THE CRICKET 



ventral nerves and muscles (cf. Fig. 6). To all of these organs 

 oxygen is carried by the air-tubes. The problem which has to 

 be met is how to force the air in and out of these fine tubes. 

 This seems to l)e done by bringing the sides of the body together, 

 thus causing the fluids of the body to press on the air-tubes. 

 When the spii'acle is open, the air is forced out of it ; when the 

 spiracle is closed, the air is forced 

 from the larger stems into the 

 finer tubes lying in the tissues. 

 There is a spiral spring (consisting 

 of a thickening of the cuticula) in 

 the inside of the main stems of the 

 tracheffi that restores the cylindrical 

 form of the tube after compression. 



Circulation in the Cricket. — In 

 the higher animals the food dis- 

 solved in the food-canal is carried 

 by tubes, called blood-vessels, to the 

 heart, from which it is pumped to all 

 parts of the body. The blood-ves- 

 sels carry not only fluid food but 

 also oxygen and bring away waste 

 from the tissues. In insects, as we Fig. 6 

 have seen, oxygen is carried by a 

 special S3'stem of air-tul^es. The 



digested food passes through the wall of the food-canal into 

 the cavity of the body. In the body-cavity it is kept in slug- 

 gish circulation by the heart, — a long dorsal tul>e full of holes, 

 — the pulsations of which drive the body fluids forward. 



Excretion in the Cricket. — In every chemical laboratory 

 there are obtained, in addition to the useful things sought, 



Alinu'iitary canal of 

 cockroach, x ^. Miall and 

 Denny. 



