- 30 



is not fully extended) directly into the under surface of the basal portion of the 

 proboscis. On the upper side the labium is hollowed out to form a channel into 

 which the labrum-epipharynx, hypopharynx and maxillae fit, when the proboscis 

 is -retracted, and usually when it is extended. To each side of the tip of the 

 labium is attached a labella (in section in fig. 1, a'). The two labellae are 

 short, fleshy, and hang below the level of the lower wall of the labium, often, 

 when they are in -use, seeming to be at an angle to it; thus they have more 

 the appearance of distinct organs than they have in Bombylius. They are, 

 however, nothing more than two hanging lateral wings of the labium, joined to 

 it only by flexible chitinous walls, not by true joints. The labellae fold together 

 so that the interior face of the one presses upon the interior face of the other, 

 when they are not in use, and when the proboscis is folded in its channel beneath 

 the insect's head. This seems to be the normal, or resting position, of the 

 labellae, from which they are brought into action by a pair of extensor muscles, 

 one in each labella (as seen in fig 1, a' in section); their further action will be 

 described more fully later. Each labella has on its inner side pseudotracheae, all 

 branching from a main pseudotracheal stem, which extends anteriorly and 

 posteriorly along the upper side of each labella. The anterior portion of this 

 pseudotracheal stern has about twenty-four branches, the posterior portion about 

 eighteen branches. The branches of these pseudotracheal stems extend, approxim- 

 ately parallel to one another, from the upper to the under margin of each 

 labella, and are much finer than, but of similar structure to. the pseudotracheae 

 of Bombylius. 



The structure of the so-called fulcrum, which occupies the central portion of 

 the basal part of the proboscis, the use of the pseudotracheae, the mode of 

 expanding the labellae, and other points pertaining to the structure and use of 

 the proboscis in Eristalis will be discussed later, and more appropriately, in that 

 part of this paper which will be devoted to a . comparison of some of the cor- 

 responding organs in Culex, Bombylius and Musca. 



ANATOMY OF THE MOUTH-PARTS OF MUSCA. 



The proboscis of Musca domestica, the common house-fly, and that of M. 

 vomitoria, the blow-fly, have attracted the attention of naturalists, from the time 

 when Aristotle wrote, down to the present day, and to attempt anything like a 

 complete notice of the different papers which, wholly or in part, treat, more or 

 less fully, more or less scientifically, of this subject, would be an extravagant 



