462 
Head of Psylla mali 
In the region where the dorsal surface of the epipharynx is applied to the 
ventral wall of the inner portions of the maxillary sclerites, a system of ridges 
and grooves appears on the approximating surfaces, so that when the parts are 
applied to one another, three channels are formed; a larger median one with 
two smaller ones, one on each side (text-fig. 1). The median channel receives 
the maxillary setae and each outer channel a mandible. Eventually the 
maxillary sclerites lose their triangular shape in cross section and become 
flattened against the dorsal wall of the buccal cavity, but the epipharynx 
which is now reduced in size is still applied to their under surfaces (PI. XXVII, 
figs. 2-3). 
By this time, a deep groove or fold has appeared in the centre of the dorsal 
wall of the buccal cavity, and as the sections are traced backwards, it will be 
seen that this groove, shallow at first, passes into a deeper fold in the ventral 
wall of the body between the coxae of the first pair of legs (PI. XXVII, figs. 4-6). 
This groove or fold extends upwards into the body and actually pierces the 
nervous system which, in this region, assumes the form of two cords one of 
which, in the sections, lies on each side of the groove. The cavity or lumen of 
this groove forms a chamber in which a loop of the setae lies when they are 
not in use, and consequently an appropriate name for this structure will be 
the setal chamber. 
These structures have not been described in detail by previous writers on 
the anatomy of the Psyllidae. Witlaczil (1885) indicates the presence of the 
setal chamber. He says (p. 603): 
‘‘Die Stechborsten, welche in einer Rinne der Unterlippe liegen, und am 
Ende derselben heraustreten, sind hier aber so lang, dass sie an der Basis der 
Unterlippe eine Schlinge bilden, welche in einem Hautsacke liegt, der durch 
eine Einstulpung der Hypodermis gebildet wird und das Centralnervensystem 
an der Grenze von Unterschlundganglion und Bauchmark durchbohrt." 
This author also indicates that a similar structure has been described by 
Dujardin and Mark in the Coccidae. Stough (1910) draws attention to the 
similarity between the psyllid and coccid mouth parts, and says that in the 
latter “the setae are retracted into a special pouch when not in use." He then 
dismisses the subject by stating: “The parts exclusive of the labium are 
similar'in general structure to the same parts in P achy psylla." Crawford 
(1914) does not mention any special structure for accommodating the loop of 
the setae, although in one of his figures (PI. XXVIII, fig. 15) he shows that 
the setae describe this loop before entering the labium. 
To return to the consideration of the serial sections of the head; in the 
region where the setal chamber has become clearly distinguishable, it is seen 
that the configuration of the surrounding parts has now altered. The maxillary 
sclerites have completely disappeared and only the tip of the epipharynx re¬ 
mains (PI. XXVII, fig. 4). The buccal cavity too has become almost obliterated, 
but the setae still remain at the buccal end of the setal chamber. Further 
back still, the region of the buccal cavity proper has now been passed, and the 
