THE PUPAL STAGE OF CULEX. 59 



A similar change occurs in the intestine. The epithelium divides 

 into a thin outer and a thick inner layer. The latter becomes 

 loosened, breaks up, and appears to be digested. 



In the rectum more complex changes occur, though here also the 

 superficial portion of the epithelium is thrown off, but it breaks up 

 later and more slowly than elsewhere ; in fact, the disintegration 

 and digestion appears to commence in the anterior part of the 

 stomach, and progress gradually backwards. Before the epithelium 

 shows any sign of disintegration in this region, the rectum becomes 

 differentiated into two parts : an anterior very wide part, the " rectal 

 pouch," and a narrower hinder portion, to which alone I shall apply 

 the term " rectum " from this stage onwards. The wall of the 

 rectal pouch rises up into four very large and prominent papillae, 

 the " rectal glands " : one ventral at the anterior end of the pouch, 

 just below the opening of the intestine into it ; one dorsal and 

 posterior, and two lateral, intermediate in position between the other 

 two. Nerves and tracheae push their way into the axis of each 

 papilla. The epithelium of the papillse undergoes division into two 

 layers as elsewhere, but the distal layer, which is ultimately shed, is 

 very thin, and the basal or permanent epithelium consists of very 

 large columnar cells, while the opposite is the case everywhere else, 

 and especially in the rectum, where the permanent epithelium is so 

 thin that I had difficulty in detecting it, and Chun (9) states that it 

 is absent in Musca and other insects. 



Besides this shedding of epithelium, changes of form occur in 

 various parts of the alimentary canal. 



The anterior part of the oesophagus expands, especially in the 

 female, and acquires a thick chitinous lining. In cross section it 

 becomes triangular, and the sides and roof, all of which are convex 

 inwards, are supplied with muscles arising from the walls of the 

 head, which by their contraction increase the size of this cavity, and 

 serve to produce the sucking action by which the imago draws the 

 blood of its victims up through its proboscis. This apparatus is not 

 well-developed in the male. 



The posterior part of the cesopltagus gives off ventrally a large 

 diverticulum ("crop"), which runs backwards under the stomach as 

 far as the hinder end of the thorax, its walls developing numerous 

 small sacculations along its two sides. Sometimes a forwardly- 



