Apr. 5, 1924 
Anatomy of the Apple Maggot 
7 
In the facts thus outlined a basis might be found for believing that the fly 
maggot owes its form and structure to an early but secondary aquatic life when 
it floated half submerged on the surface of water. In such an environment the 
enlarged dorsal tracheal trunks would serve as floats, the dorsal spiracles would 
facilitate breathing above the surface, and the closing of the lower spiracles, with 
the reduction of the lateral tracheal trunks, would follow as logical modifications. 
The maggot form is probably not necessary, or the only one possible, in the 
present environments of most muscoid larvse, but certainly, once acquired, it is 
one well adapted to many conditions of living and gives the muscoids a wide 
range of possible environment with few modifications of structure. 
THE IMAGINAL BUDS 
The larva of insects with complete metamorphosis consists of two sets of cells, 
one set including all of those cells that constitute the larval organs, the other 
those that will form the adult organs which replace larval parts. In general, the 
adult cells are much smaller than the cells of the larva, and, in the larva, they 
are centered in small groups called imagined, buds , imaginal discs , or histoblasts. 
It is now pretty well established for the Diptera that there are four imaginal buds 
in each segment, and that, all together, they form four rows of discs in the hypo- 
derm, two in line with the wings and two in line with the legs. This applies 
theoretically also to the head, but the condensation of the segments in this region 
obscures the primitive arrangement of the buds. Each bud consists of both 
ectodermal and mesodermal cells, the first regenerating the hypoderm and all 
parts derived from it, the second regenerating new muscles and perhaps other 
mesodermal tissues. The alimentary canal is reformed from special cells situated 
in its own walls. 
Most of the histoblasts do not begin their regenerative growth until the last 
larval instar. But, in the case of appendages that have disappeared entirely in 
the larva, the histoblasts may take an early start, and, in the higher Diptera, 
some of these begin their development in the embryo. Such buds must neces¬ 
sarily push inwardly in their growth, and, as a consequence, they come to lie in 
pockets of the hypoderm, called peripodal cavities . In the muscoid larvse these 
pockets of the leg buds form long, stalked pouches entirely closed at their outer 
ends (PI. 3, F, L lt L 2 , L 3 ). All such organs that begin in this precocious manner 
must later be everted in order to complete their development. This takes^place 
in the pupal stage. 
The saclike buds of the legs and wings growing inside the body were observed 
in many insects long before their true nature was known. Weismann (53) 
showed that they evert to form the external appendages and gave them the 
name of “imaginal discs.” In Corethra he noted (54) that they arise from the 
hypoderm, but he failed to see this in Calliphora. 
Kiinckel d’Herculais (27) first demonstrated that the buds of external parts 
are always of ectodermal origin. He proposed the name “histoblast” as a term 
more generally appropriate than Weismann’s ‘‘imaginal discs.” He, moreover, 
enumerated 12 pairs of buds in the Syrphidae—4 pairs in the head, 6 pairs on 
the thorax, and 2 on the abdomen which form the genital armature. 
Ganin (11) added the fact that there are two pairs of buds on each of the 
abdominal segments, and that these buds regenerate the abdominal hypoderm 
and its appendages wherever appendages occur. He showed also that each bud 
consists of both hypoderm and mesoderm. 
Viallanes (48) likewise recorded the presence of four buds on each of the ab¬ 
dominal segments, and pointed out that these fall in line with those of the thorax, 
forming thus four rows of buds along the entire length of the body in the larva 
that regenerate the external parts of the adult. Kowalevsky (25), studying 
Muscoid larvse, found a dorsal and a ventral pair of buds on each of the first 
