PHYSIOLOGY OF THE INVERTEBRATA. 229 



As a general rule, eacH stigma does not open directly into a 

 tracheal tube, but into an ante-cbamber whence the trachea 

 takes its origin. This ante-chamber is frequently provided 

 with a series of little plaits, folds of its lining membrane. 

 These plaits or epiglottides also act as filters. 



The principal trunks of the tracheal system have three 

 coats. The internal one (in contact with the air) is a con- 

 tinuation of the integument. The middle one is a spiral coat 

 of chitin, which serves to keep the tube open. The external 

 coat is of connective tissue. 



As in the Myriapoda the air in the tracheae is kept in 

 motion by the movements of the abdominal walls. These 

 rhythmical movements are frequent ; on an average twenty- 

 five in Zucanus cervus (the stag-beetle), eighty in Apis,* and 

 from fifty to fifty-five in the Loeusta mridissima of Linnseus. 

 Notwithstanding the high development of their respiratory 

 apparatus and the activity of their lives, insects resist 

 asphyxia for a long time. The author kept a stag-beetle for 

 six days in an atmosphere containing 60 per cent, of chlorine, 

 with the result that it was still living after the expiration of 

 that time ; and many insects resist the action of an atmosphere 

 containing from 40 to 70 per cent, of carbonic anhydride. 

 This may account for the fact that insects and other cold- 

 blooded animals were able to withstand an atmosphere so 

 laden with carbonic anhydride as was that of the early ages 

 of the world's history. And if the descendants of the primi- 

 tive insects are now capable of living in an atmosphere con- 

 taining only six volumes of carbonic anhydride in io,000 

 volumes of air ; it is but one of many instances where natural 

 selection (the survival of the fittest) and the direct action of 

 the environment have worked hand in hand together. 



Concerning the vitality of insects, it may be remarked 

 that M. Lyonnet states that certain caterpillars revived after 

 ■ being submerged in water for eighteen days. 



* In Apis (the bee), Newport observed forty in a state of rest, but they 

 . rose to one hundred and.twenjjy with muscular exertion. 



