Apr. 5, 1924 
Anatomy of the Apple Maggot 
11 
invaginate, forming deep crescentic slits immediately ectad to the rudiments 
of the cerebral ganglia. Next, the slits approach the midline where their outer 
ends unite to form a single, median slit, while their inner ends remain separate 
and increase much in size and depth. Thus is established the Y-shaped form, 
which is retained with subsequent modifications throughout the larval life. This 
stage is shown in figure 4, A, where only the unpaired basal part of the frontal 
sacs ( FS ) appears in a median section. 
After the union of the frontal sacs in Melophagus, according to Pratt's account, 
the involution of the head takes place. The lips of the mouth are rolled inward, 
closely followed by the common base of the frontal sacs which are thus carried 
forward, downward, and finally posteriorly into the new mouth along with the 
walls of the head (fig. 4, B). There is thus established not only a new mouth, 
but also a new section of the digestive tract. The latter becomes the anterior 
part or atrium ( Atr ) of the larval pharynx. The frontal sacs (FS), now arising 
from the rear part of the newly established atrium, grow rapidly and soon attain 
the form they have in the young larva. 
Fig. 4.—Diagrams of the development of the larval head parts in Melophagus ovinus (from Pratt, but 
relettered with parts omitted). A, a young embryo with head still in normal position, the frontal sac 
(FS) opening on frontal surface behind the labrum (Lm). FS, frontal sac; LbB, labial bud; Lm r 
labrum; Mcl, labial muscles; CE, oesophagus; Phy, pharynx; Vent , ventriculus; x , x external fold of hypo 
derm at base of head. B, an older embryo after the involution of the head. The new mouth is estab¬ 
lished by the folds x, x, the atrium (Atr) by the walls of the headinvaginated in front of the folds, carrying 
to the interior the frontal sac FS and the labial buds (LbB) and burying the labrum (Lm) and its 
muscles ( Mcl) deep in the body. The labral muscles thus become the dilators of the pharynx. Compar¬ 
ing these figures with those of figure 3 shows a complete parallelism between this individual embryonio 
development and the evident phylogenetic modification of the dipteran maggot 
The more recent paper by Wahl (51) on the formation of the head in the 
cyclorrhaphous Diptera contains an exhaustive account of the head development 
as it takes place in Calliphora. In the blow-fly and in other species studied by 
Wahl (Eristalis, Allium) the frontal sacs originate in a single median evagination 
from the anterior part of the larval pharynx, forming a suprapharyngeal cavity, 
from which the two sacs extend backward into the thorax. Probably an earlier 
stage that would show the separate origin of the cephalic sacs is omitted in these 
species. 
Pratt believed that the larval pharynx (fig. 4, B, Phy) results from the invagina¬ 
tion of the larval head. Wahl showed that the larval head forms only the anterior 
part of the pharynx, the part which he calls the head atrium and from which 
arises the common base of the frontal sacs (FS ). 
It would seem that there must be some morphological difference between the 
dorsal pouch of the atrium (fig. 3, B, DP), in the branches of which are developed 
the wing plates of the pharyngeal skeleton (PI. 3, C), and the true frontal sacs 
(FS), which contain the buds of the imaginal eyes and antennae. The chitinous 
wing plates of the pharynx are shed, as are all other cuticular parts, with each 
