TRACHEAL RESPIRATION 117 



meet the call for oxygen even during muscular exertions. This 

 result, which was controlled and secured in several ways 

 (Krogh, 1920.2), can be shown to be valid for a large number 

 of tracheates. 



This comes out clearly when the influence of size is con- 

 sidered. The animal under discussion had a length of 60 mm 

 and weighed 3.4 g. Assuming all linear dimensions multi- 

 plied by 10, the tracheae would be 60 mm long with a cross 

 section of 6.7 cm 2 . These dimensions would provide for 10 

 times the diffusion found, but the animal's weight would 

 increase 1,000 times and the metabolism at least 100 times. 

 Diffusion would be quite inadequate. Conversely conditions 

 would be greatly improved in an animal having 1/10 the linear 

 dimensions (6 mm long) which is much nearer the general size 

 of insects than the Cossus larva studied. 



The animals which show no respiratory movements and in 

 which the whole of the gas transport takes place by diffusion 

 in the tracheae are mainly the following: the Onychophora 

 (Peripatus), the tracheate Arachnoidea, Myriapoda, and Chilopoda, 

 almost all terrestrial insect larvae, all pupae and a very large 

 number of small imagines. Many imagines have such a high 

 rate of metabolism, especially during flight, that a diffusion 

 transport becomes inadequate when the weight reaches about 

 0.1 g or perhaps even less. The house Ay weighing about 

 15-20 mg is supposed never to make respiratory movements, 

 but the bee, weighing 100 mg, does so regularly. 



Many insects, even fairly large ones like the cockroach 

 (Periplaneta orientalis) or the walking stick {Dixippus morosus), 

 do not use their ability to make respiratory movements until 

 their metabolism rises above a certain point, either as a result 

 of muscular movements or at rest under the influence of a 

 high temperature. Up to that point the gas exchange is 

 brought about by diffusion only, and it is the rule that the rate 

 of diffusion is definitely cut down by regulation so as to require 

 a larger pressure difference. 



The supply of oxygen to the long legs in several insects 

 presents difficulties, and the oxygen concentration in their 



