MOSQUITO: THE MID-GUT 111 



' the contents of the ventral sac were coloured red, that of the stom- 

 ach yellow, so that there could be no doubt but that the second meal 

 had been almost entirely taken up by the stomach'. They also confirmed 

 this by the examination of living insects similarly fed, the coloured 

 contents being plainly visible through the abdominal wall. Such an 

 observation might, as they point out, have an important bearing on the 

 development of any parasite taken up with the blood, but the fact must 

 not be lost sight of that the conditions of the experiment were highly 

 artificial, and it does not follow that the same result would be obtained 

 if the experiment could be repeated with blood for both feeds. As a 

 matter of fact mosquitoes under natural conditions will not, as a rule, 

 feed till the last meal is completely digested, and in cold weather one fre- 

 quently finds that an interval of as much as four days is necessary. 



The oesophageal valve, the supposed homologue of the proventriculus, 

 is an invagination of the posterior end of the oesophagus into the com- 

 mencement of the mid-gut. It forms a marked annular 



Oesophageal Valve 

 thickening due partly to a great increase in the circular 



muscles at this point, and partly to the presence, as pointed out by 

 Nuttall and Shipley, of small protuberances, varying apparently in 

 number in different species, which represent the caecal appendages of the 

 larval insect. When examined in section (Plate XXIII, fig. 5) the valve 

 is'seen to consist of a double fold of the total thickness of the wall of the 

 oesophagus, which is tucked inside the lumen of the commencement of 

 the mid-gut. The circular muscles which surround the canal at this point 

 presumably act in regulating the rate of passage of the blood from the 

 diverticula to the digestive chamber. No such valve is found in Tabanus, 

 in which the blood is passed on at once into the gut, and is not stored 

 in the crop. 



The mid-gut consists of two portions, which probably differ in their 

 physiology as well as in their anatomy. The first part is tubular, and 

 lies for the most part of its length in the thorax, and is 

 continued downwards from the oesophageal valve in 

 continuity with the oesophagus. It is a rather thick tube of uniform 

 diameter, and is marked at its anterior end by the thickening due to the 

 valve and the caeca. This portion is termed by Thompson the cardia. 

 At the outlet from the thorax, or a little behind this, it dilates to form 

 an oval chamber about three times the breadth of the thoracic portion. 

 This is the ' stomach ', and it is here that the blood accumulates 

 during the process of digestion ; it is here also that the parasite 

 of malaria is most frequently found. It terminates about the fifth segment 



