THE METAMORPHOSIS OF FLIES. 715 
we can rightly indeed regard it as a new formation; the passage 
to the real new formation is here indicated, and we finally see also 
the appendages of the thorax, which in the larva are completely 
4 wanting, certainly developing in the same manner; the simple 
growth of the antenne at each moulting of the larva, the forma- 
tion of the. fly’s beak out of the under lip of the larva, and the 
outgrowth of wings and limbs at places in the hypodermis previ- 
ously wanting them, are also only modifications of one and the 
: same occurrence ; the protrusion of the hypodermis. 
: So far it certainly appears legitimate to regard the formation of 
Corethra as a moulting process. The entire hypodermal skin it is 
which remodels itself, in part shortening, in part elongating and 
: drawn out, but which never loses its continuity. 
But this does happen in the Muscidæ. The hypodermis of the 
anterior larval segments must die, in order that the imaginal disks 
lying within the body cavity on the upper surface, and a new tho- 
rax and head may be composed; evidently a phenomenon which 
exceeds the idea of a moulting. It is a mistake to exceed known 
facts. It were a greater error to conclude from the fact that the 
imaginal disks of the Muscidz are already formed in the embryo, 
that the body of the imago was already formed, as in Corethra, 
in which the wings and limbs begin to grow after the last moult- 
ing. The imaginal disks of Musca are not simply rudiments of 
the appendages of the segments, but really the segment itself; but 
in Corethra the germs of the segments of the imago are already 
present in the egg, not alone in the form of isolated disks, but as 
the complete segments of the larva, which afterwards give rise to 
the appendages, in order to build up the segments of the body of 
the adult insect. “Ihe body of the imago is also on the contrary 
more perfectly formed in the larva of Corethra. 
It is by no means surprising that the appendages of the seg- 
ments of Corethra begin to develop after the last moult of the 
larva. Should they arise earlier, then they would already appear 
during the life of the larva as external parts, the chitinous skin 
which in the process of moulting would separate anew would press 
to the half-formed appendages, and the larva would thus be no 
More a larva, the metamorphosis no more complete, but an incom- 
plete one. A development of the appendages beginning before 
the last moulting would, in a metabolic insect with the mode of 
Pe eponnt of Corethra, be only conceivable if the same previ- 
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