470 
Head of Psylla mali 
At their point of attachment to the operculum, the muscles assume a tendon¬ 
like character, but where they are attached to the clypeus they become much 
more expanded in outline. 
III. The Pump Muscle (PI. XXVI, fig. 2, PL XXVIII, figs. 5, 6). 
This muscle is in reality a paired one, but where it is attached to the 
chitinous membrane of the salivary pump, the two parts are closely applied 
to one another and assume a tendon-like character. As the muscle is traced 
towards the dorsal surface of the head, it is found to divide into two halves, 
each of which runs into the angle formed by the junction of the posterior 
support of its side with the transverse bar of the tentorium. The muscle is 
consequently V or Y-shaped in outline, the upper extremities of the arms 
forming the points of attachment to the tentorium, and the base or stem 
representing the point at which the muscle is inserted into the chitinous 
membrane of the salivary pump. 
IV. The Supporting Muscles of the Tentorium. 
(a) Two muscles of moderate size stretch from the front of the head— 
vertex—to the ends of the transverse bar of the tentorium at the point where 
it joins on to the anterior supports. These two muscles lie immediately dorsal 
to the cerebral ganglia. 
( b) Two muscles, much smaller in size than, but attached to the front of 
the head at the same point as (a), extend to the transverse bar and are attached 
to it one on either side of the median line, close to the centre. At the point 
where they join on to the front of the head, muscles (a) and (b) are closely 
applied, if not actually joined to one another. 
(c) Two large double muscles, one on either side of the head, stretch from 
a point of attachment to the head formed by an invagination of the occipital 
sclerite, to the dorsal surface of the tentorium. They are joined to the latter at 
a short distance beyond the region where the posterior supports become con¬ 
tinuous with the transverse bar. 
(d) Two muscles, one on either side of the head, are attached at one end 
to the same point of attachment to the head as (c) but dorsal to them, and at 
the other end to the lower portion of the posterior supports of the transverse 
bar. 
From their line of direction, the forces exerted by all these muscles, upon 
contraction, will be antagonistic to any downwardly directed traction exerted 
upon the chitinous framework. Since the framework is a fixed structure, it is 
clear that these muscles will not be concerned with moving any portion of it, 
but their function will be to counteract any displacing movement of the frame¬ 
work brought about by the pull exerted upon it by other muscles connected 
with the structures comprising the mouth parts, and thus serving to maintain 
the rigidity of the tentorium. 
