5a MEDICAL ENTOMOLOGY 



larger than the trachea. The labella, while they require comparatively 

 little supply of air, require a rich supply of nerves for their complex 

 mechanism. 



The labial salivary gland does not occur in the Stomoxydinae, having 

 disappeared with the pseudotracheal membrane. 



It has been stated that in the proboscis of Haematobia there are to 

 be found vestiges of a pseudotracheal membrane. In all other respects 



the mouth parts are similar to those of Stomoxys, such 

 Haematobia _. ... 



differences as there are, as for instance a rather less 



marked spindle shape in the labium, and a smaller disproportion 

 between the length of the rostrum and haustellum, indicating that 

 the fly is not so far advanced in specialization as the rest of 

 the group. At the distal end of the labella there are five small 

 triangular flaps (Plate XII, fig. 1), each composed of an inner and 

 an outer wall, continuous with the walls of the labella, and enclosing 

 between them a space which is continuous with the haematocoele. 

 The wall consists for the most part of a thin transparent membrane, 

 strengthened by small oval chitinous squamae, and resembles that 

 found elsewhere in the non-chitinized area of the proboscis in other 

 Stomoxydinae. In the middle line of the inner wall, however, these 

 squamae are elongated transversely, and arranged in a row so as 

 to form a shallow gutter ; as the region of the teeth is approached they 

 are gradually replaced by definite pseudotracheal rings, similar to those 

 of Musca, except that the channels do not appear to be quite so narrow 

 at the neck. Presumably the channels in the flaps and on the inner 

 walls of the labella are used in the same way as those of Musca, to suck 

 up the blood, when the haematocoele of the labella is distended. A 

 similar condition is found in Bdellolarynx. 



The proboscis of Glossina (Plate XI, fig. 2) is modified from the Musca 

 type in very much the same manner as that of the Stomoxydinae, but 

 the specialization has gone very much further. The homology can, how- 

 ever, be followed fairly closely, and the mechanism is practically the 

 same. 



The proboscis is held in front of the head when at rest, but is 

 concealed by the palps, which extend to its tip. On account of the great 



elongation of the haustellum, which in this genus is at 

 Glossina 



least three and a half times as long as the rostrum, the 



proboscis is very conspicuous even in the resting position. 



The rostrum has the shape of a very short pyramid, and is almost 

 entirely occupied by the fulcrum, this being more compact and more 



