LARVAL MOUTH-ORGANS 85 
head by an elastic membrane which has some small chitinizations imbedded in 
it. A spine-like projection of the side of the head prevents the antenna from 
turning outward. 
Several hairs or hair-tufts are inserted on the head integument of the larva. 
Three pairs of these are situated dorsally on the front or epistoma. One pair is 
inserted close to the antenne and is usually the best developed; the other two 
pairs are inserted nearer the median line and more or less approximated. These 
hairs or hair-tufts differ in development, and also in position, in different forms 
and are valuable for specific diagnosis. 
The mouth-parts consist of labrum, mandibles, maxille with their palpi, and 
labium, grouped around the pharyngeal opening. 
The labrum plays an important réle in bringing food to the mouth and on 
this account is highly developed and complex; its position is on the under side 
of the anterior portion of the head. Three parts of the labrum can be dis- 
tinguished, two lateral portions and a median one which projects freely from 
the anterior margin of the head and has been called the palatum. The lateral 
pieces have inserted upon them dense masses of long yellow hair, the so-called 
mouth-brushes. By the action of powerful retractor muscles and their relaxa- 
tion the side-pieces are jerked backward and forward and communicate to the 
mouth-brushes a lashing motion. The labrum and adjoining parts are described 
by Thompson as follows: 
“The relations of the pre-antennal region of the head are hard to describe. 
As noted, the dorsal surface is convex and heavily chitinized. The ventral face 
slopes sharply inward and downward toward the entrance to the pharynx, a point 
almost in the center of the head. Thus the pre-antennal region is in reality a 
broad rostrum which overhangs the mouth-parts, situated near the transverse 
line below, slightly forward of the entrance of the pharynx. Its cross-section 
would be the segment of a circle, the arc being represented by the convex dorsal 
surface. In the mid-ventral line, about halfway between the anterior shelf-like 
fold and the entrance of the pharynx, a crest is formed. This bears four stout 
sete and is flanked by setose areas. Here the thin chitin characteristic of the 
ventral face of the rostrum generally, is strengthened by a triangular sclerite. 
I regard this structure as an epipharynx. It receives a pair of slender muscles 
(Raschke, 1887) which arise on the top of the head and probably function as 
retractors. 
“The border line and shelf-like fold already described, mark the boundary 
between the heavier chitin of the dorsal and the thinner chitin of the ventral 
face of the rostrum, above and in front. But on either side, near the antenne, 
the thicker chitin involves part of the ventral face of the rostrum. These in- 
flected areas of heavy chitin I call the black-spot areas, because each bears a 
conspicuous patch of pigment. From either area a narrow-linear ‘line’ of 
heavier chitin traverses the ventral face of the rostrum to strengthen the sclerite 
of the epipharynx. In the bay of thin cuticle bounded in front by the anterior 
shelf and border line, and posteriorly by the black-spot areas, lie the small, 
median palatum and the larger, lateral flabelle. These are protuberances, 
densely clothed with fine hairs, the flabelle having in addition a peculiar ar- 
rangement of long yellow setz. Black-pigmented apodemes that are continuous 
externally with the black-spot areas enter each flabella and the two flabellx are 
united by a transverse rod. To the apodemes the flabellal muscles are attached, 
two for each flabella, an inner and an outer, both retractor in function and act- 
