480 
Head of Psyllq mali 
successfully and by moving the setae up and down in the wound to the extent 
of their limited range of movement, cause a continued laceration of the cells 
in their vicinity. It is possible too, that the mandibular setae move indepen¬ 
dently of the joined maxillary setae. 
This suggestion of the true function of the protractor and retractor muscles 
is supported by the case of certain stages in the life-history of Chermes. Burdon 
(1905) in describing these stages, says that once the insect has established 
itself in a suitable position, she never quits the spot, but remains there until 
her death, so that during her lifetime she probably only performs the com¬ 
plete operation of thrusting in the very long setae once. As far as is known to 
the writer, the anatomy of the mouth parts of this species has not been de¬ 
scribed, but there is no reason to suppose that were this done, the same com¬ 
plicated system of protractor and retractor muscles would not be found, such 
as has been described in many allied Aphidae. Consequently, from the peculiar 
habits of Chermes, these muscles would be used almost entirely in the way that 
has been suggested as their true function, namely, in maintaining the supply 
of sap by the continued laceration of the tissues. 
Another possible criticism which may be put forward if the hypothesis is 
to be applied in general to piercing insects, and in particular to other Hemip- 
tera, is that in some cases the labium is bent whilst the later stages of pene¬ 
tration are being effected. This is more particularly so in those cases where 
the setae are not longer than the proboscis, and the bending of the labium is a 
device, as suggested by Awati (1914), by which more deeply seated tissues are 
reached. But as a question of mechanics, the protractor force of the labium 
would be increased and not diminished by this flexion, for in this condition 
the tendency of the increase of pressure of the body fluid within the labium 
would be to cause the organ to straighten out, and thus would serve as an 
additional protractor force to that produced by the normal elongation of the 
organ following upon its increased turgescence. 
A further objection which may be urged is that the force which would be 
exerted in the way that has been suggested, would not be sufficient to produce 
the desired result. But in support of the hypothesis, examples might be quoted 
of the use of the internal pressure of the body fluid to effect an even more 
difficult result. A good example, which is distinctly d propos to the case, since 
the structure concerned is in connection with the head, is the ptilinum which 
is present in so many of the freshly emerging imagines of the Muscidae and 
other Diptera. As is well known, this structure consists of a sac formed by the 
eversion of the integument of the front of the head, and by the expansions 
and contractions of this sac, produced by variations in the internal pressure 
of the body fluid contained within it, the newly emerged insect, although it is 
in a delicate condition owing to the fact that its chitinous exoskeleton has not 
yet completely hardened, is able to force a passage large enough for its whole 
body through, in some cases, a thick layer of moderately hard earth in which 
the pupa, from which it emerged, was buried. 
