38 THE ROYAL SOCIETY OF CANADA 



into which they were liberated. Although Mitzman (1913) found 

 that the flies would take the initial bite six to eight hours after emer- 

 ging from the pupa, I was unable, except in one case, to induce the 

 flies selected for the purpose to take their first feed for at least twenty- 

 four hours, although they were repeatedly placed on the skin. They 

 usually fed readily between twenty-four and forty-eight hours after 

 emergence. 



The fly usually inserts the proboscis where it alights and does 

 not waste time selecting a favourable place. Sometimes it will move 

 the tip of the proboscis about a little on the chosen place. Under 

 natural conditions no indication of their approach is given, and no 

 notice would be taken of their presence except for the pain occasioned 

 by the act of feeding. Indeed, the fly may not infrequently insert 

 its proboscis and feed without the production of any noticeable sen- 

 sation of pain; this depends upon the place of insertion and is, no 

 doubt, governed by the local distribution of the peripheral nerve 

 supply. Before inserting the proboscis the fly takes a firm "stance," 

 to use a golfing term. The proboscis is then lowered from its normal 

 horizontal to a vertical position, and driven into the skin. In the 

 act of driving the proboscis into the skin there are two distinct move- 

 ments. It is rapidly moved up and down and at the same time 

 partially rotated from side to side through an angle of 180 degrees. 

 While this main movement is taking place the labella at the distal 

 end of the haustellum are exserted and the teeth they bear are per- 

 forming the real cutting movement. Stephens and Newstead (1907), 

 in their detailed description of the anatomy of the proboscis of Stom- 

 oxys, have very aptly compared this movement to that of a carpenter's 

 auger with cutting flanges on either side, the only difference being that 

 the movement in Stomoxys is not continuously rotating but from side 

 to side. I was able to observe the action of the labella under the 

 Zeiss binocular microscope by feeding Stomoxys on gelatin. The 

 depth to which the proboscis is inserted varies and depends upon the 

 thickness of the epidermis, that is, the nearness to the surface of the 

 blood supply. Generally from one-third to one-half of the haustel- 

 lum is inserted. I have observed the proboscis plunged to the depth 

 of two-thirds of the labium, that is, to the swollen region, in a guinea 

 pig's ear. 



After the initial prick there occur occasional painful sensations 

 during the operation of blood-sucking, which would otherwise be 

 painless, as it sometimes is. In one case pain was experienced one 

 minute and three minutes after insertion, in a meal of four minutes 

 duration; in another case pain was experienced four minutes, six 

 minutes and seven minutes after insertion in a meal of nine minutes. 



