Breathing — Food-Canal 6 1 



the bow ; its contraction draws the latter inwards 

 and brings the two edges of the slit into contact (fig. 



42) (I, 35). 



Respiration. — The set of muscles which contract 

 the cavity of the hind-body have already been de- 

 scribed. These are directly concerned in the act of 

 breathing. It has been observed that the hind-body 

 contracts by the action of these muscles causing 

 approximation of the upper and lower walls, and 

 expands by the elasticity of its tissues. The closing of 

 the spiracles is of great importance in the breathing 

 of insects, for it is believed to ensure the passage of 

 fresh air into the finest tubes of the tracheal system, 

 owing to pressure from the abdominal muscles on the 

 air within the tracheal trunks. Were the spiracles 

 constantly open, this air would be simply driven out 

 again and would never reach the minute thin-walled 

 tubular branches where the exchange of gases goes 

 on most actively, oxygen being supplied to the blood 

 and carbonic dioxide given off. The branching of 

 the air-tubes throughout the body in insects is corre- 

 lated with the absence of a branching tubular blood- 

 system. The blood, diffused through the body-spaces, 

 is everywhere in contact with the air-tubes (l, 37). 



Tracheal Gills. — Some Stoneflies, which spend 

 much of their time in wet hiding-places, are believed 

 to breathe the air dissolved in water by means of 

 tracheal gills — bunches of filaments which contain 

 air-tubes, attached in pairs to the various body- 

 segments. This method of breathing, very rare in 

 adult insects, is exceedingly common in many im- 

 mature forms whose habits are aquatic. These will 

 be referred to at length in later chapters. 



Food-Canal. — The organs concerned with feeding 

 and digestion occupy a large space in the body-cavity 

 of most insects, forming a long, more or less coiled, 



