PROBOSCIS OF HIPPOBOSCA 65 



The distal end of the external wall of the labium is separated from 

 the rest by a short interval of thin and flexible membrane, the margin 

 proximal to which is a little thickened. The tendons which traverse 

 the length of the labium, arising from the muscles of the bulb, are 

 inserted into this separated area, so that a contraction of these muscles 

 results in drawing the portion of the wall distal to the membrane up- 

 wards. This everts the prestomal teeth, and also draws the line of 

 teeth on the external surface upwards through the wound, the method of 

 action being in all essential points the same as that in Glossina. The 

 teeth are thrust back into position for the next cut by the distension of 

 the space between the wall in the labellar area and the labial gutter. 

 These actions must, of course, be repeated with extreme rapidity. 



The palps are elongate and ovoid in shape, and consist of a single 

 joint. Their external surfaces are convex, and are beset with stout hairs, 

 while the internal surfaces are flattened. The two are in close contact 

 with one another in the position of rest, and are the only part of the 

 apparatus which can be seen without dissection. On the inner surface 

 of each there is a deep longitudinal gutter, the two gutters forming 

 a canal in which the narrow part of the haustellum rests. 



The membrane from which the palps arise is attached above to the 

 distal edge of the clypeus, and corresponds to the anterior wall of the 

 rostrum. But on account of the position of the haustellum, all of which, 

 except that part which lies between the palps, is within the head in 

 the position of rest, the membrane, in order to reach its attachment to the 

 distal end of the haustellum, has to be invaginated within the head 

 around the latter. In sections, therefore, the haustellum appears to be 

 surrounded by a tube of membrane, which commences at the aperture 

 behind the palps, and terminates by becoming attached to the distal end 

 of the bulb. 



The parts corresponding to the rostrum are provided with a muscul- 

 ature very closely resembling that of Musca, but having the bundles more 

 clearly differentiated from one another than in that form (Plate XIII, fig. 

 3). The actions of the muscles are also the same, but the final result, 

 instead of being a protrusion of the rostrum from the head and the 

 extension of the whole proboscis, is only the protrusion of the haustellum 

 from between the palps. The fulcrum is rotated on its attachment to 

 the clypeus, so that the pharynx comes into line with the haustellum, 

 and at the same time the buccal cavity which connects the two is 

 straightened. In the resting position the buccal cavity is bent backwards 

 in its lower part, on account of its length. 

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