THE STRUCTURE AND FUNCTION OF THE PROBOSCIS 63 



The spaces between the rows of teeth form the gutters, 

 leading into the prestomum, which have already been mentioned. 

 The gutter is bordered by the teeth of the inner set, whilst its 

 floor is formed by the branches of the intermediate and outer 

 sets of teeth, and the integument investing these structures. 



The arrangement described is only found near the centre of 

 the prestomum. On either side of the two or three central 

 pseudo-trachese each tooth of the outer set is represented by two 

 bars of chitin, which are not united at their distal extremities 

 (see Fig. 22). Still further from the centre the outer set is 

 lacking, while at either end of the series both the outer and 

 intermediate sets are lacking. 



The arrangement of the teeth varies greatly in different 

 species. C. erytJirocepJiala has as described three teeth in each 

 row, ^. carnaria has four, and L. ccesar has three. In M. domes- 

 tica, F. canicularis and O. aitthrax there seems to be only one 

 definite series corresponding to the inner set of C. erytJirocephala. 

 In these species the sets which are lacking seem to be represented 

 by modified plates of chitin. 



When fluid food is being taken the teeth merely aid the con- 

 veyance of the fluid into the mouth by assisting in the formation 

 of the gutters. They may be used however under suitable 

 conditions in scraping the surfaces of hard substances to render 

 their solution more easy. In order to bring the teeth into action 

 as scrapers the lobes of the suctorial disc have to be more widely 

 separated than they usually are when liquid food is being taken. 

 When the teeth are in action as scrapers the prestomal cavity is 

 open to the surface, and if sucking efforts are made probably 

 large particles can pass into the mouth. 



(B) Feeding experiments. 



If hungry flies are fed on drops of syrup or other fluids thev 

 rapidly suck up large quantities. The general behaviour of flies 

 during the act of feeding is described later. In all cases the 

 suctorial disc is inflated and the lobes spread out so that the oral 

 surfaces of the labellae are nearly in one plane. If viewed from 

 its oral surface the disc presents the appearance seen in PI. XIV, 



