460 
Head of Psylla mali 
THE EXOSKELETON OF THE HEAD. 
The Head Capsule. 
The head of P. mali is of the usual psylline type. It is somewhat flattened 
in appearance, antero-posteriorly, is wide between the eyes, and the depression 
of the head so commonly met with in the Homoptera, is carried to extreme 
limits, so that the buccal region and the oral appendages extend backwards 
between the coxae of the first pair of legs. Consequently, the labium appears 
as if it arose from the prosternum. The front of the upper portion of the head 
is formed by the vertex, a quadrate sclerite with a strongly marked median 
suture. The compound eyes are attached to the lateral margins of the vertex. 
The frons is very small and is almost hidden from view by the genal cones 
which arise from the ventral margin of the vertex and project downwards in 
front of the head almost obscuring the facial portion. The antennae are 
articulated to the head just below the eyes, between the lateral margins of the 
vertex and the genal cones. The front of the head or the “face 55 proper, which 
from the deflexed condition of the head occupies a ventral position, is formed 
by the clypeus and labrum. The clypeus is a domed, pyriform sclerite attached 
at its upper broad end to the frons, and gradually tapering off into the labrum, 
which terminates in a comparatively insignificant epipharynx. The side of the 
head is formed by the gena, a sclerite, the posterior margin of which is not 
very clearly defined, but soon becomes involved with the episternite and coxa 
of the prothorax, and shades off almost imperceptibly into the conjunctival 
membrane connecting it with the prothorax. The gena is continuous along its 
anterior margin, with the under side of the genal cones, of which the latter 
are really protuberances. The articulation of the head, above, with the thorax 
is effected by the junction of the lower margin of the occiput with the flexible 
membrane of the neck. The occiput is a long narrow sclerite stretching between 
the posterior margins of the eyes and forms the back of the upper portion of 
the head. 
The Buccal Region and Labium (Plate XXVII). 
Owing to the extreme depression of the head, the whole of the oral region 
lies buried between the coxae of the first pair of legs, so that the relation of 
the sclerites which form the boundaries of the buccal cavity cannot be readily 
distinguished from the exterior (PI. XXVIII, fig. 1). This can therefore be best 
described by considering a series of transverse sections of the head. After the 
region of the frons has been passed, the clypeus appears forming the ventral 
wall of the head, and, owing to its domed character it appears semicircular in 
outline (PL XXVI, fig. 2). Its lateral margins are carried to some distance into 
the head, where they become continuous with the incurved ventral margins of 
the genae, which form the sides of the head. Thus at the junction of these 
sclerites, a groove—the clypeal groove—is formed in the surface of the 
face. 
