Apr. 5, 1924 
Anatomy of the Apple Maggot 
13 
“retraction” or “involution,” since they are due to different rates of growth in 
neighboring parts; but the results in the case of the head of the maggot are 
equivalent to an involution of the head of the embryo. 
The frontal sacs of the mature larva of Rhagoletis (PI. 3, A, E, F, FS ) are long- 
stalked pouches extending backward from the ends of the pharnygeal wings ( C) 
into the metathorax where they lie against the anterior surfaces of the retracted 
brain lobes (PI. 3, F, Br). At this stage each sac is distinctly differentiated into 
two parts, a terminal transverse enlargement containing the bud of the com¬ 
pound eye ( oh ), and a swelling of the stalk which contains the bud of the antenna 
(ab). The buds are thickenings of the walls of the sacs produced into their 
lumina. Each optic bud receives a nerve from beneath the brain lobe of the same 
side. 
In the embryo of the sheep tick, Melophagus ovinus, as described by Pratt (41), 
a tonguelike lobe filled with vertical muscle fibers projects above the mouth and 
in front of the origin of the frontal sacs. This lobe is evidently the labrum (fig. 
4, A, Lm ). During the involution of the head it is drawn into the mouth, along 
with the other parts, and eventually comes to form the roof of the anterior part 
of the larval pharynx, just posterior to the root of the frontal sacs (fig. 4, B). 
Its muscles ( Mcl) now stretch between the roof of the pharynx and the anterior 
parts of the frontal sacs, and apparently become the pharyngeal dilators of the 
larva (PI. 3, A, D, F, DiM cl). These muscles consist of two lateral masses (PI. 
3, D) attached above to the inner faces of the wing plates (C) of the pharyngeal 
skeleton, and below to the flexible, invaginated roof of the pharynx ( DPhy ). 
Their anterior ends lie against the rear wall of the transverse base of the dorsal 
pouch (PI. 3, C, DP) of the atrium. The contraction of these muscles and the 
antagonistic flexibility of the pharyngeal roof give the sucking action to the larval 
pharynx. 
A ventral pair of imaginal bud sacs arises from the floor of the atrium in the 
larva (fig. 3, B; PI. 3, A, F, LbB) f which are evidently the buds that are to form 
the proboscis and the lower parts of the head of the adult. In the embryo of 
Melophagus, according to Pratt ( 41 ), these buds arise in the normal external posi¬ 
tion ventrad to the mouth (fig. 4, A), and are withdrawn into the atrium by the 
subsequent involution of the larval head (fig. 4, B). Giacomini {12) describes 
the labial buds of Eristalis tenax as arising on each side of the opening of the 
salivary duct. 
It is evident, now, that if the fly were to have mandibles, buds of these 
appendages should be found somewhere at the inner end of the atrium. But 
no cyclorrhaphus species possesses mandibles in the adult stage, and no mandible 
buds have been noted in the larvse. The oral hooks of the maggot have often 
been called “mandibles,” but if they were true mandibular appendages, they 
should have been carried into the mouth in advance of the root of the dorsal 
pouch of the atrium and the bases of the ventral buds. 3 Instead, they arise from 
a region that is derived either from the original neck or the extreme base of the 
head. The structure and musculature, too, of the hooks is entirely nonmandib- 
ular. They are solid chitinous organs shed with each molt of the larva and are 
not renewed by the pupa. They arise by wide vertical bases from the cuticle 
at the inner ends of the lateral pockets of the mouth (PI. 1, B), where each 
loosely articulates by a single median knob on its base with the anterior end of 
the triangular plate (PI. 3, C, A) of the pharyngeal skeleton. The motion of the 
hooks is vertical and both work together, though each has its own set of extensor 
and flexor muscles (PI. 3, F). Since nearly all the muscles of the maggot degen¬ 
erate in the pupal stage, the presence of well-developed, special larval muscles 
for the oral hooks is not evidence that the hooks are derived from primitive 
3 Regarded as parts of maxillae by de Meijere (Zool. Jahrb., Sysb., Bd. 40, 1915-17, p. 177-322). 
