A. J. GrROVE 
470 
The insect having, by means of the tactile hairs at the tip of the proboscis, 
selected the spot at which it desires to pierce the plant, the labium would be 
drawn up for a short distance by means of the elevator muscle, exposing a 
small portion of the pointed ends of the stylets which would protrude from 
the tip. The internal pressure of the body fluid within the labium would then 
be increased by the forcing of more fluid into it from the body, with the result 
that the tip of the labium would grip the setae, and, with the consequent 
elongation of the organ, the projecting portions of the setae would be driven 
into the plant. This operation completed, so that the tip of the labium would 
be in contact with the substratum, the internal pressure would be slackened, 
and as a result of this slackening two things would happen. First, the grip on 
the setae would be relaxed, and second, the decreasing turgescence of the 
organ would result in its becoming shorter so that the tip of the labium would 
have travelled a short distance upwards along the setae. The elevator muscle 
would also assist in this. The tip of the labium would now be in the right 
position again to take a grip of the setae in order to force in a further portion. 
An increase in the internal pressure of the body fluid within the labium again 
occurring, the grip on the setae would be re-established, and the resulting 
elongation of the organ would force the projecting portion of the setae into the 
host. By the continued repetition of these processes, the setae could be driven 
into the host to any desired length, limited only by the length of the stylets. 
On this hypothesis, it can be readily understood, how in those cases where 
the setae are much longer than the proboscis, they can be forced into the 
plant, and the portion not contained within the labium, and which under 
ordinary circumstances remains in the form of a loop outside the proboscis 
(e.g. ’Chennes) or is lodged in a special setal chamber ( e.g. Psylla), is brought 
into use. But although the hypothesis gives a feasible explanation of some of 
the hitherto almost inexplicable cases, it also presents some difficulties. The 
first to occur to the mind of the critic is that it does not take into account the 
elaborate mechanism of protractor muscles contained within the head, to 
which, previously, the function of performing the operation of piercing has 
been attributed; and it may be argued that it is scarcely likely that these 
structures would be so developed unless they had a definite function to per¬ 
form. It has been suggested by many writers that, when the setae have been 
forced into the plant, they do not remain in a state of rest in the wound which 
they have made, but are kept in constant motion in order to lacerate the 
tissues, and so maintain a supply of fluid. In the hitherto accepted view, this 
was effected by alternate slight contractions and relaxations of the protractor 
and retractor muscles, resulting in slight protrusions and retractions of the 
setae. On the hypothesis which has just been elaborated, this would represent 
practically the entire function of these protractor and retractor muscles. The 
suggestion is, that when the setae have penetrated to the required depth, with 
the result that they now have completely straightened out from base to tip, 
the forces exerted by the contractions of these muscles can now operate 
Parasitology xi 31 
