MOUTH PARTS : TABANUS 25 



posterior and lateral aspects, by a shield-shaped plate of chitin, the 

 mentum ; anteriorly there is a thin gutter-shaped plate which may be 

 termed the labial gutter, in which the piercing parts rest. These two 

 plates are connected together by a loose membrane passing between their 

 lateral edges. The labella are bilaterally symmetrical oval lobes, continu- 

 ous with one another posteriorly, but diverging in front. Their external 

 walls, continuous with the walls of the labium, are strengthened along the 

 distal border by thin plates of chitin, the proximal one of which is 

 indented to receive a prominent spur of the mentum, the two forming a 

 loose joint. The internal walls of the labella, which are normally in 

 contact with one another, are formed by a complex structure known as 

 the pseudotracheal membrane, by means of which the fly is enabled to 

 absorb fluid from moist surfaces. This structure, which is of common 

 occurrence in the Diptera, will be described in detail as it occurs in 

 Mtisca, and it will be sufficient to note here that its occurrence in Tabanus 

 is in accordance with the observation often made, that these flies can 

 drink water. 



The muscles of the labium lie within the cavity enclosed by the wall, 

 and serve to control the position of the labella. They arise in three sets 

 from the internal surface of the mentum, and are inserted into the 

 internal surfaces of the chitinous plates of the labella. When all three 

 sets contract at the same time the labella are diverged from one another, 

 and also retracted ; by means of the anterior and posterior sets the 

 fly is able to adjust the oral surfaces to the surface on which it 

 wishes to feed. Retraction of the whole of the labella takes place when the 

 piercing stylets are introduced into the wound ; the intrinsic muscles 

 are assisted in retraction by a smaller pair inserted into two conspi- 

 cuous cornua at the proximal end of the mentum. 



When the internal or oral surfaces of the labella are to be used 

 for the absorption of fluid from a moist surface, the space between the 

 inner and outer walls of the organ is distended with blood forced down 

 from the head and body of the fly by respiratory movements. The 

 details of this important mechanism will be discussed with regard to 

 Musca, but it is well to note that, in the case of Tabanus and in fact all 

 the Diptera, the space enclosed by the labellar walls is continuous with the 

 general body cavity. 



In the space between the inner and outer walls of the labella there is 

 a labial salivary gland, which resembles that of Musca, and need not be 

 described here on that account. 



The channel between the, epipharynx and the hypopharynx leads into 

 4 



