468 
Head of Psylla mali 
supports, but is not very clear on the matter. Stough (1910) is rather vague, 
and makes no mention of the transverse bar beyond stating that the structure 
which Meek calls the tentorium is designated by Berlese as the transverse 
branch of the ipostoma, but figures structures which may correspond with the 
anterior and posterior supports. Crawford (1914) does not deal with the endo- 
skeleton of the head, but in his figure of the mouth parts of Psylla alni ameri- 
cana, structures are indicated which from their position would correspond with 
the anterior and posterior supports. 
The function of this framework is to serve as a suitable attachment for the 
various muscles connected with the setae and salivary pump. 
The Salivary Pump (PI. XXVIII, figs. 4-6). 
The salivary pump of P. mali resembles in the details of its construction, 
similar organs which have been described in many Hemiptera. Seen from the 
anterior aspect, the pump consists of a hollow vase-shaped receptacle which is 
attached at its base to the wedge-shaped hypopharynx, and occupies a position 
in the head immediately behind the lower part of the pharynx. This pump 
chamber is closed above by a flexible chitinous membrane, to which is attached 
the pump muscle. If a transverse section of the head through the line of the 
pump is examined (PI. XXVIII, fig. 4), it is seen that, in section, the chitinous 
membrane takes the form of a W, the outer arms of which are expanded and bent 
over and become continuous with the walls of the pump chamber. The pump 
muscle is attached to the upwardly directed centre of the W. The cavity of the 
pump chamber is in continuity with a fine channel—the efferent salivary duct— 
which traverses the hypopharynx, and which opens out on the tip of that struc¬ 
ture. The pump chamber also receives, on its posterior face, the afferent salivary 
duct (PI. XXVIII, fig. 5), a median structure formed by the junction of the 
ducts from the salivary glands, and which enters the chamber at a point a little 
below where the chitinous membrane becomes continuous with the posterior 
wall of the pump chamber. The whole apparatus is held firmly in position in 
the head by reason of the continuity of the chitin of its base with the hypo¬ 
pharynx, and the walls of the pump chamber are composed of thick chitin to 
ensure its stability. 
The action of the pump is probably similar to that which has been de¬ 
scribed in other forms, the propulsive force being derived from the elasticity 
of the chitinous membrane. The necessary valvular arrangement in the case 
of the afferent duct is probably effected by the prolongation of the duct into 
the cavity of the pump chamber. The valve in connection with the efferent 
salivary duct is more difficult to imagine. Traversing the hypopharynx as it 
does, the walls of the duct are not collapsible, and no separate valve-like 
structure can be detected. 
It is possible that a valve in connection with the efferent salivary duct may 
not be necessary, for the secretion contained in the afferent duct is always 
more or less under pressure, by reason of the activity of the salivary glands, 
