70 CHIRONOMUS PRASINUS. 



small and large intestine, the posterior part of which, surrounded 

 by a strong muscular coat, may be considered as a rectum. With 

 reference to the embryonic origin of this canal, it may be stated 

 that the oesophagus and intestine (including both its divisions) are 

 derived by invagination from the epiblast layer of the embryo, and 

 have received the names respectively of stomodeum (fore gut), 

 and proctodeum (hind gut), while the stomach, or mid gut, is 

 derived from the hypoblast, and corresponds to the mesenteron or 

 primitive digestive cavity. It has at least four coverings, though 

 these are not equally present in every part. Its internal wall is a 

 structureless cuticle, continuous with that of the integument, and 

 extending the whole length of the canal. It is reflected around 

 the termination of the oesophagus, where the latter hangs in 

 the proventricular cavity.* It passes by without entering the 

 cavities of the proventricular cgeca and the malpighian tubes. It 

 is this membrane which in other insects is often solidified at the 

 upper part of the canal and developed into rows of strong, horny 

 teeth, as is well known in the cricket, etc. Here, however, this 

 is not the case, but it forms a smooth membrane more or less 

 separated from the epithelial layer to which it owes its birth.f 



The epithelial coat comes next in order, and is continuous 

 with the hypodermis of the integument. It secretes the cuticular 

 lining just described. It varies greatly in character in different 

 parts. In the oesophagus and small intestine it is difficult to 

 trace, having, I think, become atrophied to a great extent. The 

 occasional occurrence of nuclei between the two membranous 

 coats of the canal in these parts, attests the original continuity 

 of this all-important structure. 



It is, however, in the proventriculus and stomach that the 

 epithelial coat attains its greatest development, where it functions 

 as a glandular layer of cells, secreting probably an alkaline fluid 

 concerned in the process of digestion.^ The cells differ some- 



* See Fig. 12, and compare this organ in Psychoptera paludosa, page 72 

 and Plate IX., Fig. 9a. 



t This cuticle has been hitherto overlooked in the Blow-Fly by both 

 myself and Mr. Lowne ; I have, however, recently ascertained its presence 

 in that insect to my entire satisfaction. 



X Plateau denies the existence of an acid gastric juice in insects. He says 

 that the contents of the alimentary canal in every case give alkaline or neutral 



