A. J. Grove 
461 
As the sections are traced backwards, it is seen that in the region of the 
bases of the setae, two hollow, flattened, strongly chitinised structures appear 
just dorsal to the setae and between the maxillary setae and the nervous 
system. These structures are the buccal folds (PL XXVI, fig. 4). As the buccal 
region is approached, the buccal folds extend inwards towards the median 
line until in the region of the termination of the pharynx, the innermost wall 
of each fold gradually approaches and eventually becomes continuous with 
the strongly chitinous posterior wall of the pharynx. In the meantime, the 
clypeal groove has become deeper, and in the region of the commencement of 
the labrum-epipharynx, the closed end of the groove bifurcates so that in 
section the groove has now taken the form of a Y. The stem of the Y repre¬ 
sents the old clypeal groove, and at its base the cavity is open to the exterior. 
The inner arm of the Y extends upwards into the head in the direction of the 
pharynx, and the outer arm towards the outer edge of the buccal fold. Even¬ 
tually these various structures become continuous with one another. The 
innermost wall of the inner arm of the Y joirjs on to the end of the ventrally 
directed arm, of its side, of the A-shaped posterior wall of the pharynx. The 
outer wall of the inner arm of the Y becomes continuous with the inner margin 
of the ventral wall of the buccal fold. The inner wall of the outer arm of the 
Y fuses with the outer margin of the ventral wall of the buccal fold, and the 
outermost wall of the outer arm with the lateral margin of the dorsal wall of 
the buccal fold. The total result of this anastomosis is the formation of the 
buccal cavity and maxillary sclerites (PI. XXVI, fig. 5), and the differentiation 
of the hypopharynx and epipharynx. The buccal cavity takes in section the 
shape of an inverted triangle with an incompleted apex (w) and lies buried 
between the anterior portions of the coxae of the first pair of legs. The space 
between the incompleted sides of the w is filled by the epipharynx, which is 
formed by the fusion of the labrum with the ventral or anterior wall of the 
pharynx. The middle portion of the base of the w is occupied by the hypo- 
pharynx, which arises, as indicated above, by the fusion of the inner margins 
of the buccal folds with the posterior wall of the pharynx. The remainder of 
the cavity is occupied by the maxillary sclerites, which lie one on either side 
of the epipharynx, and in section take the form of scalene triangles with their 
shortest sides applied to the dorsal surface of the buccal cavity. 
The setae, as they pass out from the head, enter the buccal folds and are 
received into furrows on the inner sides of the maxillary sclerites. Continuing 
backwards, it will be seen that when the hypopharynx has been passed, the 
upper margins of the maxillary sclerites approach one another in the median 
plane until they meet over the top of the epipharynx. At this point the upper 
portions of the maxillary sclerites are so shaped that they fit into one another 
and interlock (text-fig. 1). Also, the ventrally directed portions of the sclerites 
embrace the epipharynx and fit into grooves on the sides of that organ, with 
the result that these three structures fit closely together in a manner similar 
to that of the pieces in a jig-saw puzzle. 
