MUSCA: LABIAL SALIVARY GLAND 41 



the labrum-epipharynx, and is rather broader in its transverse than in its 

 antero-posterior diameter; its lateral borders are produced a little for- 

 wards so as to come better into apposition with the epipharynx. Its 

 proximal half is closely fused with the anterior surface of the labium, 

 a feature which is generally found in the non-biting flies, for it is only 

 when the saliva has to be injected into a wound that the hypopharynx 

 becomes free. The salivary duct perforates the whole length of the 

 hypopharynx in the usual manner, as in Tabanus. At the upper end the 

 hypopharynx projects into the rostrum posterior to the epipharynx, and 

 is attached in a similar manner to the membraneous wall of the buccal 

 cavity. 



The labium is roughly cylindrical in shape, broader in the middle 

 than at the two ends, and a little flattened on the anterior surface. Its 

 wall is composed of two plates of chitin, united by a loose membrane. 

 Of these by far the larger is the mentum, an oblong plate, concave on 

 its anterior surface, which forms the posterior and part of the lateral 

 walls. On the anterior surface there is a very thin and narrow plate of 

 chitin, depressed below the rest of the surface, and united in its upper 

 half with the hypopharynx. On either side of this there is a thin but 

 rounded rod of chitin, the two forming the boundaries of the shallow 

 groove in which the labrum-epipharynx and the hypopharynx lie. These 

 rods take an important part in the articulation of the labium with the 

 labella. 



Within the cavity of the labium there are the intrinsic muscles which 

 move the labella, and the labial salivary gland. The latter is similar to 

 that found in Tabanus, and to that of Philaematom- 

 yia. It is situated at the lower end of the labium, " a 



at the point where it divides into the two labella, 

 and consists of a rounded mass of large cells, averaging about forty 

 microns in their long diameter, spherical or oval in shape, and closely 

 compressed together. They lie near the anterior surface of the labium, 

 and have their narrow ends directed towards the surface. The proto- 

 plasm is finely granular, and contains usually a single large nucleus, 

 occasionally also an additional small one. At one side of the nucleus 

 there is a permanent vacuole, generally cresentic in outline, and lined by 

 a clear non-staining layer said to consist of chitin. The cells communi- 

 cate with the exterior by means of fine ducts which arise intracellularly, 

 and unite to form a single channel on each side, opening at the side of 

 the oral pit. The secretion is thus poured out on to the internal or oral 

 surfaces of the labella. Its function is not known. 

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