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 Fio. 390. Section of Sphinx 



embryo, showing at ft the ectoderm 

 . invaginated, and forming the germ 



It Will much Simplify Our Conception Of of a stigma and trachea W- After 



Kowalevsky. 



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 r in 



ky- 



cc- 



FIG. 392. Structure of a trachea, diagram- 

 matic : portions of the peritrarheal membrane 

 (hy) and chitinous intima (cc) removed to 

 FIG. 391. Portion of a trachea of a caterpillar, show the structure; in the chitinou* intima 

 with its branches B, <~\ D: a, peritracheal mem- or endotrachea (cc) can be seen the spiral 

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



