SOFT PARTS AND MECHANISM OF THE PROBOSCIS. 401 
There are two distinct ways in which the sucker of the 
proboscis comes into operation, in drinking and in feeding on 
solid substances; in the former act the oral sucker is applied 
to the surface of the fluid, which necessarily passes by capillary 
attraction into the pseudo-trachee. The mouth-opening de- 
scribed by Reaumur is alternately opened and closed. When 
it is open, a current passes directly into the mouth; but when 
it is closed, fluids may flow into the prestomum from the 
pseudo-tracheal channels, driven into it by their elastic walls, 
which exhibit quasi-peristaltic movements as the air passes into 
and out of the air-channels. When, however, the insect feeds 
on solids, the oral sucker is applied by its edges to the food, 
and the pseudo-trachez distribute the salivary secretion of the 
lingual glands over the anterior surface of the sucker; this 
dissolves the food, assisted, perhaps, by the disintegrating 
action of the stomal teeth, which, as Suffolk [58, p. 339] 
remarked, leave lines upon the food material; and the fluid, 
with small particles in suspension, is drawn into the oral 
cavity. Under these circumstances, as I formerly suggested 
[62], the pseudo-trachee probably conduct it into the pre- 
stomum and mouth and act as strainers. 
That the little pouches of thin integument in the forks of the 
rings, which Anthony regarded as suckers, are the main agents 
of suction, as he appears to suggest, is an untenable hypothesis ; 
but that they may assist in the passage of fluid into the pseudo- 
trachee is, I think, exceedingly probable. Flies undoubtedly 
collect moisture from damp foliage and smooth surfaces which 
have only a film of moisture on them, and it appears to me pro- 
bable that these pouches may be alternately emptied and filled 
by the alterations in the tension of the oral lobes which occur 
during the ingestion of food. The disposition of the sacs is such 
that it is easy to see that they may alternately open on the sur- 
face, and into the pseudo-tracheal channels. Moreover, capillary 
attraction may assist in filling the pseudo-trachez through the 
minute orifices of these pouches. 
The mouth frequently contains air, whether drawn into its 
cavity by suction or expelled from the alimentary canal, and 
27—2 
