64 THE STRUCTURE AND FUNCTION OF THE PROBOSCIS 



fig. I, the adjacent sides of the lobes being pressed together so 

 that the prestomal cavity is almost completely closed. 



If the flies are fed on shallow drying drops of somewhat con- 

 centrated syrup containing finely ground Indian ink deposited on 

 glass, proboscis marks, recognizable as white areas where the ink 

 deposit has been removed, can frequently be observed (PL XIV, 

 fig. 2). These areas correspond with the shape of the inflated pro- 

 boscis showing that the margins, and probably the greater part, of 

 the suctorial disc are closely applied. The firmer the application 

 of the disc to the surface supporting the food the more completely 

 are the walls of the prestomal cavity pressed against one another. 

 Hence under these circumstances no material can enter directly 

 into the mouth but has to be conveyed into it through the agency 

 of the pseudo-tracheae and collecting channels. 



If the head of a blow-fly is removed and the proboscis erected 

 by slight pressure on the head and fixed in that condition with 

 plasticine it is possible to obtain an excellent view of the 

 expanded disc. In this position each lobe of the disc is convex 

 in its transverse diameter and the entrance to the prestomal 

 cavity is recognizable as a logitudinal sulcus slightly expanded 

 near its centre. By applying a cover-glass to the oral surface 

 of the disc it can be readily shown how pressure exerted on the 

 disc closes the prestomal cavity in proportion to the degree of 

 the pressure. 



Under natural conditions flies probably seldom have the 

 opportunity of feeding on large drops but suck up thin films of 

 moisture and consequently feed with their proboscides so closely 

 applied that the longitudinal prestomal sulcus as well as the 

 longitudinal fissures of the pseudo-tracheae are to a great extent 

 obliterated. Under these conditions it seems impossible that 

 food should enter the mouth except through the interbifid 

 grooves, and that this is actually the case can be proved by 

 experiments with suitable fluids. If flies are allowed to suck 

 at films of partially dried Indian ink they often remove from the 

 glass only those portions which lie immediately under the inter- 

 bifid grooves. In such cases beautiful patterns like gratings are 

 left on the glass. PI. XIV, fig. 3, is a photograph of a portion of 

 one of these patterns. The fly has applied the proboscis firmly to 



