118 



RESPIRATORY MECHANISMS 



tracheae may become very low (Krogh, 1912). In the 

 harvestmen (Opiliones) there are special spiracles on the tibiae 

 of the long legs (H. J. Hansen, 1893). 



The Gastrophilus larva was mentioned on p. 107 where an 

 account is given of its haemoglobin-storing cells. These cells 

 are connected with the spiracles through 4 relatively enormous 

 tracheae (Portier, 1911, p. 318). The air, to which the larvae 

 will have access only occasionally, is probably very poor in 

 oxygen. 



Diffusion regulation. Diffusion of gases in air in a system of 

 rigid tubes would seem to be a process not readily lending itself 

 to regulation on the part of the organism. Nevertheless 

 mechanisms are present at both ends of the tracheae providing 



a very effective adaptation 

 of the diffusion to the re- 

 quirements of the moment. 

 The spiracles through 

 which the tracheae commun- 

 icate with the outside air are 

 as a rule not simple openings, 

 but more or less complex or- 

 gans sui generis. A com- 

 paratively simple type of 

 spiracle is shown in the some- 

 what diagrammatic Fig. 66. 

 The rim and funnel of the 

 spiracle cannot be wetted by 

 water, which is thus pre- 

 vented from entering the 

 tracheae. The hairs at E 

 form a very effective filter preventing the entrance of dust, 1 

 and finally we have at Vh-Vb a mechanism provided with 



1 A large diversity of spiracle types is found among tracheate insects, and 

 several of the structural traits are probably of respiratory significance. 



In the larvae of the lamellicorn beetles living in earth or manure the spiracles 

 appear closed by a thin membrane, at one time supposed to be continuous, 

 but in reality pierced by numerous exceedingly minute openings. These will 

 provide a very effective filter and in spite of their small aggregate area offer, like 



Fig. 66. Section through spiracle 

 of Cossus larva. H hair filter, Vh-Vb 

 closing mechanism. (Claus.) 



