GENERAL NOTES ON INSECTS. 345 
lick; and a great number of larve 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 cesophagus, 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 ceca, 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, z.¢. 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. 
(6) From the beginning of the mid-gut blind outgrowths sometimes 
arise (in some Orthoptera, etc.), which are apparently 
digestive. They are sometimes called pyloric caca. 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 (trachez), 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 
