484 
Head of Psylla mali 
are considered, it will be seen that these difficulties are, perhaps, not so great 
as to be insurmountable. In describing the maxillary setae of P. mali, it was 
shown that the maxillary setae as they leave the head, pass out one on either 
side of the hypopharynx, and then are immediately received into a groove 
or channel formed by the apposition of ridges on the inner faces of the epi- 
pharynx and maxillary sclerites, which in this region are so constructed that 
they interlock, and, consequently, are firmly held together. The stability of 
these sclerites would also be assisted by the oral muscles YII ( b — e), the action 
of which would be to press the sclerites together against the roof of the buccal 
cavity. The effect of the enclosing of the setae in this channel in this way, 
would be that the termination of the pharynx and the proximal end of the 
suction-canal would be maintained in intimate relations with one another. 
Presuming then that the sap from the host rises up the suction-canal imme¬ 
diately the setae are inserted into the plant by the action of capillarity, 
assisted by whatever help may be derived from the pressure of the sap, it will 
follow that the proximal end of the canal and the orifice of the pharynx will 
be bathed in the fluid. As a consequence of this, any interstices in that portion 
of the canal where the interlocking of the setae is not so perfect will be filled 
up, and there will be direct “physical” continuity between any liquid within 
the pharynx and that contained in the suction-canal. If now a reduction of 
pressure, or tension, is produced inside the pharynx by the raising of the 
operculum by the pharyngeal muscles, a pull will be exerted upon the column 
of fluid in the suction-canal in a way similar to that which it has been 
suggested is exercised by the osmotic pressure in the leaves of a plant upon a 
contiguous column of sap. This, combined with the peristaltic action of the 
pharyngeal muscles, will ensure a continuous flow of fluid along the suction- 
canal and pharynx into the oesophagus. This then is the way in which it is 
suggested that the transference of the sap from the host-plant to the insect 
takes place. 
The second problem, that of the injection of the salivary secretion into the 
host, also presents some points of difficulty. The chief of these is again, the 
lack of physical continuity between the termination of the efferent salivary 
duct and the efferent salivary canal which traverses the maxillary setae. It 
is not easy to see, therefore, why the salivary secretion when it is ejected from 
the efferent salivary duct by the action of the salivary pump, should pass down 
the salivary canal and enter the wound. It appears just as probable that it 
would be poured into the buccal cavity, or even into the suction-canal. 
Another point to be considered is that if the suggestion made in considering 
the process of suction is true, that the sap rises in the suction-canal by means 
of capillarity, it will apply equally to the salivary canal, so that the propulsive 
force exerted by the salivary pump will not only have to eject the secretion, 
but also overcome the resistance of the capillary attraction. This makes it 
imperative that there should be as little leakage of force as possible, and that 
the connection between the two channels should be as close as possible. As 
