Formation of the imago in Muscidæ.[[118]]—In the flesh, and undoubtedly the house, and allied flies the germs or imaginal buds of the legs and wings arise in the same way as in Corethra. But in the Muscidæ, the buds are situated far within the interior of the body, the peripodal cavities appear closed, and the peripodal membrane stands in connection with the hypodermis merely by means of a delicate thread-like stalk. This connecting cord, which was first detected by Dewitz, and whose interpretation was entirely right, shows in its interior, as Van Rees proved, a narrow cavity.
Though the earliest stages in the development of imaginal buds in the embryo of the Muscidæ are still unknown, yet we shall not go far astray if we refer them, like the imaginal buds of Corethra, to hypodermal invaginations. We must, then, regard the stalk-like connection just mentioned as the long drawn-out neck of this invagination.
In general, the development of the appendages (Figs. 626, 627) goes on as described in Corethra. The rudiments of the legs enlarge and show at an early date the first traces of the later joints. They are so packed in the peripodal cavities that the single joints of the extremities appear as if pushed in “like the joints of a travelling cup.” (Van Rees.) The evagination of the completely formed buds of the limbs, which occurs on the first day after the beginning of pupation, goes on in such a way that the stalk of the imaginal bud (Figs. 626, B; 627, B) shortens, while its cavity widens so that the limbs finally, as in Corethra, pass out through the widely opened mouth of the peripodal invagination, which at the same time gradually completely disappears. The peripodal membrane is converted into a thickened part of the hypodermis in the region adjoining the base of the leg, and from this thickened hypodermal portion, the formation of the hypodermis of the entire imaginal thorax goes on, as the larval hypodermis is gradually destroyed.
Fig. 627.—Imaginal buds of muscid flies in process of development: A, brain (c) and ventral ganglion (v) of a larva, 7 mm. long, of C. vomitoria; h, head-rudiment; rc, portion of ventral cord; pd, prothoracic rudiment; vc3, third nerve; md, mesothoracic rudiment. B, mesothoracic rudiment more advanced, in a pupa, just formed, of Sarcophaga carnaria, showing the base of the sternum and folds of the forming leg, the central part (f) representing the foot. C, the rudimentary leg of the same more advanced; f, femur; t, tibia; f1, f5, tarsal joints. D, two buds from a larva, 20 mm. long, of Sarcophaga, attached to tracheæ; msw, mesonotal and wing-germ; mt, metathoracic rudiment. E, r, mesothoracic germ of a 7 mm. long larva attached to a tracheal twig.—After Weismann and Graber, from Sharp.
We must here settle the question as to the first origin of the mesodermal portions of the rudiments of the appendages. We can already distinguish in the imaginal buds of the fully grown muscid larva a clear separation between an ectodermal and an inner mesodermal part. Ganin derived the mesodermal part through a sort of differentiation and separation of the innermost layer of the ectodermal part, and Van Rees has, in general, confirmed this view. Kowalevsky, on the other hand, is inclined to the view that the mesodermal part of the imaginal bud is derived from the embryonal cells of the mesoderm. He finds scattered throughout the mesoderm, under the hypodermis of the larva, so-called wandering cells (Fig. 632, A, w), which are different in appearance from the leucocytes and from the elements from which the formation of the mesodermal parts of the imaginal rudiments proceed. Kowalevsky is inclined to believe that there are in each segment rudiments of the imaginal mesoderm, but which are so delicate and indifferent that we cannot find them in the first stages of their origin. From these mesodermal imaginal rudiments the above-mentioned wandering cells of the mesoderm are derived, which afterwards come into connection with the ectodermal portion of the imaginal buds.
Still more complicated and difficult to understand is the development of the head-section of muscids. We must remember that in muscid larvæ the head-section exists in its most rudimentary form, being the result of extreme modification and degeneration. The small size of the head is also due to the fact that it is more or less retracted within the thoracic region. Then, as shown by the researches of Weismann, in the last embryonal stages, the forehead, mandibles, and the whole region of the head around the mouth invaginate and form a sunken cavity (Fig. 628, p), in which the chitinous supports of the hooks characteristic of muscid larvæ are soon developed. This sunken part of the head, at whose inner end is the œsophagus, is called by the not entirely appropriate name of “pharynx,” and it must at present be remembered that the hollow space thus named is not a part of the digestive canal. It is an invaginated section of the head, and the formation of the head of the imago mainly depends on the evagination of this region.
The first rudiments of the most important parts of the head (eyes, antennæ, and forehead), occur in the youngest larvæ as paired masses of cells which lie in the thorax next to the two halves of the brain (for this reason called by Weismann “brain-appendages”), which are from their first origin connected with the pharynx, and may be regarded as the imaginal buds of the head. These appear very soon in later stages in the shape of elongated sacs widening at the hinder end (Fig. 628, A and B, h), which from their origin are to be regarded as evaginations of the pharynx. Very soon epithelial thickenings appear in the wall of this sac-shaped brain-appendage, in which the rudiments of the parts of the future head may be recognized.
Disk-shaped thickenings in the hinder widened part of the brain-appendage form the rudiments of the compound eyes, which therefore may be called the eye-buds. On the basal surface of the eye-buds is situated a nervous expansion which is connected by a nerve with the supraœsophageal ganglion. This nerve becomes the optic nerve of the perfect animal, while the optic ganglion is clearly separated from the brain.
In the anterior, more cylindrical or tube-like part of the brain-appendage we find the “frontal buds” (ss), on which the antennal rudiments (at) soon bud out, in exactly the same way as the rudiments of the limbs arise from the imaginal buds.