110 MEDICAL ENTOMOLOGY 



projecting into the lumen. The cells are more regular and conspicuous 

 in the posterior part of the tube, where the intima is thinner. There 

 is a thin layer of muscle fibres externally, best developed at the posterior 

 end. The lumen of the tube may appear wide in sections, when the 

 cells are very much flattened, or it may be contracted, in which case 

 the cells are more cubical. Evidently the tube is capable of consider- 

 able dilatation. 



The crop is of a totally different form to that of the other flies 

 referred to, and consists of three sacs with excessively thin walls, termed 



the oesobhageal diverticula. Two of these open into 

 Crop or Diverticula 



the oesophagus on the dorso-lateral aspect, and one on 



the ventral side. Their shape is extremely variable, as they have no 

 rigidity of their own, and are simply adapted to the structures among 

 which they lie. When they are dissected out of the body they contract 

 to form irregular ovoids. Usually they are filled up with minute bubbles 

 of gas, but in newly-hatched mosquitoes they are collapsed. The two 

 dorsal sacs are small, and do not extend beyond the thorax as a rule, but 

 the ventral one is very large, and passes into the abdomen, even as far as 

 the fifth segment, and occupies a very large proportion of the abdominal 

 cavity. 



The wall of the sac consists of an extremely delicate chitinous intima 

 in which a few scattered nuclei can be made out, and an external coating 

 of extremely fine muscle fibres, which run transversely around the sac, 

 but do not completely encircle it. The nature of the wall is best 

 made out in sections of newly-hatched mosquitoes, in which the sacs 

 have not been distended with air or blood. 



The function of these diverticula is the same as that of the crop in 

 Tabanus, with this difference, that in the mosquito the blood is retained 

 rather longer in them. They are, in fact, true ' food reservoirs ', as was 

 pointed out by Nuttall and Shipley. In mosquitoes killed during the act 

 of feeding they are always found to be full of blood, while a little later, 

 depending on the rate of digestion and therefore on the temperature, the 

 blood is almost entirely confined to the mid-gut. The great distension of 

 the body of a mosquito as it completes its meal is not due solely to the 

 distension of the mid-gut, but also to the distension of the diverticula 

 of the oesophagus. 



Nuttall and Shipley record some interesting observations with refer- 

 ence to the function of these sacs. They fed mosquitoes on a mixture 

 of blood serum and sugar, tinted with carmine, and again in twenty- 

 four hours on the same mixture without the colour. On dissection, 



