478 
Head of Psylla mali 
Returning now to the consideration of the mechanism present in the insect, 
is it possible to apply either—or both—of these methods? From what has 
been recorded above, it is clear that the first method can be ruled out at once, 
at any rate as far as P. mali is concerned. In this insect the setae are free 
after they leave the epipharynx until they are lodged within the setal chamber, 
and although, within the proboscis the bending of the setae would be controlled 
to some extent by the groove in which they lie, yet in the proximal region of 
the labium they could still bend in one direction, namely through the open side 
of the groove. There is therefore no structure present which would serve as 
a closely fitting tube to restrain the bending of the setae. 
At first sight, the alternative mechanical device suggested scarcely seems 
to offer a solution, for it is not at all obvious what structure in the mechanism 
of the mouth parts could possibly take the place of the gripping instrument— 
forceps or pincers— used in the illustration. 
The only portion of the mouth parts which, from its position with relation 
to the setae, could possibly function in this way, is the labium or proboscis. 
In the description of the labium of P. mali it was stated that as the tip of the 
organ is reached, the character of the groove changes somewhat. In this 
region, the walls of the groove and of the labium itself, are more strongly 
chitinised, and the lumen of the groove becomes smaller so that the setae are 
more closely enclosed by it. In fact, at the tip of the proboscis, the relation of 
the groove to the setae is such, that a very little compressing force would be 
necessary to ensure the latter being held in a firm grip. 
It is evident therefore that the tip of the labium will provide the instrument 
for gripping the setae, but the next problem to solve is the origin and nature 
of the driving force which is to act through the labium to thrust the setae into 
the plant. There appear to be two possibilities. The first is, that the force 
might result from downward pressure exerted by the closer approximation of 
the body of the insect to the substratum, the labium being held rigid the while. 
The objections to this are that it is cumbrous, and that the labium is not 
particularly rigid throughout its whole length, the first segment particularly 
being composed of relatively thin chitin. The other possibility is that the 
force arises as a result of increased pressure of the body fluid contained within 
the labium. This increased internal pressure would cause the organ to become 
turgid and elongated, since the flexible portions between the segments of the 
proboscis, and also part of the flexed first segment, would become straightened 
out. Consequently, provided the body of the insect was held rigid, the tip of 
the labium would be pushed down towards the substratum. Another conse¬ 
quence of this increased internal pressure would be that the grip of the tip of 
the labium would be increased, for the pressure would tend to bring the 
margins of the groove closer together, thus further reducing the space enclosed. 
In this way, the two desiderata, the grip on the setae, and the downward 
driving force, would arise simultaneously. On this hypothesis, the sequence 
of events during the operation-of piercing would be as follows: 
