OF SUCTION IN LYGUS PABULINUS, 127 
ducts, one on each side, issuing from the salivary reservoirs in 
the thoracic region. ‘They are narrow and Jong, and run straight 
into the head, where they join together and form the common 
salivary duct. They are always full of secretion, and as they 
are thin and elastic, their walls are distended. The secretion is 
therefore under pressure. 
The Function of the Pump. 
The structures of the pump, described above, will show that it 
is used to force the salivary secretion forward into the efferent 
salivary duct and thence into the ejection-canal of the maxillary 
stylets. Hence it has been called a force-pump. 
The ejection-canal pours the secretion straight into a wound in 
a leaf made by the stylets. The secretion does not leak out since 
the canal is air-tight, which will be seen from cross-sections. 
The shape of the cylinder in different species depends upon the 
degree of force required to drive the secretion through the 
efferent duct. The more minute this duct the more cylindrical 
is the pump: the return-stroke of the posterior wall is more 
powerful in proportion to the force required to pull out the 
piston. The posterior wall of the pump may be compared to 
a bow. 
In the normal position the posterior wall is apposed to the 
anterior, and the capacity of the chamber is reduced to a 
minimum. When the pump muscles, which are attached to 
the handle, contract, the piston is pulled out and with it the 
posterior wall. The capacity of the chamber slowly increases 
and a partial vacuum is produced. Now, either the secretion 
from the efferent duct or that from the afferent duct must flow 
into the chamber to fill up this vacuum. The former case is 
impossible, as the efferent duct is very short and empties itself 
into the ejection-canal as soon as it is charged with the secretion, 
and when empty its lumen would tend to collapse owing to the 
thinness of its walls. On the other hand, the salivary ducts are 
always full of secretion under pressure, and their walls are always 
distended by their contents. As soon, then, as a vacuum is pro- 
duced in the chamber, the secretion in the afferent duct flows 
forward into it. Thus, every time the muscles contract and the 
vacuum is produced, fresh secretion from the reservoirs flows into 
the pump-chamber. 
When the muscles relax, the posterior wall of the cylinder 
begins to return to its former position, the return-stroke being 
effected by its own elasticity. The fluid in the chamber 1s 
gradually being compressed, and is forced along the line of least 
resistance, which lies through the efferent salivary duct. This 
duct is empty now, while the afferent duct is full of secretion 
under pressure. The secretion therefore must flow into the 
efferent duct. The space of the chamber is gradually being 
reduced, and the entrance of the afferent duct into the chamber 
