TRACHEAL RESPIRATION 119 



a muscle by which the. size of the opening can be regu- 

 lated and even completely closed. Closing mechanisms are of 

 several different types and in some cases, including the large 

 cockroaches, they are labiate and make up the very outside 

 opening where they can be easily observed. This fact was 

 utilized by Hazelhoff, who in 1926 published a beautiful 

 study of their function. 



Hazelhoff found that in the animal at complete rest the 

 spiracles would remain closed at all temperatures up to about 

 26°C, while at higher temperatures they are opened more and 

 more. Special experiments, in which the animals were sub- 

 jected to sudden pressure variations in the surrounding atmos- 

 phere, showed however that some air could pass between the 

 closed lips. Any struggling on the part of the confined cock- 

 roach caused the spiracles to open up after a latent period of a 

 few seconds, and when the struggling ceased they would close, 

 again after a few seconds. Experiments with varied atmos- 

 pheres showed clearly that the spiracles react to CO2 and only 

 indirectly and feebly to oxygen lack. One per cent C0 2 has a 

 perceptible effect; with 2% the spiracles are kept permanently 

 open, and with 3% they are widely open. Further experi- 

 ments in which single spiracles were exposed to a jet of air 

 with or without a suitable concentration of CO2 were con- 

 clusive in showing that the spiracles react by a kind of local 

 reflex to the CO2 concentration and that the organs which are 

 sensitive to C0 2 are situated just inside the spiracle lips. 

 Similar experiments were made on a number of different 

 insects and spiders, and always with the same result in such 

 forms which permanently or temporarily ventilate by diffusion 

 only. 



The regulation mechanism in a flea (Xenopsylla cheopis) was 

 carefully studied by Wigglesworth (1935) and found to differ 



the stomata in plant leaves, only the slightest resistance against diffusion, which 

 through sufficiently narrow openings in a thin membrane is proportional not 

 to the area but to the circumference. This fact also explains why extremely 

 narrow slits in several types of spiracles can allow a sufficient diffusion of gases. 

 Inside such openings, which themselves constitute an efficient filter, there is 

 often a very dense felt-like covering of hairs on the spiracle wall, the significance 

 of which remains obscure. 



