477 
A. J. Grove 
the form of a backwardly directed loop. The authors suggest that the trophi 
enter the skin by means of the same elastic force which induces the sharpened 
end of a piece of whalebone to pierce a soft body if the other end is pushed 
towards the surface. This is criticised later by Cragg (1913) who, after a 
careful discussion of the subject, arrives at the conclusion that piercing is 
effected in stages, and that the extremely rapid protraction and retraction of 
the maxillae lacerates the tissues and paves the way for the entry of the other 
mouth appendages. This, however, does not help very much in the elucidation 
of the problem of piercing in the Hemiptera. 
As a final addition to the difficulties of the problem enumerated above, it 
may be mentioned that the writer (Grove, 1909) in describing the anatomy ol 
Siphonophora rosarum stated that in that insect only the maxillary setae were 
provided with protractor muscles. This led the writer to suggest a method for 
the protrusion of the setae, which in the light of fuller knowledge requires 
considerable modification, but the interest of the absence of the mandibular 
protractor muscles remains. 
From all these considerations therefore, it would appear that the penetra¬ 
tion of the host is not effected in many Hemiptera, and certainly not in the 
case of P. mali in the way which has hitherto been accepted, namely by means 
of a driving force exerted along the length of the stylets as a result of the 
contraction of the protractor muscles. The question now remains to arrive 
at some other interpretation of the mechanism and method of action of 
the parts involved, which will fit in more closely with the mechanical laws 
concerned. 
The problem which has to be solved, resolves itself into the mechanical 
means necessary to force a long, extremely slender, flexible, pointed body to 
some depth into a moderately hard substance. It may be illustrated by con¬ 
sidering how a piece of fine, flexible steel wire can be driven most readily into 
a piece of moderately hard indiarubber. It is obvious at the outset that the 
result will not be attained by applying one end of the wire to the surface of the 
rubber, and exerting pressure at the opposite end. This will merely cause the 
wire to bend, and very little penetration will be effected. Since it is the bending 
of the wire which interferes with the attainment of the desired result, the 
obvious remedy is to introduce some mechanical device which will overcome 
this. This might be effected in two ways; either by enclosing the wire within 
a closely fitting rigid tube so that bending would be impossible, or bv applying 
the driving force to such a small portion at a time that the possibility of bend¬ 
ing would be practically eliminated. This latter method would, in practice, 
be effected by the use of a pair of strong forceps or pincers; the wire would be 
firmly gripped at a short distance from the pointed end, and the projecting 
portion forced into the rubber. This accomplished, the grip of the forceps 
would be released, moved a little further up the wire, the grip applied once 
more, and a further portion forced in. This series of operations would then be 
repeated until the wire had been driven in to the required depth. 
