PROBOSCIS OF MUSCA: MOVEMENTS 45 



its margin a square-shaped incision. At each side of this there is a stout 

 chitinous rod, those of the two sides diverging from one another 

 as they pass downwards ; the rods have a thinner portion about 

 the middle of their length, so that one may speak of the two halves 

 as the proximal and distal portions of the fork of the mentum. The 

 furca lies between the arms of the fork. It is a thickening of the wall 

 of the labella, and is at every point closely attached to the membraneous 

 wall in this region, so that the labella must always follow it in its move- 

 ments. 



The movements of the proboscis are best discussed together with 

 the musculature (Plate VIII, fig. 1), taking each joint in turn. It has 

 already been stated that the proboscis is completely 



retractile, a fact which will be familiar to all who have H ov me " t8 p f 



the Proboscis 



watched the movements of the common house fly. 



The mechanism of extension and retraction will be described first. 



Extension of the rostrum on the head is brought about by the rotation 

 of the fulcrum downwards and forwards on the fixed point provided by 

 the attachment of the anterior arch to the epicranial wall. This is 

 brought about, not by direct muscular action, but by the distension of 

 the two large lateral air sacs which are contained within the wall of the 

 rostrum, behind the fulcrum. If one may use the simile, the rostrum is 

 thrust out and straightened as one might extend the finger of a glove by 

 blowing into it. The motive force is provided by the muscles which act 

 in respiration. 



Extension of the haustellum on the rostrum is brought about by the 

 contraction of a pair of muscles which arise from the lower end of the 

 fulcrum and are inserted into the expanded upper ends of the labral 

 apodemes. In the resting position the apodemes diverge like the arms 

 of a V, while in the extended position they are almost parallel to 

 one another. As the origin and insertion of the muscles are approxi- 

 mated the labrum-epipharynx, and with it the labium, is straightened 

 on the rostrum ; an instance, as Kraepelin remarked, of the application 

 of the parallelogram of forces. 



Retraction of the rostrum within the head cavity is brought about by 

 two pairs of muscles, a short pair running between the posterior cornua 

 of the fulcrum and the internal surface of the wall of the head capsule 

 below the antennae, and a long pair passing between the upper end of 

 the mentum and the region of the occipital foramen. The two pairs 

 acting together rotate the fulcrum in a backward and upward direction, 

 and at the same time pull it within the head. 



