45 







is continuous basally, sometimes with folding, into the under wall of the head. 

 The labium is usually somewhat fleshy, containing muscles and two longitudinal 

 trachea! stems, and is hollowed out along the upper side to form a trough for 

 containing other mouth-parts. At each side of the tip, the labium, probably in 

 all cases, has more or less developed labellae, on the inner surface of which are 

 usually the channels termed pseudotracheae. These pseudotracheae may be absent, 

 however, as in Culex. The labellae are fleshy in Musca, less so in Eristalis, and 

 least so in Bombylvus and Culex. While the labium of all insects is undoubtedly 

 the product of a coalesced pair of appendages, as Savigny supposed, and its parts 

 can be still further safely hornologized, in many insects, with particular portions 

 of the maxillae, and altho it has been divided in Orthoptera and some other 

 insects into subrnentum, mentum, glossa, etc., I do, not believe that in .the 

 diptera, our knowledge of the structure of the labium suffices for us to enter 

 safely on homologies of its parts, and I have, consequently, refrained from doing 

 so in the proceeding pages. 



The mode of separation and approximation, of the labellae is somewhat diffe- 

 rent in different diptera, and is, perhaps, worthy of a comparative notice in this 

 place. As already mentioned (p. 18), the labellae of Culex are attached to 

 the tip of the labium by a true joint, and have, each, a flexor and extensor 

 muscle. The labellae of Bombylius, described briefly on p. 25, altho not 

 connected to the labium by a true joint, have each a flexor and extensor 

 muscle. When, however, one examines, by sections, the labellae of Eristalis, 

 which I have briefly described on p. 30, one finds only an extensor muscle 

 in each labella, and in the sections of the labellae of Musca one finds no muscles. 

 How, then, is the separation of the labellae effected in Musca, for both in Musca 

 and in Eristalis their normal condition seems to be that of opposition ? Examining 

 the sections of the labellae of Eristalis (pi. 3, fig. 1, ') one sees that a chitiuous 

 band passes through the middle of each labella, and that, to the outer side of 

 this band, the extensor muscle is attached. The bands serve as supports for the 

 labellae, and as attachments for their extensor muscles; and these bands are 

 themselves jointed at their bases to the inner or upper chitinous wall of the 

 labium. An examination of cross-sections of the labellae of Musca shows this 

 same chitinous band, but no muscle. The reason for the absence of the muscle 

 is that, as has been already described (p. 36), the muscle does not extend into 

 the cavity of each labella, but is only inserted on the base of these chitinous 

 bands. The labellae of both Musca and Eristalis are opened, then, at least in part, 

 by muscular force ; but probably the more complete separation of the two labellae 

 is caused by the same inflation witli air, which, as previous writers have supposed, 



