126 MEDICAL ENTOMOLOGY 



habits are entirely different, as they take one large meal and then rest 

 while digestion proceeds, while Musca and its allies feed intermittently. 

 In the Orthorraphic flies, which characteristically take one large meal 

 from the same wound, the process of excretion is seen only when the flies 

 are caught in the act of sucking. When examined in the fasting state 

 only globules of small size are found, and these do not cause any marked 

 bulging of the cell. If examined after a full meal, before digestion 

 has proceeded far, say within the first four or five hours, the cells 

 are found to be flattened out by the distension of the gut and the 

 stretching of the wall until they resemble a squamous epithelium ; 

 the longest axis of the cells then lies in the direction of the circular 

 muscle fibres. Even in Tabanus, in which the cells are invaginated 

 into the lumen in the form of villi in the resting condition, this is 

 the case, as the first effect of the distension of the gut is to flatten 

 the folds until the epithelium is disposed in a single layer of colum- 

 nar cells; later, with an increase in the contents of the gut, these 

 become flattened, gaining in one diameter what they lose in the other. 

 The discharge of the secretion into the lumen is, therefore, a sudden 

 one, taking place under the stimulus of the entry of blood, and is 

 complete within a very short time after the meal is ingested. Probably 

 the great increase in tension in the gut and its wall materially assists 

 in the process. 



The appearances seen in sections of the mid-gut during this process 

 are to a large extent artificial. The digestive fluid contains substances 

 which are precipitated by the alcohol used in the preparation of the 

 tissues, and these appear in the section as granules, surrounding 

 the clear spaces which represent the vacuoles. It is, of course, pos- 

 sible that some of the granules seen are pre-existent in the cell. 



The action of the digestive fluid on the red blood cells results 

 first in haemolysis, and subsequently in the formation of a brownish 



_. x .. . black pigment from the undigested residue. This 



Digestion or blood 



occurs wherever the blood corpuscles reach the 

 periphery, and thus come into contact with the secretion as it is 

 discharged from the cells. But the blood in the gut is kept in a 

 state of constant movement by the contraction of the longitudinal 

 and circular muscle fibres of the wall, so that the layer of haemolized 

 cells and the resulting pigment are moved away as soon as they are 

 formed, and their place taken by fresh cells, until the whole has 

 been digested. In some parts of the gut from which the figures on 

 Plate XXVI were drawn layers of pigment formed in this way can 



