V] THE ALIMENTARY AND EXCRETORY SYSTEMS 109 



gizzard is probably also of use in compressing the triturated 

 particles into a form suitable for passing into the mid-gut. 



Posteriorly, the gizzard narrows to a short portion which, histo- 

 logically, closely resembles a part of the oesophagus. This is deeply 

 sunk into the anterior end of the mid-gut, the arrangement suggesting 

 the top of a "safety" ink-pot. It is known as the oesophageal 

 valve. 



The Mid-gut (fig. 49). 



The mid-gut of the Dragonfly, both in the larva and imago, is 

 exceptional in possessing no caeca. It is very clearly marked off 

 from the gizzard above, and from the small intestine below, the 

 change in the character of the epithelial cells taking place quite 

 suddenly along a definite transverse plane. The following description 

 applies to the structure and functions of the mid-gut in the 

 larva [98, 187]. 



The mid-gut is generally of a distinct yellowish colour, owing 

 to the presence of yellow pigment granules in its epithelial cells. 

 Anteriorly there is a short annular zone, which takes no part in the 

 process of digestion. The cells of this region remain normal (i.e. 

 they resemble the cells of the other part of the mid-gut during the 

 period of rest). Their function is simply to elaborate the anterior 

 end of the peritrophic membrane (see below). 



The rest of the mid-gut is made up of the following layers : 



(a) A layer of mostly large, columnar epithelial cells, bounding 

 the internal cavity. These cells have large spheroidal nuclei (A, nu). 

 The yellow pigment granules (yp), already mentioned, occur towards 

 their inner or free border. This free border ends in a peculiar 

 delicate lamina (ib), destined to form part of the peritrophic mem- 

 brane. There is no chitinous intima. Externally, the epithelial 

 cells rest on a fine, delicate, but distinct basement membrane (bm). 



(b) A weak tunic of circular muscle-fibres (cm), external to the 

 basement membrane. 



(c) A still weaker incomplete tunic of longitudinal muscle- 

 fibres (lm), forming a loose investment external to (6). 



In order to understand the very remarkable physiology of 

 digestion in the Dragonfly larva, and the extraordinary changes 

 undergone by the cells of the mid-gut during the process, it must 



