THE AIR-TUBES 



431 



a small, confined space absorbs all the oxygen. Insects can survive 

 for many hours when placed in an exhausted receiver, or in certain 

 irrespirable gases. " Cockroaches in carbonic acid speedily become 

 insensible, but after twelve hours' exposure to the pure gas they 

 survive and appear none the worse." (Mi- 

 all and Denny, p. 165.) Insects of the 

 swiftest flight breathe most rapidly, their 

 great muscular activity requiring the ab- 

 sorption of an abundance of oxygen. 



Warmth, plenty of food, besides muscu- 

 lar activity, increases the demand for oxy- 

 gen and the quantity of carbonic acid 

 exhaled. 



a. The tracheae 



Fir,. 390. Section of Sphinx 

 embryo, showing at * the ectoderm 

 invaginated, and forming the germ 

 of a stigma and trachea (t). After 

 Kowalevsky. 



It will much simplify our conception of 

 the nature of the air-tubes when we learn 

 that they originate in the embryo as tubular ingrowths of the 

 integument (ectoderm), these branching and finally reaching every 

 part of the interior of the body. They are elastic tubes, and being 



filled with air are silvery in 

 color, though at their origin 

 near the spiracles they are red- 

 dish or violet bluish ; or, in 



FIG. 392. Structure of a trachea, diagram- 

 matic : portions of the peritracheal membrane 

 (hy) and chitinous intima (ce) removed to 

 Fir,. 391. Portion of a trachea of a caterpillar, show the structure; in the chitinous intima 

 with its branches B, C, D: a, peritracheal mem- or endotrachea (ec) can be seen the spiral 

 brane; b, nucleus. After Leydig, from Gegenbaur. thickenings or ticnidia. After Lang. 



