LAEVAL 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 antennas 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, maxillae with their palpi, and 

 labium, grouped around the pharyngeal opening. 



The labrum plays an important role 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 

 setae 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 

 (Easchke, 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 antennae, 

 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 flabellae. These are protuberances, 

 densely clothed with fine hairs, the flabellae having in addition a peculiar ar- 

 rangement of long yellow setae. Black-pigmented apodemes that are continuous 

 externally with the black-spot areas enter each flabella and the two flabellffi 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- 



