458 Head of Psylla mali 
but in order to avoid any confusion with regard to the significance of the terms 
used in the body of the paper, it will be useful to indicate here the meaning 
ascribed to them and the reasons for adopting them. Of the two main questions 
indicated above, that connected with the homology of the setae in the Hemip- 
tera, has perhaps been more definitely solved. Muir and Kershaw (1911-1912) 
by their researches into the embryology of Pristhesancus papuensis and 
Siphanta have shown that in both Homoptera and Heteroptera the anterior 
setae represent the mandibles, and the maxillary plates and the posterior setae 
together, represent the maxillae/ The anterior setae, which owing to the 
extreme depression of the head in P. mali, have come to occupy a ventral 
position, will therefore be termed the mandibles (or mandibular setae) and 
the posterior setae, the maxillary setae. 
The homologies of the sclerites which go to the formation of the head 
capsule, and which surround the buccal cavity, cannot be dismissed so easily. 
It is not intended here to enter into the discussion, but it is evident from what 
has been found in P. mali, which exhibits some special and probably advanced 
features, that the problem -can only be solved by a wider survey of the different 
types of head found within the order than has already been made. The forma¬ 
tion of the dorsal portion of the head presents little difficulty. This is usually 
termed the epicranium, a useful term when speaking of this portion as a whole, 
but Crawford (1914) states that in the Psyllidae this portion of the head is 
formed by two sclerites, the vertex and occiput, and that in some cases the 
latter contributes largely to the formation of the side of the head. These 
terms will therefore be adopted, the vertex forming the region between the 
eyes, and the occiput, the narrow posterior region of the head, where it articu¬ 
lates dorsally with the neck. When the sclerites which surround the mouth 
parts are considered, it is evident that some confusion both as regards homo¬ 
logies and nomenclature prevails. Muir and Kershaw (1911a) endeavoured to 
show that the mouth parts were homologous throughout the Homoptera and 
Heteroptera and figured nine examples of which three belonged to the 
Homoptera and six to the Heteroptera. On their interpretation the buccal 
region is made up of the clypeus, which has two lateral sclerites (these appar¬ 
ently alternatively termed the lorae, and corresponding to the mandibular 
sclerites of Meek), the labrum and the epipharynx; and the latero-ventral 
portion of the head is formed by the maxillary plates which are, judging from 
the figures, continuous with the genae above, but show no articulating suture. 
(In a later paper (19116) the authors describe these maxillary plates as com¬ 
pletely amalgamated with the head capsule and leaving no suture.) The 
fundamental point upon which the authors base their interpretation, is the 
articulation of the mandible with the head capsule. In the attempt to apply 
their results to P. mali difficulties at once arise, for in this insect the mandible 
has no skeletal articulation with the sclerites of the head. The extent to which 
the head has been deflexed has also resulted in modifications of the arrange¬ 
ments of the parts in the buccal region, and possibly in the reduction of some 
