GENERAL NOTES ON INSECTS. 345 



lick ; and a great number of larvae devour the flesh and 

 vegetables in which they are hatched. 



Many modifications of mouth organs, and of the ali- 

 mentary canal, are associated with the way in which the 

 insect feeds. 



The alimentary canal consists of fore-gut, mid-gut, and 

 hind-gut, but in many cases it seems very doubtful if the 

 mid-gut has its typically endodermic character. 



The fore-gut conducts food, and includes mouth cavity, 

 pharynx, and oesophagus, the latter being often swollen into 

 a storing crop, or continued into a muscular gizzard with 

 grinding plates of chitin. 



The mid-gut is digestive and absorptive, often bearing a 

 number of glandular outgrowths or caeca, and varies in 

 length (in beetles at least) in inverse proportion to the 

 nutritive and digestible quality of the food. 



The hind-gut is said to be partly absorptive, but is chiefly 

 a conducting intestine, often coiled and terminally expanded 

 into a rectum with which glands are frequently associated. 



In association with the alimentary canal are various glands : 



(a) The salivary glands, which open in or near the mouth. They 



are usually paired on each side, and provided with a 

 reservoir. They arise as invaginations of the ectoderm 

 near the mouth. Their secretion is mainly diastatic in 

 function, i.e. it changes starchy material into sugar by 

 means of a ferment. Along with these may be ranked 

 the "spinning glands" of caterpillars, etc., which also 

 open at the mouth. They secrete material which hardens 

 into the threads used for the cocoon. 



(b) From the beginning of the mid-gut blind outgrowths sometimes 



arise (in some Orthoptera, etc.), which are apparently 

 digestive. They are sometimes called pyloric caeca. In 

 other cases (some beetles) there may be more numerous and 

 smaller glandular outgrowths resembling villi in appearance. 



Respiratory system. The .body of an insect is traversed 

 by a system of air-tubes (tracheae), which open laterally by 

 special apertures (stigmata), and by means of numerous 

 branches conduct the air to all the recesses of the tissues. 

 In animals which breathe by gills or lungs the blood is 

 carried to the air; in insects the air permeates the whole 

 body. But how does the air pass in and out? In part, no 

 doubt, there is a slow diffusion ; in part the movements of 

 the wings and legs will help; but there are also special 



