OF SUCTION IN LYGUS PABULINUS. 021 
is no space left in the lumen of the pharynx when the operculum 
is in the normal position, but a strong vacuum is produced in it 
when it is pulled out. 
It is lined with a thin and soft chitin which is continued 
dorsally into a ligament to which the divaricators are attached. 
Posteriorly it loses its character as an operculum (text-fig. 22) ; 
it is no longer introversible though it remains flexible. The “ V” 
and the operculum form together, in this posterior part, one 
structure, which is more or less circular and is continued in the 
esophagus. In this region the muscles are directly attached to it 
instead of to the ligament. As it passes anteriorly into the 
pharyngeal duct it also loses its flexibility and introversibility. 
It fuses with the epipharynx (labrum) which now covers the 
ventral wall, and the muscles are again no longer attached to it. 
Thus in both directions (anteriorly and posteriorly) the pharynx 
proper loses its character as such, both in respect of the “ V” 
and the operculum. 
There is a hinge-like fold of chitin upon which the operculum 
turns inside out. It will be understood from the figure (text- 
fig. 20). 
The supports of the pharnyx proper are :— 
(i.) The hypopharynx. 
(ii.) The clypeal folds. 
(iii,) The body of the tentorium with its dorsal arms. 
(iv.) The posterior arms of the tentorium, 
All these supports make the “ V” immovably and firmly fixed 
in its proper place, while the operculum remains all the time 
flexible and introversible. 
The function of the pharynx proper is very important since it 
is the chief organ of suction. The pharyngeal duct cannot start 
suction as there ave no muscles attached to its dorsal wall, which * 
is no longer introversible; as explained above the operculum in its 
normal position is closely apposed to the “ V,” and thus there is 
no empty space between them. The muscles contract, the oper- 
culum is pulled out, and a strong vacuum is produced. To fill it 
up, the sap, which is now present in the pharyngeal duct, is sucked 
into it, 
Gil.) The Hsophagus. 
There is no sharp external demarcation between the pharynx 
proper and the cesophagus, the transition is gradual, the “ V ” and 
the operculum losing their characteristic structures. In some 
other species there is, at the entrance to the cesophagus, a constric- 
tion, which is due to the fact that the thick chitin of the pharynx 
proper ends suddenly where the cesophagus begins. ‘The nerve- 
mass, consisting of the supra-csophageal ganglia, the lateral 
commissures, and the sub-csophageal ganglia, surrounds the 
esophagus, which then opens straight into the stomach through 
the cardiac valve. 
