730 MR, P. R. AWATI ON THE MECIIANISM 
form starch into sugar and thus help its digestion. Plateau (41) 
has proved that the saliva of insects has this effect. It is 
doubtful whether the sap is coagulated by exposure to air, but if 
so, the saliva may prevent its coagulation. A third possibility 
is that saliva may make the sap less viscid. 
(4) Turgidity of the Cells. 
This may be important indirectly in forcing the sap mixed 
with saliva into the suction-canal of the maxillary stylets. The 
cells of a tissue are always turgid and their walls are stretched to 
their utmost capacity. They collapse if pricked, and squirt out 
the sap with some force. The maxillary stylets pierce the cells, 
the sap of which may thus be forced into the empty suction- 
canal, since the other canal is full of the salivary secretion. I 
attach no great weight to this possible factor in suction. 
(5) Capillarity. 
This factor may now come into action, and by its means the 
sap would begin to ascend in the suction-canal if this were open 
at the top. The sap reaches the pharyngeal duct, which hangs 
into the suction-canal, and is thus immersed in the sap. 
(6) Suction. 
The divaricators of the pharynx begin to act. Those of the 
anterior part of the clypeus contract first. The operculum is 
pulled out and a vacuum is formed between it and the “ V.” 
The sap is sucked into the pharynx from the pharyngeal duct. 
The muscles relax, and the operculum regains its former position. 
Meanwhile the muscles lying posteriorly contract, the operculum 
is pulled out, and the sap is forced onwards. The muscles lying 
still more posteriorly, do the same thing, and the same process is 
repeated. Thus there is a wave of contraction of muscles passing 
backwards, and the sap is continuously forced on towards the 
stomach. It is prevented from flowing back, because there is no 
empty space behind, since the operculum regains its normal 
position with relaxation of the anterior muscles. 
Thus, in the pharynx there are two complementary factors 
which force the sap onwards towards the esophagus :— 
(i.) The vacuum produced by the raising of the operculum. 
(il.) Elasticity of the operculum, which enables it to regain 
its normal position. 
(7) The Gsophagus. 
The sap is forced into the cesophagus, the walls of which are 
soft and flexible and have attached to them the constrictor 
muscles. As the sap distends the walls, the muscles contract, 
and a peristalsis is produced which forces the sap onwards towards 
the cardiac valve. 
