14 
Journal of Agricultural Research 
Vol. XXVIII, No. 1 
structures. They are evidently secondary movable appendages of the cuticle 
brought forward by the involution process from the back of the head or the 
neck to the. present position at the sides of the new larval mouth. 
A recent paper by Bishoff (7) reopens the question of the morphology of the 
oral hooks in the Cyclorrhapha, but gives no convincing evidence of suggested 
homologies. Finding some fly larvae, an' unidentified species living in mush¬ 
rooms, having four instead of two mouth hooks, Bischoff presents their case as 
one of a maggot having both mandibles and maxillae. He gives no discussion 
to substantiate such an identification, though he explains at some length that 
the specimens are not in a molting condition and that neither pair of hooks can, 
therefore, be explained as those about to be cast off with the slough. Next, he 
discovers a species having only two oral hooks and a pharyngeal skeleton similar 
to that of his four-hooked species. Such a combination he finds in Coenosia. 
Then, by comparison, he decides that the single pair of Coenosia represent the 
inner hooks of the other species, and hence the mandibles . The weakness of 
the argument lies in the assumption that any larval mouth hooks are ever either 
mandibles or maxillae—the very thing of which we lack the proof. Bischoff makes 
no reference to the work of Pratt (41) or any other on the actual development of 
the head and the mouth parts. He is cautious, however, about extending his 
conclusions to all the Cyclorrhapha, and promises a special paper on the 
cephalopharyngeal skeleton of this group. 
In first-instar Muscoid larvae there is a small, median, chitinous tooth in the 
roof of the atrium, whicli some writers have suggested might be the true man¬ 
dibles fused into a single rudimentary appendage. The fact, however, that the 
tooth is situated before the root of the dorsal pouch shows that it, too, belongs 
to the rear part of the head. The tooth is shed with the first larval molt and 
does not reappear. 
The upper edge of each lateral plate of the pharynx (PI. 3, C, B) and the lower 
edge of each wing plate ( C) are deeply incised parallel with their margins. On 
the lateral plate a cartilagelike ridge (b) arises in the marginal notch and turns 
obliquely downward across the outer surface of the plate. On the wing plate a 
similar ridge extends from the notch around the posterior edge of the plate, 
forming a prominent marginal thickening (c). These ridges serve for the attach¬ 
ment of muscles (PI. 3, F). The posterior third of each lateral plate and the 
posterior and dorsal parts of each wing plate are but weakly chitinized and fade 
off into transparency. 
The external musculature of the pharynx is very simple, consisting of four 
sets of muscles. These are the dorsal protractors (PI. 3, F, DPMcl ), the lateral 
protractors ( LPMcl ), the extensors of the oral hooks ( EMcl ), and the flexors of 
the oral hooks ( FMcl ). The dorsal protractors are inserted on the posterior 
surfaces of the wing plates (C) and along the dorsal edges of their marginal 
ridges (c). Each arises along the mid-dorsal line of the prothoracic segment ( 1 ) 
in a row of fingerlike divisions that criss-cross with those of the opposite side* 
The lateral protractors (LPMcl) are inserted along the ventral edge of the margi¬ 
nal ridge (c) of each wing plate, and go forward, downward, and outward to 
the latero-ventral walls of the prothorax, as indicated by the arrow in Plate 4, 
C. The protractors pull the entire pharyngeal structure forward, thus everting 
the mouth and fully exposing the oral hooks. There are no specific retractors 
of the pharynx, its retraction being accomplished by the general contraction of 
the muscles of the body wall (PI. 4 C). The lateral and ventral muscles of the 
first body segment are inserted anteriorly around the base of the larval head, 
but six slender dorsal muscles are inserted on the membranous roof of the atrium 
between the prongs of the triangular plates (PL 3, C, a ). The motions of a live 
