A. J. Grove 473 
terminal joint, moving it forwards; or by the muscles of one side acting inde¬ 
pendently of those on the other, move the joint laterally to either side. 
IX. The Muscles of the Labial Furrow. 
Three sets of muscles are associated with the posterior portion of the labial 
furrow. Each arises from the dorso-lateral margin of the furrow, and 
(a) runs upwards and forwards, and becomes attached to the middle of 
the side of the thorax, 
( b ) runs forwards but slightly downwards, becoming attached to the 
latero-ventral wall of the thorax, and 
(c) runs almost vertically upwards and slightly forwards to the dorsal 
region of the body. 
These muscles would counteract any displacing movement of the labial 
furrow, in whatever direction it should occur. 
THE MECHANISM OF PIERCING. 
In most of the papers which have been previously written dealing with the 
mouth parts of Hemiptera, it is rather curious that, although the structure of 
the parts concerned is often described in great detail, yet the actual means by 
which the piercing of the host is effected has been more or less overlooked or 
taken for granted. In many of these cases a complex system of apodemes 
comprising the endoskeleton, and protractor and retractor muscles associated 
with the piercing stylets have been described, and this, in most cases, has been 
considered sufficient. The protrusion of the setae has been attributed to the 
contraction of the protractor muscles, and the penetration of the host has 
♦ 
been taken as a natural mechanical result of this protrusion. If the mechanism 
of piercing is examined more closely, however, it will be seen that this result 
does not always necessarily follow, and P. mali affords a good example of this. 
It has been described above, that the setae in P. mali, after leaving the head, 
do not follow a straight course into the proboscis, but describe a loop, which is 
contained within a special receptacle—the setal chamber—and then pass into 
the groove on the dorsal surface of the labium which they traverse to its tip. 
As a consequence of this, the direction of the distal portion of the setae, i.e. 
the portion contained within the proboscis when the latter is in its usual 
position, is at right angles to that of the bases of the setae. If the parts remain 
in this relation to one another during the act of piercing—and, as in its normal 
position the proboscis is nearly at right angles to the substratum, this is most 
probably the case—it is difficult to see how a force exerted along the line of 
the bases of the setae, which is the line of action of the force exerted bv the 
contraction of the protractor muscles, can effect the protrusion of the tips of 
the setae with sufficient force to pierce the epidermis of the host. This could 
only happen if the setae were confined within a closely fitting tube through¬ 
out their whole length, which is not the case. If the mechanics of the problem 
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