A. J. (trove 
475 
the proboscis is not completely enclosed by it, but the setae lie in a groove, 
which is open anteriorly, in the anterior face of the labium. The extent to 
which the setae are enclosed varies in different species and also in different 
parts of the proboscis, but in general, the setae are more firmly held at the.tip 
of the labium than in the more proximal portions. It is usually suggested that 
the groove serves to support the setae whilst they are being driven into the 
host, and to prevent bending; but again, if the mechanics of the problem are 
considered, the matter is not so simple. The setae are extremely fine, delicate, 
and flexible structures, and when their tips are resting against a resistant 
surface, the effect of exerting a force upon them such as would be derived from 
the contraction of the protractor muscles, would be to cause them to bend; 
and although at the tip of the proboscis the groove is sufficiently contracted 
to enclose them in a moderately firm grip, yet in other parts of the labium 
where the groove is not so strongly developed,* the bending of the setae could 
not be controlled, and they would protrude through the open portion of the 
groove, with the result that a great part of the driving force would be wasted. 
This difficulty must be materially greater in those cases such as the Chermidae 
and P.syllidae , etc. where the setae are much longer than the proboscis. 
Awati (1914) states that in Lygus pabulinus the labium is bent during the 
act of piercing in order to enable the setae to penetrate further into the host. 
His description of the process is as follows (p. 728): 
“ (1) The stylets inside a leaf. 
The insect begins to feel the surface of a leaf with the sensory hairs at the 
tip of the proboscis. It is trying to find a good place for piercing. When it 
is found, the proboscis is applied to it, the stylets are driven forward, the 
protractors contract, and the epidermal layer is pierced, however thick it 
mav be. 
«/ 
(2) The bending of the proboscis. 
Once in, the stylets are pushed down into the tissue by direct mechanical 
force, i.e. by the bending of the proboscis. This feature has already been 
alluded to. It increases artificially the protrusion of the stylets and enables 
them to reach the tissues which contain food....” 
In describing the labium, Awati says (p. 696): “ 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 has a 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 
