GLOSSINA: THE MID-GUT 117 



delicate and easily ruptured, but which possesses sufficient coherence to 

 enable one, by careful dissection, to draw out a considerable portion of it 

 intact and containing the food. 



The significance of this membrane will be evident from its relations. 

 It is a continuous sheet with no apertures, and therefore the digestive 

 fluids from the cells, and the digested products from the food, must 

 pass through it as through an osmotic membrane. 



The peritrophic membrane, according to Berlese, is composed of 

 chitin, secreted by special cells situated at the junction of the proven- 

 triculus with the mid-gut, to which is added a deposit of mucous sub- 

 stance secreted by the cells of the mesenteron. It appears to grow 

 downwards from the mesenteron till it reaches the anus. 



The alimentary canal of Musca is very similar to that of Philaematomyia. 

 The mid-gut is a little longer and more convoluted, and the ventriculus 



broader. The latter is narrower at its anterior end 

 i i i i i r i Wusca 



than elsewhere, but it is not separated from the pro- 



ventriculus by a definite neck as is the case in Philaematomyia, so that 

 almost the whole of the part lies in the thorax. The Malpighian tubes 

 are not usually dilated at the ends and are often of a yellowish or 

 greenish colour. It is rather remarkable that the colour of the tubes 

 should differ in different species. 



It should be noted that the part of the gut sometimes referred to 

 as the ' distal intestine' is the hind-gut. The proximal intestine belongs 

 to the mid-gut or mesenteron. 



The alimentary canal of Stomoxys is like that of Philaematomyia, and 

 is of about the same length. The narrow portion anterior to the dilated 

 ventriculus is longer, and extends through the thorax. 

 This is, in fact, a peculiarity found in all the blood- 

 sucking Muscids which have been examined, and is probably related to 

 their habit of taking in a large meal at one time. The Malpighian tubes 

 show an interesting feature, first pointed out by Tulloch in the 

 case of Stomoxys, but probably of common occurrence, namely, that 

 the dilated distal ends of one pair pass to the dorsal region and lie 

 in a constant position near the heart and pericardial cells. As these 

 two tubules arise by a common stem, it is possible, as suggested by 

 Minchin, that the tubes should be regarded as dorsal and ventral, not as 

 lateral. 



In Glossina there is a great elongation of the mid-gut, which has 

 brought about a corresponding complexity in the coils into which it is 

 thrown. The proventriculus, according to Minchin, is saddle- shaped, 



