114 MEDICAL ENTOMOLOGY 



It will be noted that the positions of the crop and the proventriculus 

 are the reverse of those in Tabanus, a fact which throws some light 

 on the development of the parts. It seems probable that in some remote 

 ancestor the crop was a simple dilatation of the oesophagus, such as is 

 found in many modern insects, and that this dilatation became divided into 

 several portions, one or more of which passed backwards towards the 

 abdomen, remaining connected with the oesophagus only by a narrow 

 duct. In Tabanus the crop has developed from a dorsal sacculation, 

 in the Muscidae from a ventral one. In the mosquito the develop- 

 ment is seen at an early stage, in which three of the original sacculations 

 are present, one of them, the ventral, being predominant, and extend- 

 ing to the abdomen when distended. All these structures function in 

 the same way, receiving the blood from the pharynx and passing it on 

 to the mid-gut for digestion, but the rate at which the crop is emptied 

 seems to differ in the different forms. In some, as in Tabanus, the 

 contractions of the crop appear to be as rapid as those of the pharynx, 

 so that the blood is passed on to the gut at once, while in others, among 

 which are Musca and Philaematomyia, the food is retained for a time, 

 and passed on gradually. The object of the crop is to enable the fly 

 to absorb a very large amount of food at one time evidently a great 

 advantage in the case of those flies which do not remain permanently 

 in association with the host on whose blood they feed. 



The proventriculus is a small disc-shaped body, flattened from 

 above downwards, but placed rather obliquely in the thorax, with its lower 



. , edge directed downwards and forwards. The dorsal 

 Proventriculus . . 



surface is smooth and convex ; the ventral surface is 



irregular on account of the attachment of the oesophagus and the duct 

 of the crop. 



The structure of the proventriculus is best understood from the 

 study of a longitudinal section, such as that shown in Plate XXIV, 

 fig. 2. It consists of a mass of cells, part of which are derived from 

 the mid-gut and part from the fore-gut. The external layer is continuous 

 with the mid-gut and is composed of columnar cells which are longest 

 in the anterior portion. Inside this there is a thick plug of smaller round 

 cells which belong to the fore-gut. The oesophagus is, in fact, tucked 

 inside the dilated end of the mid-gut, the portion which is invaginated 

 having a greatly thickened wall. The opening by which the oesophagus 

 and the mid-gut communicate lies in the middle of this plug, and is 

 not shown in the section figured, as this is not precisely through the 

 middle line. The invaginated portion is button-shaped, and, since the 



