THE PROBOSCIS. -15 



forwards, occupying exactly the same position as in the Lep- 

 doptera. Whatever., their function may be, it is probably 

 called into activity when the proboscis is retracted, for they 

 are turned back when it is exserted ; they are probably connected 

 with the sense of taste or smell.* 



The second joint consists of two portions, one dorsal and the 

 other ventral, the former being the roof, and the latter the floor 

 of the cavity of the month ; so that they might well be named 

 lips, if the term had not been unfortunately applied to the ter- 

 minal lobes of the proboscis ; especially as the upper consists of 

 the labriun and terminal lobes of the maxilla?, and the inferior of 

 he united labium and inenturn. I shall call them respectively 

 the operculum, and the canula. 



The canula is thick and fleshy, being filled with muscles ; it 

 is terminated by the lobes of the proboscis, and is deeply grooved 

 on its dorsal surface, the groove forming the mouth, and 

 concealing the tongue. The edges of this groove overlap the 

 operculum, and completely cover its anterior extremity, meeting 

 in front of it, so that it is held firmly in its place, but by a 

 little violence it can be made t o start upwards and assume a 

 position at an angle with the canula, without any rupture of 

 membrane. It never leaves the groove, however, during life, 

 although in those diptera in which the maxillso, mandibles, and 

 tongue form lancets, the canula is capable of being folded 

 back, to bring the lancet into play. 



The operculum, Plate II, Fig. 7, consists of a central and two lateral 

 portions. 



The labrum or central part is tubular behind, and is united by a mem- 

 branous tube to the anterior extremity of the cavity of the pharynx, a 

 small semicircular piece of chitine protecting its bend ; the food passes from 

 the mouth by this means into the pharynx. The anterior two- thirds of 

 the labrum is open below and forms the roof of the mouth. 



The homologies of the lateral portions of the operculum have already been 

 discussed.f They are firmly attached to the back of the labrum and form, 



See Section IV. page 55, and XV. page 93. f Page 43. 



