FORM, GROWTH, AND CHANGE 23 



side the crop are situated the paired salivary glands (s gl), 

 the fluid secreted by which passes through a set of fine tubes 

 or ducts into a median duct that opens into the mouth behind 

 the base of the tongue. The gullet and crop are like the air- 

 tubes lined with an extension of the cuticle ; so is the small 

 gizzard (or proventriculus) lying immediately behind the crop ; 

 the chitinous lining of the gizzard is thickened into teeth and 

 ridges and beset with hairs, so that this organ serves for crush- 

 ing, pressing and straining the food. Next comes the tubular 

 stomach (or ventriculus) which has no chitinous lining, its inner 

 or mucous coat consisting of a sheet of glandular and absorbent 

 cells. This region (Fig. 12 s) is often known as the mid-gut. 

 Just behind the gizzard, the stomach gives off a series of blind 

 tubes (Fig. 12 pc) within which digestive juices containing fer- 

 ments that act on the food substances are formed, and whence 

 they are poured into the food-canal. Behind the stomach is 

 seen a narrower tube the intestine (Fig. 12 i) which again has a 

 chitinous lining, as has the somewhat swollen terminal region of 

 the digestive tube known as the rectum (Fig. 12 r), whence the 

 waste residue of the food-stuffs is passed out of the body through 

 the vent or anus. 



A remarkable feature of the insect's food-canal is that at 

 either end an extensive region is lined with a continuation of 

 the outer cuticle the gullet, crop and gizzard in front, and the 

 intestine and rectum behind. These two regions are known as 

 the fore-gut and the hind-gut respectively, and their living lin- 

 ings the sheets of cells on whose inner surface the cuticle is 

 formed must be regarded as inpushings of the skin. The 

 cuticular lining of these regions is shed at each moult along with 

 the exoskeleton. The digestive system is therefore directly 

 concerned in the changes that accompany growth, and in those 

 cases where the adult insect has a manner of feeding differing 

 widely from that of the young, these changes may be necessarily 

 very great. 



At the junction of the hind-gut with the mid-gut a number of 

 fine tubes with cellular walls are given off so as to float freely 

 in the great blood-space. These (Fig. 12 k t) are usually called 

 Malpighian tubes, being named after the eminent Italian anato- 

 mist of the seventeenth century, Marcello Malpighi, who made 

 monumental studies on the minute structure of insects. They 



