124 MEDICAL ENTOMOLOGY 



The posterior portion of the tract, derived from the proctodaeum, 

 serves only to conduct the residue resulting from the digestive process, 

 and the secretion of the Malpighian or urinary tubules, to the 

 exterior. 



The salivary glands probably provide some secretion which is con- 

 cerned in digestion, directly or indirectly. Little is known regarding its 



exact properties or function, but one thing is evident 

 Salivary glands , . . , 



trom anatomical considerations, namely, that in blood- 

 sucking insects it is necessary for the saliva to be conveyed to the 

 wound. The salivary system does not come into communication with 

 the alimentary tract anywhere except at the prestomum. In the 

 Diptera the conveyance of the saliva is arranged for by the elongation 

 of the hypopharynx ; in the Hemiptera and Siphonaptera, as will be 

 explained in later chapters, the same end is attained by different means. 



It has been stated that solid particles are not ingested by the cells of 

 the mid-gut, that is, that the food substances are rendered soluble before 



they are absorbed. The proof of this lies in the 



presence of the peritrophic membrane, which has 

 the peritrophic ., . .... 



membrane already been described as it occurs in the alimentary 



tract of Philaematomyia. The presence of such a 

 structure, completely separating the cells of the gut from the ingested 

 particles, renders necessary the assumption that the soluble products 

 of digestion arrive at the cells after passing through the membrane. 



In the Muscid flies, as in many other insects and their larvae, the 

 peritrophic membrane is a well-defined structure, which possesses sufficient 

 cohesion to enable one to separate it from the gut wall, and passes to 

 the distal end of the tract. In the Orthorraphic flies, on the other hand 

 (at least in Culex and Tabanus, which have been carefully examined), no 

 such definite membrane exists, there being only a delicate layer of 

 homogenous tissue, which stains faintly with eosin, between the blood in 

 the gut and the cells ; this layer is so fine that it can only be distin- 

 guished under the most favourable conditions. According to Schaudinn, 

 who noted its presence in the mosquito, it is formed as a secretion of the 

 chitinogenous cells of the fore-gut, given off by them as the fly feeds, and 

 passed into the mid-gut as a protecting envelope surrounding the mass of 

 blood. But in these Orthorraphic flies the gut is a distensible organ, 

 and is increased to many times its normal diameter when the insect has 

 ingested a full meal, and the layer of secretion, only a thin one at the 

 most, becomes so thinned out by the stretching of the wall that it can 

 no longer be recognized. It can, as was pointed out by Schaudinn, be 



