8 
Joumal of Agricultural Research voi. xxviii, No. 1 
seven segments of the abdomen and corresponding pairs on the eighth surround¬ 
ing the anus. These abdominal buds, he says, are mere islands of small cells 
among the larger cells of the larval hypoderm. They regenerate the new 
hypoderm of the pupa, and those of the last segment form also the new epithe¬ 
lium of the rectum and the rectal papillae. 
Van Rees {47) describes three pairs of buds on each of the abdominal segments 
' in Calliphora, two dorsal and one ventral. The second dorsal buds, he says, are 
very small, however, and lie close to the others with which they soon unite when 
they begin to expand. Wahl {50) finds also three pairs on the abdominal seg¬ 
ments of Eristalis, two dorsal and one ventral, but the two dorsal pairs widely 
separated and situated close to the opposite edges of the segment. This is ap¬ 
parently an exceptional condition. Giacomini {12) describes only two pairs of 
buds on each of the abdominal segments of Eristalis tenax L. and says that they 
have the form of small invaginations in the mature larva. 
Pratt { 40 ), discussing imaginal buds of all insects with complete metamor¬ 
phosis, sees so much significance in the presence of four rows of them that he 
concludes that the ancestors of insects had four series of segmental appendages 
along the entire length of the body. Such a theory is unwarranted if we regard 
the buds as the regenerative centers of each segment as a whole, and not as the 
histoblasts, primarily, of an appendage. The presence or absence of a leg or a 
wing is incidental, though the invaginated form of the bud is due to the append¬ 
age which the bud is to form. Yet all appendage-forming buds are not invagi¬ 
nated, as has been shown by Needham {86) for some beetles, where the bud is a 
flat disc of cells which grows outward to form the appendage beneath the larval 
skin. 
This discussion would be more in place in the section on metamorphosis were 
it not for the fact that an understanding of the imaginal buds in general is neces¬ 
sary for an understanding of the morphology of the head and pharynx of the 
maggot. The metamorphosis of the higher flies begins in the embryo. Normally 
metamorphosis has to do with the transformation of the larva into the adult, 
and, in its more primitive manifestations, the changes do not begin until the 
end of the last larval stage. In the Diptera, however, the reconstructive growth 
has started at successively earlier and earlier stages during evolution, till, in the 
higher forms, some of the imaginal parts now begin to develop in the embryo. 
THE LARVAL PHARYNX AND THE FRONTAL SACS 
The mouth of the larva opens into a large antechamber of the alimentary 
canal which is functionally the larval pharynx. An elaborate chitinous skeleton 
(PI. 3, B) is developed in its walls and forms the dark chitinous structures that 
show through the skin in the anterior part of the maggot's body, and which 
support the oral hooks and their muscles. Most of the illustrations published 
in systematic and economic papers on Muscoid larvae show only the parts of the 
pharyngeal skeleton that are chitinized strongly enough to be seen through the 
overlying muscles and body wall. Since such figures do not give the true form 
or structure of the plates, they have no morphological value and convey no idea 
of the functions of the parts. 
An intelligible description of the pharynx and its skeleton must be based on 
a clear understanding of two well known facts in the development of the head 
of the maggot and of the adult fly. The first is that the head of the fly is formed 
largely from two sacs inverted deeply into the body from the frontal region of 
the larval head. The second is that, in the higher Diptera, the true head of the 
larva has been invaginated, not retracted, into the mouth and has carried with 
it the two frontal sacs and their points of origin in the hypoderm. Hence, the 
so-called head of the larva is really only the neck and possibly a small part of the 
