474 
Head of Psylla mali 
are considered, it will be seen that the only effect which would be produced 
by the contraction of the protractor muscles when the tips of the setae are 
resting against a resistant substratum, would be to increase the curve which 
the setae make when they pass from the head into the setal chamber. Even 
assuming that the proboscis was deflected during the act of piercing so that it 
lay more or less in the same line with the bases of the setae, this difficulty 
would still remain by reason of the loop of the setae which lies in the setal 
chamber. 
The more the problem is studied, the more obvious it becomes that the 
considerations put forward above are not the only difficulties in the way of the 
conception that the protractor muscles are the sole means by which the setae 
are forced into the host plant. Many instances have been recorded in which 
the setae are thrust to varying, and sometimes considerable, distances into the 
host, in order to reach particular tissues from which the insect desires to obtain 
its food. If, keeping this fact in mind, the possible range of movement of the 
bases of the setae as effected by the contraction of the protractor muscles is 
considered, it will be seen that this range of movement is quite small com¬ 
pared with the distance to which the setae enter the host. An extreme case 
of this is met with in the case of Chermes abietis in which, according to Burdon 
(1908), the setae, which are considerably longer than the entire body, are 
almost completely inserted into the host plant. How could this possibly be 
brought about by the agency of the protractor muscles, whose range of action 
must be very much less than—in fact but a mere fraction of—the distance to 
which the setae penetrate? It may be argued that the act of piercing is not 
effected by a single contraction of the protractor muscles, but that with the 
first contraction, the setae are only forced into the tissues to a distance which 
is equal to the range of movement of the bases of the setae. This effected, the 
protractor muscles will relax and by a simultaneous contraction of the re¬ 
tractor muscles and a slight downward movement of the head, the bases of 
the setae will be restored to their original position. The protractor muscles 
would then again contract, and by a repetition of this cycle of movements, the 
setae would be driven in to the required depth. It will be shown later however, 
that for other reasons it is improbable that the force exerted by the contraction 
of the protractor muscles plays more than an insignificant part, if any at all, 
in the actual operation of piercing. 
Whilst considering the difficulty arising from the limited range of action 
of the protractor muscles, it will be well to indicate another difficulty of the 
same kind. It has been shown by a number of authors that in many cases in 
the Hemiptera, both Heteroptera and Homoptera, the bases of both mandi¬ 
bular and maxillary setae are directly attached to the head capsule by rods 
or plates of chitin, which have been termed the mandibular and maxillary 
levers. These attachments will further limit the range of movement of which 
the bases of the setae are capable. 
A further point of difficulty is that the portion of the setae which lies in 
