46 



Retraction of the haustellum, or rather flexion of it on the rostrum, 

 is accomplished mainly by a pair of muscles which pass between the 

 anterior arch and the distal ends of the labral apodemes, and act in 

 opposition to the extensors of the haustellum. 



In addition to these main muscle bundles there are many more, on 

 both the anterior and posterior sides of the fulcrum, the functions of 

 which are difficult to determine on account of their small size. Their 

 distribution and attachments are described in Kraepelin's paper on 

 Musca domestica, and in Hansen's account of Stomoxys. 



In the labium there are three pairs of muscles, two of which act 

 upon the joints between it and the labella. (Plate VIII, fig. 1, and 

 Plate X, fig. 2.) The first of these is situated in front of and be- 

 tween the other pair, and runs from the upper end of the mentum, 

 and from the posterior surface of the upper end of the labial gutter, 

 to the posterior rods of the discal sclerite. The second, situated 

 behind and rather external to these, is inserted into the lateral arms 

 of the furca. These two pairs of muscles, which are not very clearly 

 differentiated from one another in sections, have a double function. 

 When they contract together the result is that the external wall of the 

 labella is rendered taut, and, through their continuity, the internal wall 

 also ; further contraction would result in withdrawing the pseudotracheal 

 membrane and exposing the prestomal teeth. When the anterior mus- 

 cles alone contract, the discal sclerite is rotated until it becomes per- 

 pendicular to the labium. When the posterior pair contracts the exter- 

 nal wall of the labella is withdrawn, and the pseudotracheal membrane 

 exposed. Owing to the lax nature of the wall of the labium, contrac- 

 tion of both muscles will also reduce its long diameter and increase 

 its transverse one. 



The third muscle is transverse in direction, and runs between the 

 labial gutter and the mentum in the lower part of the labium. It 

 will, therefore, counteract the last mentioned action of the two longi- 

 tudinal muscles. 



The movements of the labella are somewhat complex. In the 

 resting position the space between the two walls is practically non- 

 existent ; the pseudotracheal membranes of the two 



^ sides are in contact with one another, and the free ends 

 of the discal sclerite are directed forwards, the labella 

 lying slightly behind the end of the haustellum, while the latter is folded 

 against the retracted rostrum. When they are in use, however, the 

 space between the inner and outer walls of each labellum is distended 



