flaps of the proboscis, halfway down which it is joined 

 by the salivary duct (S.D.). At the entrance to the 

 stomach it is bifurcated, and one limb of the bifurca- 

 tion is extended backwards to the bilobed crop. By a 

 valvular apparatus at the entrance to the stomach the insect 

 can direct the liquid driven by the pump (p) in its trunk 

 either into the stomach or crop. 



The proboscis (P) is a highly elastic muscular organ with 

 universal movement. At the end are two flaps or labella 

 (only one of which is shown, L), which it can open out like 

 the leaves of a book and apply the medial surfaces to 

 the material it feeds upon. From the middle line or hinge 

 minute chitinous channels pass outwards to the margin. 

 These are seen in the diagram of the front view of a fly's 



FIG. 3. 



Diagram of alimentary canal and mouth parts of house-fly. 

 p, Pump. I, Labella. g, Gullet. SD, Salivary duct. 

 s, Stomach, c, Crop. I, Intestine, r, Rectum. 



head (Fig. 4, CH), in which the labella are shown opened 

 out and facing forwards. These tubular structures are 

 strengthened at frequent intervals by chitinous rings like 

 a trachea, but are not complete tubes, being open to the 

 surface by a minute linear channel with lateral bays in it. 

 At the base of the trunk a number of muscle fibres are 

 attached to the gullet by the peristaltic contraction of 

 which fluid is pumped up from the mouth and propelled 

 into the stomach or crop. 



The structural arrangement of the flaps of the trunk acts 

 as a filter. Graham Smith (1911) is of opinion that solid 

 objects larger than 0' 006 mm. seldom pass into the gullet. 

 According to Nicoll (1911), the ova of such tapeworms 

 as do not exceed 0-035 in their smallest diameter maybe 

 swallowed. These ova must, therefore, get into the mouth 

 direct. When feeding on a liquid the fly applies the labella 

 to the surface and sucks the liquid through the " strainer" 



