466 
Head of Psylla mali 
the basal portion of the seta—such as has been described in the case of the 
mandibular setae—is, in the maxillary setae, either very much reduced or 
has become completely obliterated. Its presence could be detected in the 
buccal region, but beyond that it was not visible. 
In many Hemiptera, both Homoptera and Heteroptera, the basal portions 
of both mandibular and maxillary setae are attached to the chitinous walls of 
the head by rods or plates of chitin which have been termed the mandibular 
and maxillary levers respectively. In P. mali , only the maxillary levers are 
present. These take the form of slender curved rods of chitin which pass from 
the inner face of the base of each maxillary seta, and, curving behind the seta, 
pass to and fuse with the outer lateral margins of the buccal folds (PI. XXVIII, 
fig. 3). The mandibular setae are free throughout their entire length, and are 
devoid of any chitinous connection with the head capsule. 
When the setae are examined from the lateral aspect in a cleared specimen 
of the insect, a remarkable feature with regard to them at once becomes 
obvious. This is the interesting course they describe after leaving the head 
and before they become lodged in the groove in the proboscis. Whilst still 
within the head, the bases of the setae—both mandibular and maxillary—lie 
well to either side of the median line, but, as the buccal region is reached, they 
gradually approach more and more towards the centre, until, as they pass into 
the buccal cavity proper, they occupy a median position. Whilst still anterior 
to the hypopharynx, both the mandibles and maxillary setae lie in grooves on 
the inner faces of the maxillary sclerites, but in the region of the hypopharynx 
the maxillary setae become more or less free and lie one on either side of that 
structure,, and are separated by it. When the hypopharynx has been passed, 
the maxillary setae come together to form a single piercing organ but the 
mandibles remain separate. As has been indicated above, the setae in this 
region are lodged in channels formed between the epipharvnx and maxillary 
sclerites. By this means the three piercing organs are kept in a definite re¬ 
lation to one another, the maxillary setae lying in the centre between the 
mandibles but separated from them by ridges of chitin which jut out from the 
inner faces of the maxillary sclerites. When the region of the maxillary 
sclerites has been passed, the setae are still held in position by being lodged 
in a groove on the inner face of the epipharvnx, but now, instead of being 
separated from them, the mandibular setae lie in close juxtaposition one on 
either side of the approximated maxillary setae. The significance of these 
arrangements will be further considered when discussing the mechanism of 
suction. 
So far, the setae have proceeded in practically a direct line, but, when they 
have traversed the buccal cavity, instead of passing directly into the groove in 
the labium, they curve sharply upwards, and enter the invagination of the 
ventral wall of the body between the pro-coxae which has been described above 
as the setal chamber. When the top of the setaT chamber has been reached 
the setae, describing a complete semicircular curve, then pass into the groove 
