Apr. 5, 1924 
Anatomy of the Apple Maggot 
31 
SUMMARY 
By assembling all available information on the comparative anatomy, em¬ 
bryology, and postembryonic development in the Diptera, we may construct the 
following outline of the evolution of one of the higher flies, and thus arrive at 
a better understanding of the structure and metamorphosis of a maggot than 
could be gained from a study of the maggot alone. 
1. The larvae of primitive flies had legs and a normal head with antennae 
and biting mouth parts, and breathed through lateral spiracles connected with 
the lateral tracheal trunks. 
2. As a result of the reduction of the external appendages in the larval stage, 
the corresponding appendages of the pupa were forced to back into the body in 
order to find space for growth, thus coming to be developed in deep pockets of the 
hypoderm. 
3. As long as there was a remnant of the larval appendage, the tip of the 
pupal appendage developed within it. When the larval appendage disappeared 
entirely the bud of the pupal appendage became free in its hypodermal pouch. 
The bud was then at liberty to begin its growth in earlier larval instars or even in 
the embryo. Thus has been established the precocious origin of imaginal buds in 
the Diptera. Yet the imaginal rudiment, regardless of when it begins its growth, 
is morphologically of the pupal stage—the hypoderm pouches of the buds, the 
peripodal sacs, shed no cuticle, so far as known, till the molt of the pupa. 
4. Imaginal buds of the body wall that do not form appendages, such as most 
of those of the abdomen, remain as flat discs of cells in the hypoderm, typically 
a dorsal and a ventral pair in each segment in line with the appendage-forming 
buds of the thorax. 
5. In the higher Diptera the imaginal buds of the antennae and the compound 
eyes are developed in a single pouch on each side of the head, the so-called frontal 
sacs. 
6. By an invagination of the frontal region the roots of the frontal sacs were 
carried inward. These new cavities then united basally, forming a median 
bilobed pouch with the true frontal sacs arising from the inner ends of its wings. 
7. The head of the larva suffered a reduction after the loss of its appendages, 
and then suffered an involution resulting either from a retraction of the mouth 
or from a forward growth of a fold from the base of the head. The inturned 
parts thus came to form an anterior addition to the pharynx, known as the head 
atrium; and the median frontal pouch, carried in at the same time, formed the 
dorsal pouch of the atrium. The two wings of the pouch still carry the bases 
of the original frontal sacs, which last are buried deep in the thorax along with 
the brain lobes retracted in advance of them. 
8. By the involution of the head, the labrum and the buds of the mouth parts 
were also carried inward. The labrum now forms the roof of the atrium behind 
the root of the dorsal pouch, while its muscles constitute the dilators of the new 
larval pharynx. The buds of the mouth appendages arise from the floor of the 
atrium. 
9. The maggot early lost its true mandibles, but later acquired substitutesvin 
the form of hooks located at the sides of the new mouth. Since this region came 
from the back of the head, or from the neck, the oral hooks appear to be secondary 
cuticular organs. They move in a vertical plane and are solid structures shed 
with each larval molt. 
10. In connection with the oral hooks there was developed a chitinous skeleton 
in the walls of the atrium, the dorsal pouch, and the original larval pharynx, thus 
unifying all these parts in a structure designed to support the hooks and to give 
attachment to muscles for moving them. This skeleton is also renewed with 
each larval molt. - 
