704 MR. P. R. AWATI ON THE MECHANISM 
in the middle part of the head and show a curious structure in 
cross-sections, appearing as areas of glandular tissue surrounded 
by a chitinous ring which disappears in more distal sections. 
The Interlocking Arrangement of the Stylets (external and 
internal). (Vext-figs. 8-16, Mx.Md.Art., Md.Art.).—As shown 
by Davidson (9), the maxille and the mandibles of Schizo- 
neura lanigera are round and lie loose from each other. In 
Lygus pabulinus, however, they are never separate outside the 
head, but are interlocked. Within the head the members of both 
pairs lie loose; the maxillee and the mandibles being smooth and 
oblong without any processes (text-figs. 18-20, Md., Mx.St.). 
Anteriorly in the labral region, they have a different structure. 
The mandibles give rise dorsally to longitudinal ridges (which in 
section appear as knobbed processes), which attach them to the 
labrum or epipharynx. These disappear, however, beyond the 
labrum. This arrangement demonstrates the importance of the 
labrum in keeping the stylets in place and preventing their lateral 
sliding. 
The maxille also have small processes, of a similar nature and 
function, which attach them firmly to the mandibles. These 
structures are clearly visible in cross-sections. In the fourth 
segment of the Jabium they are more pronounced. 
It is obvious, therefore, that the mandibles and maxille are 
inseparably attached to one another, and, the space between them 
being exceedingly small, friction will ensure their simultaneously 
working up and down; the mandibles accompanying the maxille 
all the time and all the way in and out of the plant-tissues, unless 
a differential force be applied *. Inside the head they are always 
prevented from lateral movement by structures which are in- 
vaginations or infoldings of the outer walls of the head, and 
which enclose them successively at different levels. 
These folds are formed by :— 
(i.) The hypopharynx. (Text-figs. 15-15, Hyp.) 
(ii.) The buccal fold. (Text-figs. 16-19, Bu.F.) 
(iii.) The labial fold or the maxillary fold, formed by the 
infolding of the maxillary sclerite. (Text-figs. 16-19, 
Mx.F.) 
These are very prominent structures in cross-sections of the 
head. They are not, however, found in the heads of those forms 
which require lateral movements of their mouth-parts, 7. e., in the 
biting insects. 
Tuer TENTORIUM OR ENDOSKELETON. 
(Text-figs. 1, 4, 6, 7,17, 19-22.) 
This has already been described by various writers: Burmeister, 
Wedde, Léon, Bugnion, ete. Its description by Comstock and 
* The existence of such a force will be demonstrated later on. 
