A. J. Grove 
4BM 
groove of the setal chamber is more strongly developed. A fold or furrow has 
now appeared in the chitin of the body wall a little to each side of the setal 
chamber and dorsal to the coxae of the first pair of legs (PL XXVII, fig. 5). These 
folds, as they are traced backwards, curve more and more upwards and inwards 
towards the median line so that there are formed two short, almost cylindrical, 
sclerites which lie one on either side of the setal chamber and are fused to the 
body of the insect along their dorsal margins (PI. XXVII, figs. 6-7). If these 
structures are traced further back still, it will be seen that they become con¬ 
tinuous with the free portion of the labium or proboscis, the upper portion of 
which lies in a groove or furrow—the labial furrow—between the procoxae. 
The inner margins of this furrow are formed by the furcae from the prothorax 
which are described by Crawford (1914) as embracing the labium. The relation 
of the parts to one another in this region, throws some light upon the homologies 
of the setal chamber, for it is evident that the cvlindrical sclerites which have 
been indicated above as lying one on each side of the open base of the setal 
chamber are clearly portions of the first joint of the labium, and as the setal 
chamber passes right through this joint, and at its posterior margin becomes 
continuous with the groove on the anterior face of the free portion of the 
labium, it can be concluded that the setal chamber is an extension of the 
labial groove. 
O 
The labium in the Psyllidae, as already shown by Witlaczil (1885), Stough 
(1910) and Crawford (1914), consists of three joints or segments. As has been 
indicated above, the first joint is fused for a great part of its length with the 
body, and is divided longitudinally to give access to the setal chamber. The 
free portion of the labium (PI. XXVIII, fig. 2) is therefore made up of the second 
and third joints, and projects almost vertically downwards from the body, 
but is hidden from view by the proximal joints of the first pair of legs. The 
second joint is the longest, its walls are but thinly chitinised, and at its distal 
end it becomes bulged or expanded on its anterior face. The third, or terminal 
joint is much shorter and smaller in diameter and tapers off at the end. The 
» 
termination of the joint is furnished with a number of stiff tactile hairs. As a 
result of the fusion of the proximal portion of the first joint with the body, the 
labium does not trace a straight course, but is sharply flexed at the junction 
of the first and second joints. The labium varies, both in cross section and in 
the character of the groove which runs along its anterior face, from its proximal 
to its distal portion. In the region where it just becomes free, that is, at the 
junction of the first and second segments, it is flattened in cross section, and 
the groove, which is not strongly developed, lies at the bottom of a V-shaped 
depression in the anterior face of the organ. By the time the middle portion 
of the second joint is reached, the organ has become almost triangular in 
section and the groove, which is now heavily chitinised, has become enclosed 
within the body of the labium. This is effected by the gradual approximation 
of the sides of the V-shaped depression referred to above, so that now the 
groove containing the setae appears as an expansion of the closed end of a 
Parasitology xi 30 
