THE PHYSIOLOGY OF RESPIRATION IN INSECTS. 379 



exhausting by means of an air pump, but never succeeded in 

 doing so. It is true that bubbles of air do escape from the 

 thoracic spiracles, when insects are put into hot water or even 

 spirit ; but in the former case the valves of the spiracles are 

 dislocated or ruptured, and in the latter the air, which rises in 

 bubbles to the surface, is that which adheres to the exterior of 

 the spiracle and fills the vestibule. The tracheal vessels them- 

 selves remain full of air for a long time, a fact which is suffi- 

 ciently manifest when the insect is dissected after immersion 

 for an hour or more. When the waxy secretion is removed 

 the efficacy of the external valves is destroyed, and the con- 

 tracting muscles open the internal valves. Experiments which 

 give rise to solution of the wax and to muscular rigidity, are 

 therefore without value ; such, for example, as immersing the 

 insect in alcohol, or even in hot water. 



The inadequacy of the supposed expiratory movement of the 

 body wall to change all the gas in the tracheae, or even a large 

 proportion of it with each respiratory act, is so manifest, that 

 it precludes the acceptance of the expiratory theory of Trevi- 

 ranus ; if I am right in my conclusion that all or nearly all the 

 inspired oxygen must be utilised to account for the metabolic 

 activity of a lively insect. 



Passage of Gases through the Integument. Herman Dewitz 

 [159] made direct experiments on the passage of gases through 

 the skin of the larva of Smerinthus ocellatus. He used the 

 portion, which has no spiracles, to close the mouth of a test 

 tube containing air, and immersed the tube in an atmosphere of 

 carbon dioxide. He then found that the skin became very convex 

 in an hour, showing that carbon dioxide passes in more rapidly 

 than air passes out. He also reversed the experiment by 

 filling the test tube with carbon dioxide, when the skin became 

 deeply concave. In both experiments he observed on perforat- 

 ing the skin that equilibrium was restored by a rush of gas, 

 which made a loud sound. 



Dewitz does not mention the condition of the skin, whether 

 moist or dry, but it was presumably moist. 



There can be no doubt the passage of gas through the 



