Fig. 628.—Diagrammatic representation of the position of the imaginal buds in the larva (A) and pupa (B) of Musca (the wing rudiments omitted): as, eye-buds; at, antennal germs; b1-b3, germs of the legs; bg, central ganglionic cord: g, brain; h, so-called frontal appendage (Hirnanhang): m, peripodal membrane; o, opening of the frontal appendage into the pharynx; oe, œsophagus; p, so-called “pharynx”; r, rudiment of the proboscis; ss, frontal bud; st, stalk-like connection of the peripodal membrane with the hypodermis; I-III, 1st, 2d, and 3d thoracic segments.—Adapted from Van Rees, by Korschelt and Heider.
Originally (Fig. 628, A) the brain-appendages lie tolerably far behind in the thorax of the larva, so that they connect the hindermost part of the wall of the pharynx with the foremost section of the brain, which they surround in the form of a mushroom. Afterwards, however, subsequent to pupation, they move, together with the central nervous system, farther forward (B), whereby they (if we have correctly understood the descriptions of Weismann and Van Rees) laterally surround the pharynx with their anterior end, which is somewhat ventrally bent. At the same time, there becomes established a gradually widening communication (B, o) between the brain-appendage and the pharynx, which soon extends in the form of a lateral pharyngeal fissure along the entire length of the brain-appendage. As a result, the cavity of the brain-appendage and the pharynx so completely unite that the two soon form a single sac, the head-sac or vesicle (Fig. 6–9, k). The walls of this head-vesicle are the later head-wall, the most important parts of which can now be recognized (the antennæ, eyes, rudiments of the beak). It is now necessary that the head-vesicle (Fig. 629, +, +) be, by the eversion of the pharynx, turned outward in order that the head of the pupa may be completed. By this eversion of invaginated parts, the former mouth-opening of the pharynx becomes a neck-section (Fig. 629, +, +) by which head and thorax are now united. (Korschelt and Heider.)
Fig. 629.—Diagram of the changes to pupa of Musca before imago appears; the wing-germs not drawn: as, eye-buds; at, antennal germs; b1-b3, leg-germs; bg, ventral nerve-cord; g, brain; k, head-vesicle (originating from the union of the pharynx with the hypophysis, Hirnanhängen); oe, œsophagus; r, germ of the proboscis; ss, germ of the forehead; I, II, III, 1st, 2d, and 3d thoracic segments.—Based on Kowalevsky and Van Rees, with changes, after Korschelt and Heider.
The cause of the eversion of the head-vesicle, which Weismann directly observed, appears to be due to an increase of the inner pressure through a contraction of the hinder parts of the body. The anterior end of the œsophagus now becomes turned down ventrally corresponding to the conformation of the head of the imago.
It has been shown that the so-called pharynx is only an invaginated part of the outer surface of the larval head. The brain-appendage Korschelt and Heider consider to be the diverticulum of this invagination, in which the single parts of the body lie in an invaginated state. They may throughout be compared to the rudiments of the thoracic limbs. All these imaginal buds have been traced back to the invaginated parts of the outer surface of the body, i.e. the ectoderm.
It should be borne in mind that the process of development of the head of the highly-modified Muscidæ is much more complex than in the more primitive Diptera.
In their essay on the development of the head of the imago of Chironomus, Miall and Hammond arrange the dipterous types thus far examined, in the order of complexity of the invaginations which give rise to the head of the imago, in the following order:—
1. Culex. Relatively simple. Invaginations of the imaginal buds, shallow. 2. Corethra, Simulium. } Intermediate. 3. Chironomus, Ceratopogon. } 4. Muscidæ. Relatively complex. Invaginations deep, and apparently, but not really, unconnected with the epidermis.