A. J. Grove 
459 
of the sclerites. Unfortunately, Muir and Kershaw do not give the relation of 
the parts to one another as they would appear in section (except in one case, 
. Flata, in which the arrangement of the sclerites is quite different from that in 
P. mali), but only the outward aspect—which, from the configuration of the 
head, is very difficult to investigate in P. mali —so that correlation of the 
parts by general comparison is difficult. Previous writers on the anatomy of 
the head of the Psyllidae show no unanimity of nomenclature. Witlaczil 
(1885) confines himself to very general terms and does not differentiate the 
sclerites of the head beyond distinguishing the “ Vorderkopf ” or forehead 
which includes those median sclerites which—from its deflexed condition— 
lie on the ventral side of the head. Stough (1910) states that the head is 
enclosed by the epicranium, genae and frontal cones together with the antennal 
sockets and compound eyes and ocelli, and that the mouth parts are composed 
of the labrum, epipharynx, labium, hypopharynx and setae, and closely 
associated with them, the frons, clypeus, mandibular and maxillary sclerites 
(after Meek) and the tentorium. This interpretation is criticised by Crawford 
(1914), who points out that Stough has overlooked the frons proper, calling it 
the ligamentary process of the frons, and consequently has misnamed the 
clypeus proper as the frons, and introduced an extra suture into the clypeus 
so that the correct number of sclerites appear to be present. On Crawford's 
interpretation—which is arrived at from a review of a large number of species 
—the lower portion of the head in the Psyllidae is made up of the following 
sclerites: the frons, a very variable and in many instances small sclerite which 
joins on to the vertex between the genae, and carries the median ocellus; the 
genae , which form the portion of the side of the head which lies between the 
vertex and clypeus, and from which project the genal cones, two lobes or 
conical processes, which project downwards from the head, and obscure the 
“face” proper; the clypeus, a relatively large and prominent sclerite, imme¬ 
diately ventral to the frons, and which converges rapidly towards the labrum, 
which is relatively short, and terminates in an inconspicuous epipharynx. 
Among the appendages of the head, the author includes the labium which is 
three-jointed and sharply flexed, the mandibular and maxillary setae, the eyes, 
and antennae. It will be seen that the author makes no mention of the 
maxillary plates of Muir and Kershaw, nor of the lateral sclerites (lorae) to the 
clypeus, but it is evident that he is dealing only with those sclerites which are 
clearly visible from the exterior. Without therefore elaborating the discussion 
further, which, as indicated above, would necessitate a wide survey of a large 
number of types to solve satisfactorily, it must suffice to state that in this 
paper the terms frons, clypeus, labrum, gena, etc., are used in the way indi¬ 
cated by Crawford. The term maxillary sclerite is used in the paper to denote 
a structure which appears to correspond to the maxillary plate of Muir and 
Kershaw, but since the structure is not exactly a plate, the term sclerite has 
been adopted as being more appropriate. 
