400 THE ALIMENTARY CANAL OF THE IMAGO. 
angle, or they form a deeper or shallower funnel, become 
flattened or tumid; sometimes the whole, sometimes a part 
only, of the disc is swollen. The most constant phenomenon, how- 
ever, is an undulatory movement of all the channels of the lips, 
accompanied by vapid vibrations of their surface.’ 
‘The end towards which these movements tend is not equi- 
vocal; they cause the syrup to enter the proboscis. Whilst on 
this subject, I would remark that the lips touch each other in 
the centre of the disc, but appear to leave two openings, one in 
front and one behind. The anterior of these may be called the 
mouth of the fly, and one may observe currents passing into it 
when the syrup is very fluid’ [1, p. 206 (the italics are mine)]. 
Reaumur also observes [p. 205] that the channels of the 
lips (pseudo-trachez) of certain flies are black when the lips 
are moderately distended, and that when fully distended they 
become white. 
The vibratory movement of the discs and the altered ap- 
pearance of the pseudo-trachez depend upon the air, which is 
alternately forced into and allowed to flow out of the trachez 
beneath the pseudo-tracheal channels. When these are dis- 
tended, the pseudo-trachee open along their whole length, and 
as soon as the pressure of the air is diminished the recoil 
closes them. The extension of the air-channels enlarges and 
flattens the disc, or in an extreme case produces a convex 
surface, whilst the escape of air from them into the large 
trachez of the disc and haustellum diminishes the extent of 
the disc, which is rendered concave by the contraction of the 
paraphysal muscles and the tension of the tendinous cords. 
There has been much difference of opinion as to the use of 
the pseudo-tracheal channels. Some observers have held that 
they are concerned in conducting the food into the mouth, and 
that they act asa strainer. Others regard them as conduits for 
the saliva secreted by the lingual glands. Kraepelin argues in 
favour of the latter opinion, but I have repeatedly fed flies with 
blood and with syrup deeply stained with carmine, and after- 
wards found, by making sections of the proboscis, many of the 
pseudo-trachez filled with these substances. 
