696 MR. P. R. AWATI ON THE MECHANISM 
second, and, in the last segment, forms a tube enclosing the 
stylets.. Cross-sections of the last segment show one tube within 
the other. The inner tube enclosing the stylets is formed by the 
groove; and the labium is the outer tube which encloses the 
former (text-fig. 8, Lb.G.). 
The. tip of the labium is encircled with bristles which are of 
two sorts, fine and stiff. They are arranged in a definite way, 
and function as sensory hairs. The insect feels the surface of a 
leaf with them before thrusting its stylets into it. 
The labrum and the labium together keep the stylets in place. 
In the first segment, where the labial groove is too flat and 
shallow to do it and where the labium hasa bend inwards, thereby 
leaving the stylets free, the labrum encloses them in its groove. 
Distally the labium takes over this function of holding the 
stylets, since its groove becomes deeper and deeper, Thus the 
stylets are always found in the groove of one or the other, and 
are prevented from that lateral movement which would make 
piercing and sucking impossible. 
The labium protects the stylets in the groove, but its more 
important function is very ingenious. The stylets are very thin, 
needle-like structures which have, however, to pierce the tough 
and cuticularised epidermis of a leaf. As they are very delicate, 
they would bend in the act of piercing, were they not enclosed in 
the proboscis. The lumen of the tubular tip of the proboscis is 
so small that the stylets fit into it tightly (text-fig. 8). There 
is no empty space in it for them to bend. The tip of the labium 
is closely applied to the surface of the leaf; the protractor 
muscles of the stylets contract; and the stylets are forced out 
of the proboscis and driven against the epidermis, which they 
cannot fail to pierce. Once in, their forward progress to the 
required tissue is mechanical owing to the bend of the labium 
at the first joint. 
It is thus obvious that the labium is an important structure in 
the sucking apparatus of this insect ; it is one, moreover, on the 
structure of which stress has not been laid by previous writers. 
(6) The Stylets, or Stechborsten. (Text-figs. 1, 6, 23.) 
(1) MorpHonoey. 
(¢) The Maxillary Stylets or Internal Stechborsten. (Text- 
figs. 1, 5, 6, 23, Mx.St.) 
It is shown above that the stylet is a part of the maxilla and 
not the maxilla itself, Each maxilla is divided, in the embryonic 
stage, into two parts, one of which forms the stylet or Stechborste. 
The maxillary stylets are situated between the pharynx and the 
mandibles on either side. They are not, however, on the same 
level with either of them: they are just above the pharynx and 
below the mandibles. They are hollow, and their cavities are 
continuous with the body-cavity. 
