THE RESPIRATORY SYSTEM. 27 



protoderm ; it is extremely delicate, covered with reticulated 

 wrinkles, and marked with striae, transverse to the wrinkles, not 

 the 12,000th of an inch apart, which render it iridescent. This 

 membrane, as well as the inner coat of the other tracheae, assumes 

 a smoky tint in the adult insect, probably from the oxidization 

 of minute particles of pigment contained in its substance.* 



The spiracles or external openings are all protected, either by 

 folds, the edges of which are armed with minute spiculae, or by 

 a ring of spiculae formed of chitine, and the admission of air is 

 controlled by a structure somewhat resembling a pair of 

 callipers, acted upon by a special muscle, which surrounds 

 the tracheae and is connected with their internal coat.f 



The manner in which the circulation of air in the trachece is 

 carried on in insects is by no means certain ; that it is exceed- 

 ingly vigorous in the fly may be fairly assumed from the rapidity 

 with which the vapour of chloroform and other volatile sub- 

 stances act upon the insect ; and from the fact that a little turpen- 

 - tine, applied with a camel's hair brush to one of the anterior 

 thoracic spiracles, produces instantaneous insensibility. The 

 great vital activity of the fly indicates that the respiratory func- 

 tion is very perfect ; in bees again the high temperature of the 

 hive tends to the same conclusion. 



I believe the pressure of the thoracic muscles acting unequally 

 upon the main membranous trachece and their sacculi, aided by 

 the valves at the spiracles, is the chief agent by which the air 

 is propelled through the smaller tubules. If such be the case, the 

 ordinary muscular acts of the insect would be sufficient to keep 

 up a very perfect circulation, as it is scarcely conceivable that 

 any muscle can contract, without giving rise to some movements 

 of the air in the large trachea). When a fly is held by the wings 



* The disposition of the main tracheal vessels is shown wi Plate I. 

 f Landois, Kolliker Zeichrift, Band. 17, 105. 



