THE PROBOSCIS 21 



consequence somewhat difficult to decide how much of the labium is 

 properly to be regarded as composed of the appendages. The two basal 

 pieces are known as the submentum and the mentum. The latter is 

 conspicuous in the elongate proboscis of the Diptera. 



The mesial constituents of the mouth apparatus are inconspicuous 

 in the cockroach. The labrum is a small transverse flap which occupies 

 the space between the mandibles in front, and is attached to the clypeus 

 by a moveable joint. It is, in fact, a process of the clypeus, and forms 

 the dorsal boundary of the mouth aperture. There is no epipharynx in 

 the cockroach, and the hypopharynx is represented by a small chitinous 

 fold in the floor of the mouth, on the posterior surface of which the 

 salivary duct opens. 



Now it is from such a condition of affairs, both as regards structure 

 and function, that the Dipterous mouth has evolved, and from a consider- 

 ation of the change in habit one can deduce the changes 



which would become necessary in the structure of the Mam features of the 



~ , . , , , , ,. , proboscis in the 



parts, firstly, in consequence or the exchange of solid Diptera 



food for fluid, and secondly, in the case of the blood- 

 sucking forms, as a result of the necessity for making a wound in 

 the skin of the host. For the first a tubular channel up which the fluid 

 can be sucked is essential, for the mouth parts of such insects as the 

 cockroach are adapted only for the ingestion of solid particles. In the 

 Diptera this is accomplished by the development, to a very high degree, 

 of just those parts of the mouth apparatus which are rudimentary in the 

 cockroach, namely, the epipharynx and the hypopharynx. (Plate III, fig. 5.) 

 These are outgrowths, from the dorsal and ventral walls respectively, of 

 the stomodaeum, * which pass forward to the level of the terminations of 

 the other appendages ; the dorsal outgrowth is deeply grooved on its 

 under surface, and the ventral one flattened, the two being so apposed to 

 one another as to form a closed channel. The hypopharynx fulfils the 

 secondary but equally essential function of conveying the saliva to the 

 level of the distal orifice of the channel, in order that it may be mixed 

 with the food. The labrum, which lies immediately above the epi- 

 pharynx, is produced forwards and partially fused with it in order to 

 strengthen the channel. The muscle at the base of the labrum, which 

 in the cockroach lifts up the labrum in order to widen the mouth, is 

 retained, and functions in regulating the size of the distal aperture 

 between the epipharynx and the hypopharynx. 



To save confusion in the subsequent descriptions, the point at which 

 * The first part of the alimentary canal. See page 26. 



