482 
Head of Psylla mali 
force is only applied when the labium becomes turgid and is consequently 
protruded. In its retracted state, the labium is comparatively flaccid and its 
hold upon the setae is lessened. As has been stated above, the major part of 
the first segment of the labium is fused to the under side of the body, and the 
free portion of the labium lies almost hidden between the articulation of the 
first pair of legs. If therefore the thorax is raised, the labium will be raised 
with it and if at the same time the labium is in a state of turgescence and 
consequently has a firm grip of the setae, a powerful retractile force will be 
brought to bear upon the setae and will withdraw them bodily from the wound. 
In the preliminary stages, some assistance is doubtless afforded by the elevator 
muscles of the labium though it is scarcely probable that these can do more 
than help to prevent the labium from being torn out of the body. 
THE MECHANISM OF SUCTION. 
The problem of the means whereby the juices obtained from the host are 
conveyed into the alimentary canal in the Hemiptera, is one which has received 
more attention from previous writers than the operation of piercing, but there 
are some points in connection with it which have either been taken more or 
less for granted or require further elucidation. By way of illustration the 
process of suction as described in two recent papers may be given. 
Awati (1914) in describing the mechanism of suction in Lygus pabulinus, 
the Potato Capsid Bug, sums up the factors concerned (p. 731) as: 
“(i) Capillarity in the suction canal, helped by the turgiditv of the cells. 
(ii) Suction produced by the vacuum through the raising of the operculum. 
(iii) The peristalsis in the oesophagus by means of the constrictors. 
(iv) The valvular action, which prevents the sap from flowing back into 
the oesophagus from the stomach.” 
He further adds: “The pharyngeal duct hangs into the suction canal and 
is immersed in the sap. A question arises whether it is possible for the sap to 
avoid the duct and flow into the body cavity from the suction canal. The sap 
cannot do so because (i) there is no difference of pressure to force the sap into 
the body-cavity, and because (ii) there is active suction through the pharyngeal 
duct.” 
“Another important thing for suction is that the suction- and ejection- 
canals must be separate and distinct from each other. The least intercommuni¬ 
cation will stop the whole mechanism. Also one canal cannot serve two 
contradictory purposes—one of sucking and the other of ejecting saliva. The 
whole mechanism, therefore, depends upon the presence of the two canals.' 
The author also says (p. 729): “Before the sap is sucked through the 
suction canal, it seems that saliva is injected into the wound made by the 
stylets.” 
Davidson (1914) describing the same process in Schizoneura lanigera, 
says (p. 326): 
