventilation of the tracheal system combined with 

 diffusion. In a number of insects of medium size or larger — as 

 insects go — part of the tracheal system is ventilated by respira- 

 tory movements, but the gas transport in the narrower 

 branches, often of several mm length, is always brought about 

 by diffusion. The quantitative distinction between the part 

 played by mechanical ventilation and diffusion respectively 

 can be most conveniently illustrated by reference to an 

 aquatic form, the larva of a large water beetle (Dytiscus 

 marginalis). This animal has only one pair of functional 

 spiracles on the last body segment. These are brought into 

 contact with the surface, and a succession of rapid respirations 

 takes place before the animal goes down again to seek its 

 prey. The spiracles lead into two voluminous tracheae 

 running all the way up to the head and having an elliptic 

 cross section. From these tracheae a number of narrower 

 tracheae branch out to supply the organs, and all these are 

 circular in cross section. Experiments were made (Krogh, 

 1920.3) to determine the volume of ordinary respirations 

 (tidal air), of maximal respiration (vital capacity) and the 

 internal volume of the tracheal system (total capacity). In 

 the arrangement shown in Fig. 70 the depth of respiration can 

 be read off directly by the movement of the meniscus in the 



