PROBOSCIS IN THE STOMOXYDINAE 57 



not well developed, for the anterior joint is practically fixed in a position 

 of incomplete extension. 



The labrum-epipharynx and the hypopharynx are much more slender 

 and narrower than those of Musca, since they are more effectively 



concealed and protected by the deeper labial gutter. 



TU ir.uurj-rj-.LU u Labrum-epipharynx 



The canal for the food is formed m the same way, but a||d hypopharynx 



with this notable difference, that in the upper part 

 of the haustellum it becomes closed by the fusion of the sides of the 

 groove formed by the epipharynx, and the union of the dorsal lamina of 

 the hypopharynx with this. The hypopharynx is free from the labial 

 gutter except at the upper end. 



Both the labrum-epipharynx and the hypopharynx are shorter than 

 the rest of the haustellum, and do not, in the resting position, reach 

 beyond the level of the furca. They cannot, therefore, take any part in 

 the making of the wound, and have, as one would expect, thin and 

 flaccid distal ends. On the ventral surface of the distal end of the 

 epipharynx there are several sensory tubercles or hairs, which, as they 

 are situated at the opening of the food canal, probably play a part in the 

 mechanism by which the size of the opening is regulated. 



The distance between the terminations of the labrum-epipharynx 

 and hypopharynx and the margin of the prestomum, as denned by the 

 discal sclerite, is materially reduced in the position of action, when 

 the wound has been made to a satisfactory depth, by a longitudi- 

 nal compression of the mentum. The surface of the latter is marked 

 by a series of transverse ridges and furrows, which extend all round the 

 organ, and are the most marked in its narrowest part. The deepening 

 of the furrows must be accompanied by a decrease in the length of the 

 labium. This is brought about by the further contraction of the power- 

 ful muscles of the bulb, which press the furca against the fork of the 

 mentum, and so compress the latter towards its proximal end. The ridges 

 and furrows are always more marked in specimens fixed in the everted 

 position than in those in which the labella are closed. 



The tracheal structures in the proboscis are the same as those of 

 Musca and Philaematomyia. One interesting point may be noted 

 with regard to the nerves. There is one large nerve on each side of 

 the cavity of the labium, in close contact with the trachea. At the 

 upper end of the haustellum the trachea is very much the larger of 

 the two, but as they pass downward it diminishes rapidly in size 

 as it gives off branches for the supply of the muscles. The 

 nerve becomes only a little smaller, and as it enters the labellum is 

 8 



