47(5 
Head of Psylla mali 
which have, however, to pierce the tough and cuticularised epidermis of a leaf. 
As they are very delicate, they would bend in the act of piercing, were they 
not enclosed in the proboscis. The lumen of the tubular tip of the proboscis is 
so small that the stylets fit into it tightly (text-fig. 8). There is no empty space 
in it for them to bend. The tip of the labium is closely applied to the surface 
of the leaf; the protractor muscles of the stylets contract; and the stylets are 
forced out of the proboscis and driven against the epidermis, which they 
cannot fail to pierce. Once in, their forward progress to the required tissue is 
mechanical owing to the bend of the labium at the first joint. 
It is thus obvious that the labium is an important structure in the sucking 
apparatus of this insect; it is one, moreover, on the structure of which stress 
has not been laid by previous writers.” 
These statements seem to contain some contradiction and confusion of 
ideas, which is unfortunate in what is evidently a serious attempt to explain 
the operation of piercing. The author emphasises the fact that the stylets are 
very delicate and would bend if they were not enclosed within the groove in 
the proboscis and yet goes on to say that once the setae are in (the tissues of 
the plant) their further progress is mechanical owing to the bending of the 
labium. The latter statement is clearly a contradiction of the first, for the 
author does not seem to have realised that if the labium bends the setae must 
leave the groove if the advantage which he claims,, viz. the further protrusion 
of the stylets, is to accrue: for if they still remain within the groove no in¬ 
creased protrusion would result. If therefore the setae do leave the groove, 
and remain in the same straight line with that portion of them which has 
already entered the host, it is clear that they will lose that support of the 
groove throughout a great part of their length which according to the author 
is essential for the successful piercing of the host, and when a further driving 
force is applied to them they would bend. The author passes over this serious 
difficulty by stating that once the setae have entered the host, their forward 
progress is mechanical, owing to the bend of the labium. This is unfortunately 
too vague, and leaves a great deal to the imagination of the reader. For what 
is the force which causes this forward “mechanical " progress? There appear 
to be only two alternatives; either the force produced by the further contrac¬ 
tion of the protractor muscles, or some pressure exerted by the lowering of 
the position of the head and body in relation to the substratum. Either of 
these would exert a driving force along the length of the setae but, as has been 
pointed out above, the major part of this would be vitiated by the bending 
of that portion of the stylets which was not contained in the groove of the 
proboscis, and although a slight penetration of the tips of the setae might 
occur, this would be quite inadequate to the result desired. 
This bending of the labium occurs also in other piercing insects outside 
the Hemiptera, as is admirably shown by Nuttall and Shipley (1901) in their 
figure of a female Anopheles maculipennis feeding. Here the trophi are free 
from the labium for the greater part of their length and the latter is bent into 
