THE PRESTOMUM IN THE ORTHORRAPHA 35 



making of the wound, and it seems certain that the ' teeth ' at their 

 distal ends fulfil an entirely different purpose, namely, that of preventing 

 particles too large for its lumen from entering the food canal, a function 

 which would be in keeping with what we know of the nature and origin 

 of these organs. In none of the Orthorraphic biting flies are the pro- 

 cesses in the least like the cutting edges or teeth found elsewhere in 

 the Diptera. They are, on the other hand, with the single exception 

 of the hook on the labrum-epipharynx of Simulium, mere processes 

 of no greater thickness than the margin from which they arise. 

 There are no muscles attached to the bases of these organs which 

 could effectively impart to them an in-and-out thrust, and any lateral or 

 rotatory movement is anatomically impossible. Moreover, both organs 

 fall a little short of the terminations of the appendages. There are, 

 it is true, some muscles, most easily seen in Tabanus, which may 

 retract and protract the buccal cavity, but the utmost limit of pro- 

 traction cannot bring the terminations of the epipharynx and the 

 hypopharynx to the level of the other cutting points, and indeed it seems 

 more likely that the buccal cavity, and with it these outgrowths, are 

 retracted during the making of the wound, and subsequently thrust down 

 when the level of blood is reached. It should be noted that the only 

 chitinous connection between the labrum-epipharynx and hypopharynx 

 is through the thin slip of chitin by which the labrum is articulated to 

 the clypeus, a condition of affairs which renders it most unlikely that 

 these organs can make any effective thrust. 



When the food channel of either Simulium, Ceratopogon, or Phlebo- 

 tomus is examined from the side with the parts in situ, it is found that 

 the flattened distal ends of the organs are separated from one another by 

 a small interval. The processes on the distal margins are directed 

 across this interval, and those of the two organs come in contact. The 

 opening at the prestomum is therefore guarded by an arrangement which 

 resembles, and presumably acts as, a sieve, preventing the ingress of 

 large particles. The size of the aperture can be regulated by the 

 contractions of the muscle at the base of the labrum. 



In Tabanus the mechanism appears to be somewhat different. 

 The distal end of the labrum-epipharynx is much broader and stronger 

 than that of the hypopharynx, and in the normal condition of the parts 

 the hypopharynx falls a little short of the termination of the former, 

 The dorsal boundary of the prestomum is thus strong and rigid, while 

 the ventral one is comparatively soft. When the ends of the organs 

 are in a layer of blood, therefore, the latter must enter the canal from a 



