ALIMENTARY CANAL. 625 



teron or mid-gut which alone is lined by endoderm and is usually 

 small, in some cases it is even absent ; it forms (v) the chylific 

 ventricle which usually is provided with certain caecal glands ; 

 (c) the proctodaeum or hind-gut which is divided into (vi) the 

 ileum which bears the malpighian tubules, (vii) the colon and 

 (viii) the rectum and anus. The epiblastic stomodaeum and 

 proctodaeum usually retain their chitinous lining throughout 

 life. In many insects one or more of the eight parts men- 

 tioned above cannot be recognized. 



In many larvae the alimentary canal is no longer than the 

 body and is consequently straight, but in most imagos it is longer 

 and the excess in length has to be accommodated by coiling it 

 into one or more loops, a rather rare feature in Arthropods. 

 Again in the larvae of many Hymenoptera, some Coleoptera and 

 Diptera and a few others the partition between the mid-gut and 

 hind-gut persists and the larva is physiologically aproctous 

 until a late period. 



The mouth leads into a pharynx which in those insects that 

 suck fluid food has chitinous walls. Normally the walls enclose 

 a triradiate lumen, but by the contraction of numerous muscles 

 which run from the outside of the pharynx to the inside of the 

 skeleton of the head this lumen can be enlarged until in section 

 it is almost circular and the fluid food then rushes into it (Fig. 

 381). The oesophagus is a simple tube running from the pharynx 

 to the crop. It traverses the neck and enlarges into the crop which 

 has muscular walls. In certain insects which take liquid food 

 one or more food-reservoirs open into the end of the oesophagus. 

 These are present in the Diptera (Fig. 381, 13), Lepidoptera 

 and Hymenoptera and they are usually but erroneously called 

 " sucking stomachs " ; they take indeed no part in the imbi- 

 tion of the food, which is simply stored therein, until it is wanted 

 elsewhere. A good deal of the digestion goes on in the crop when 

 present. The saliva of many Insects, which is alkaline, con- 

 verts the starch there into glucose which passes through the walls 

 into the blood. The acid secretion of the caecal glands of 

 the chylific ventricle is passed forward and assists in the 

 emulsifying of fats and the conversion of albuminoids into 

 peptones. The proventriculus is well developed in many 

 Orthoptera and Coleoptera and many ants. The oesophagus 

 is in some species produced into it, like a funnel in a tumbler, 

 z in S s 



