A. J. Grove 
481 
From this example it is clear that a very considerable force can be exerted 
through a suitable organ by the variations in the internal pressure of the body 
fluid contained within it, and quite commensurate with, if not greater than, 
the force which would be derived from the contractions of the protractor 
muscles to which hitherto the whole of the act of penetration has been 
attributed. 
The progress of the discussion may now be conveniently summarised. It 
has been shown that in the case of P. mali the penetration of the setae into 
the host cannot be effected by the agency of the protractor muscles, and the 
difficulties in the way of the application of the conception (that the action of 
the protractor muscles alone can produce the result which has been ascribed to 
them) to other cases, have been enumerated. As an alternative it is suggested 
that in P. mali the forcing of the setae into the host is brought about through 
the agency of the labium, actuated by variations in the internal pressure of 
the body fluid contained within it. It is further suggested that this hypothesis 
may explain such cases as Chernies where the setae are very long and not 
wholly contained within the proboscis. Various possible objections to the 
suggestion have been indicated, and answers to them given. 
The next problem to consider is how the setae are, withdrawn from the 
host, when the insect wishes to move to another spot. From what has gone 
before, it is evident that, just as the range of movement of the protractor 
muscles is insufficient to account for the depth to which the setae penetrate, 
so equally, the range of movement of the retractor muscles is insufficient to 
withdraw the stylets, though it is certain that by their participation in the 
to-and-fro movements of the setae, they will loosen them in the wound pre¬ 
paratory to withdrawal. There remain two alternatives; either the setae are 
dragged out of the wound by the raising of the whole body; or that the labium 
is again instrumental in this operation. To the first suggestion there are some 
objections. Although the maxillary setae are connected with the endoskeleton 
of the head by means of the maxillary levers, the mandibles in P. mali are 
quite free, so that any traction exerted by the raising of the head or body 
would only be operative to any great extent upon the maxillary stylets, and 
the mandibles would be left behind, unless the retractive force of the retractor 
muscles of the mandibles was sufficient to retain them in their proper relation 
to the maxillary setae. It is scarcely probable, therefore, that this suggestion 
is a true explanation of what occurs in P. mali , though in other cases, where 
both mandibular and maxillary levers are present, it may be operative to a 
limited extent. Also in those piercing insects where the articulation of the 
head to the body is not strong, and the neck is composed of thin conjunctival 
membrane, the withdrawal of the setae could not be effected in this wav. 
Considering the alternative suggestion, viz. the instrumentality of the labium, 
it is difficult to see how the force exerted by the variations in the pressure of 
the body fluid contained within the organ, as suggested for the driving in of 
the setae, could, alone, operate in their retraction. This is because the driving 
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