8o 
THOMPSON YATES LABORATORIES REPORT 
a slight groove which continues upwards on either side for a short distance, becoming 
deeper. Its convex ventral and lateral chitinous surfaces are thickly covered with 
hairs and scales, the chitin bearing them having irregular annular markings. Its upper 
surface is of smooth chitin, upon which the stylets rest. The labium tapers slightly 
from base to apex : at its commencement it is broad from side to side, its internal 
measurement from above down being 45*6 m ; from side to side 65*2 m ; its smallest 
depth is 22*8 /u and width 42*4 fx. Its upper smooth surface is here flattened and 
on it rest centrally the hypopharynx, on either side the two maxillae (plate XVI, fig. 2). 
A little way from its origin, the labium becomes roughly round in shape owing to the 
edges of the upper surface turning upwards and inwards over the stylets forming a 
large oval channel in which they lie (plate XV, fig. 3) ; these edges are extremely 
fine and do not meet in the middle line, so that a space of uniform width is left 
running along the dorsal surface of the labium to its extreme tip. At about o* 1 6 mm. 
from the extremity of the proboscis the labium proper ends abruptly, while its upper 
concave surface is continued on to the tip of the proboscis, gradually tapering to 
a blunt point covered with fine hairs. This tip of the labium is easily broken off 
in dissections of the proboscis. 
At the abrupt ending of the main portion of the body of the labium, which 
in transverse section is somewhat oval from side to side, are attached by true joints 
two lobiform appendages — the labellae — which enclose between their inner surfaces 
the tips of the stylets and the true tip of the labium. Crescentic at their bases, 
the labellae gradually taper to form the tip of the proboscis. 
Running longitudinally on each side of the labium, and projecting into its 
substance from the inner surface of the chitinous exoskeleton, is a thick, very opaque 
chitinous ridge. These ridges commence at the base of the labium, and end abruptly 
a little distance behind the point of attachment of the labellae (plate XIX, fig. 3) ; 
from their inferior surfaces for about 34*3 /j. from their distal extremities, and extend- 
ing obliquely downwards and upwards, the chitin of the convex under surface of the 
labium proper becomes greatly thickened, forming two ventral plates, which in the 
mid-ventral line curve upwards and outwards, scrollwise, into the substance of the labium, 
ending in a short thick rod, near the centre of each lateral half of the labium (plate XIX, 
fig. 3 r). They present four borders : a proximal convex border continuous with the 
general exoskeleton of the labium ; an outer border limited by the lateral longitudinal 
ridges of the labium ; an inner, ending abruptly in its substance as a thick ridge of 
chitin ; and a lower distal border, convex, curving from within, outwards, and upwards 
towards the distal end of the lateral ridge of the labium : upon the thickened inner 
extremity of this surface, which is hollowed out for its reception, the labella articulates. 
The labellae being removed, a view of the termination of the labium seen in section 
(plate XV, fig. 2) presents the following regions : on either side a pear or kidney-shaped 
area, approximated below in the median line to its fellow ; to these areas the bases of 
