STRUCTURE OF INSECTS. 265 



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 

 probably digestive. They are sometimes called pyloric 

 caeca. In other cases (some beetles) there may be 

 more numerous and smaller glandular outgrowths on 

 the external wall of the mid-gut. 

 (c) From the hind-gut arise numerous fine Malpighian 



tubes, which are certainly excretory in function. 

 Respiratory System. — When we watch an insect — say a 

 drone-fly resting on a flower — we can observe panting move- 

 ments in the abdominal region. These movements are of 

 the greatest importance in respiration. For all through the 

 body there is a system of branching air-tubes (tracheae) which 

 open to the exterior by special apertures (stigmata) often 

 guarded by hairs, and when the abdomen contracts, some of 

 the air in these tubes is driven out, while the expansion of 

 the body sucks air in. But the exchange of gases between 

 the tissues and the tracheae, and between the tracheae and 

 the atmosphere, is probably, for the most part, a slow diffusion. 

 The trachea: seem to arise as tubular ingrowths of skin, 

 and primitively each segment probably contained a distinct 

 pair. But their number has been reduced, and many are 

 often connected into a system. With the doubtful exception 

 ■S'f one of the primitive Collembola, and the certain exception 

 of caterpillars, no insects have any tracheal openings in the 

 head region. There are rarely more than two pairs in the 

 thorax, there are often six to eight pairs in the abdomen, the 



