THE PHYSIOLOGY OF RESPIRATION IN INSECTS. 377 



The function of the abdominal muscles is to maintain the 

 air-pressure, and to expel the generative products and faeces. 

 The alternate contraction and expansion of the abdomen which 

 occur during flight are not respiratory, but a means of regulating 

 the intra-thoracic air-pressure. I have frequently convinced 

 myself that there are no rhythmic movements of the body wall 

 in the blow-fly when at rest. 



Thoracic Respiratory Movements. At first sight it will appear, 

 perhaps, as many have held, that inspiration is effected by the 

 enlargement of the thorax, and I think it possible that when 

 the intra-thoracic air-pressure is low, some air may be drawn 

 into the thoracic tracheae by the contraction of the longitudinal 

 thoracic muscles (dorsales). There is, however, no doubt in 

 my mind that the air-pressure in the tracheae is normally 

 higher than the atmospheric pressure ; the air cannot in this 

 case be renewed by expansion of the thorax. 



The Intra-tracheal Pressure. That the air contained in the 

 tracheal tubes is compressed, is shown by the fact that the 

 membranous tracheae may frequently be seen to expand when 

 relieved of the pressure of surrounding parts ; and by the con- 

 tinuous nature of the humming sound produced by the escape 

 of air from the tympanic spiracle. I have been unable to 

 determine the internal pressure, but the following facts, I think, 

 justify the assumption that it is not less than two or three 

 inches of mercury, higher than that of the surrounding atmo- 

 sphere, in a vigorous blow-fly. 



If an opening is made into the great air vessels the insect 

 becomes helpless. I believe the experiment, which has fre- 

 quently been made to determine the function of the balancers 

 by cutting them off, an operation which destroys the insect's 

 power of flight, or even of supporting its own weight on its 

 legs, really gives rise to these phenomena by reducing the 

 internal air-pressure ; as it is impossible to remove these organs 

 without laying open the great tracheal trunks in the vicinity of 

 the posterior spiracles. 



The proboscis is exserted and erected (see Chap. XL, Sect, i) 

 by the air-pressure in its great tracheal trunks, and, as 



