10 
Joumal of Agricultural Research 
Vol. XXVIII, No. 1 
semicircular base reaching back on the sides through the first third of the lateral 
plates (£). This pouch soon divides into two wings which are produced back¬ 
ward nearly as far as the end of the pharynx, and are then prolonged into two 
long, stalked sacs (PI. 3, A, E; fig. 3, B, FS). The inner surfaces of the outer 
walls of the two divisions of the dorsal pouch are chitinized and form the two 
dorsal wing-plates (PI. 3, C, C) of the pharyngeal skeleton. These wings are 
supported by wide stalks from the upper edges of the posterior lateral plates 
( B ), developed in the lateral parts of the semicircular base of the dorsal pouch 
(DP), and they are united with each other dorsally at their anterior ends by a 
narrow bridge in the upper part of the basal undivided part of the pouch. The 
stalked bags (FS) continued backward from the ends of the pouches are the 
frontal sacs which contain the buds of the imaginal antennae and compound 
eyes. 
Plate 3, D, shows a transverse section of the pharynx through the stalks of 
the wing plates. The roof of the pharynx is deeply concave behind the base of 
the dorsal pouch as here shown (DPhy) and as indicated in Plate 3, C. The 
space between the roof and the wing plates is filled with the dilator muscles of 
the pharynx (PI. 3, A, D, DiMcl ), but these will be described later. 
The earlier phases of the evolution of the larval and imaginal head of the 
Diptera have been shown to exist in the larvae of Culicidae and Chironomidae by 
Weismann (54) and by Miall and Hammond (83 ). The latter writers state that 
the invaginations that are to form the head of the fly in Chironomus do not 
begin to form, even in a rudimentary state, until after the last larval molt. 
Then they appear as simple infoldings along the edges of the frontal plate of 
the larval head. Later they unite posteriorly and the wide single pouch result¬ 
ing is produced posteriorly into the thorax. In this pouch are developed the 
antennae, the compound eyes, and the surrounding head region of the adult. 
The eversion of the parts can be observed when the larva changes to a pupa. 
According to Miall and Hammond (84) the earliest developmental stage of the 
frontal sacs in Chironomus represents the maximum attained in Corethra, while 
the sacs are still more primitive in a mosquito larva. 
In the Muscoidea and Hippoboscidae the frontal sacs reach their highest 
development, but Pratt (41) has shown that in Melophagus they still originate 
in a pair of simple thickenings in the hypoderm on the dorsal wall of the head 
in the embryo. In Eristalis and Calliphora as shown by Wahl (49, 51), the sacs 
arise from a single median bud, but it seems probable that an earlier paired stage 
is here eliminated, for the two rudiments in Melophagus soon unite. 
The paper by Pratt (41) on the history of the imaginal buds in Melophagus 
ovinus is the most important work we have on the embryology of the larval 
head in the Diptera. Our knowledge of the subject began with the work of’ 
Weismann (58,54 ), who discovered that the head of the larva is almost completely 
invaginated into the body, and that the head of the adult is formed by an ever¬ 
sion of concealed parts. But Weismann was mistaken in believing that the 
sacs in which the adult head is formed are produced by the involution of the 
larval head. He did not distinguish the two processes, already described in 
this paper, namely, the formation of the frontal sacs containing the imaginal buds 
of the antennae and eyes, and the withdrawing of these sacs into the body by 
the second process of the involution of the larval head. 
Pratt (41) shows that the cephalic buds of Melophagus appear at a very early 
stage in the embryo, before the lateral mesodermic bands have extended to the 
dorsal side, and before the endoderm has enveloped the yolk on the ventral side. 
They consist at first of a pair of simple thickenings of the hypoderm on the external 
dorsal surface of the head just behind the stomodeum. The latter is dorsal at 
this stage but soon moves forward, followed by the buds, which now begin to 
