728 MR. P. R. AWATI ON THE MECHANISM 
is closed when that critical point is reached at which the pres- 
sure inside the chamber becomes higher than that in the afferent 
duct. This closure is effected by the posterior wall itself, which 
slides over and closes the entrance to the duct. Thus there is no 
necessity for a valve, and the saliva, once in the chamber, never 
flows back into the afferent duct. 
To sum up:— 
(i.) The pump is a force-pump. 
(ii.) The return-stroke of the posterior wall is effected by its 
own elasticity, when it has been pulled from the anterior wall by 
the pump muscles. 
(iii.) The efferent salivary duct is short and empties itself into 
the ejection-canal as soon as it is charged with secretion, It is 
therefore empty when the return-stroke begins. 
(iv.) The afferent salivary duct is always full of secretion, 
which is always under pressure. It is longer than the efferent 
duct. 
(v.) No valve is required at the mouth of the afferent duct into 
the chamber ; nor are there any muscles to open it. Its opening 
and closing are automatic. 
Tur MECHANISM OF SUCTION. 
The functions of the different parts concerned in suction 
have now been described under their respective headings. The 
present section gives a connected account of the mechanism, 
summarising, at the same time, what has been said above. The 
following is a picture of the mechanism as it might be imagined 
from the arrangement of the different structures described. 
The mechanism consists of many structures in the dead co- 
ordinated with one another, from the piercing of the epidermis 
of a leaf by the stylets to the digestion of the sap in the stomach. 
(1) The stylets inside a leaf. 
The insect begins to feel the surface of a leaf with the sensory 
hairs at the tip of the proboscis (text-fig. 29). It is trying to 
find a good place for piercing. When it is found, the proboscis 
is applied to it, the stylets are driven forward, the protractors 
contract, and the epidermal layer is pierced, however thick it 
may be. 
(2) The bending of the proboscis. 
Once in, the stylets are pushed down into the tissue by direct 
mechanical force, 2.e. by the bending of the proboscis. This 
feature has already been alluded to. It increases artificially the 
protrusion of the stylets and enables them to reach the tissue 
which eontains food. It seems that the insect is not confined to 
any particular plant-tissues. The stylets may be found in the 
xylem, the phloem, or in the cortex which contains food-substances 
(text-figs. 24, 25). 
