PROBOSCIS IN STOMOXYDINAE 51 



least of insignis, the rods are almost in line with the labial gutter. The 

 posterior set of muscles then contracts, and draws the furca upwards, the 

 latter carrying with it the external wall, and through their continuity 

 the internal wall also, till the teeth are rotated outwards on their 

 bases. This turns their cutting edges outwards, and draws them through 

 the wound, and is the essential feature of the cutting act. It is possible 

 that the teeth are also used in a scraping manner, by alternating contrac- 

 tions and relaxations of the anterior set of muscles, the teeth being held 

 in the everted position by the action of the posterior set. The length of 

 the lever to the end of which the teeth are attached would make this 

 action more effective in insignis than in any of the others. Whether 

 the pseudotracheal membrane is used in the same way as that of Musca 

 it is difficult to say, but it appears probable. The proboscis in this 

 genus is not a piercing organ, and, although the cutting apparatus is an 

 extremely efficient one, the wound made must be a scarification rather 

 than a puncture. Probably the fly bores its way down to the level of 

 the blood by rapidly repeated eversions of the teeth, and by using the 

 everted teeth as a scraper, the discal sclerite being moved up and down 

 on its articulation with the labial gutter; the blood having been reached, 

 it is sucked up through the pseudotracheal channels exactly as is done 

 by Musca. 



The first two species described are comparatively rare flies and have 

 no special interest apart from their anatomy. The third is common and 

 widely distributed. It is highly probable that if a systematic examina- 

 tion of the proboscides of muscid flies known to be blood-feeders but 

 having a retractile proboscis were made, many more species would be 

 found. 



In the next group of flies, of which Stomoxys is the best known ex- 

 ample, there is a definite advance in the adaptation of the proboscis to 



the blood-sucking habit. The proboscis is a true pierc- 



. . ... , . . . . The Stomoxydinae 



ing organ, and is actually inserted into the skin. 



The most obvious change, one so marked as to obscure the essential 

 similarity, is the elongation of the haustellum. On account of its in- 

 creased length it can no longer be concealed when it is retracted, and 

 forms a conspicuous projection in front of the head when the fly is at 

 rest. The rostrum, on the other hand, is reduced in size. 



In the rostrum, in addition to the smaller size and more compact 

 nature of the part, there is an important modification of the buccal cavity. 



This has no resemblance to the buccal cavity of Musca 



The buccal cavity 

 or to that of the Orthorraphic flies, but has become 



