718 THE METAMORPHOSIS OF FLIES. 
rethra only a final modelling, while in Musca the head does not 
yet appear as a whole, and nothing of the beak is yet formed. 
But if in relation to the external form and formation of the 
tissues in the interior the Corethra larva stands nearer its imago, 
so not less as regards the systems of internal organs. Here also 
we find a complete continuity between larva and pupa; no new 
organs arise in place of the old, but the old either remains the 
same, or are wholly intact, or with only slight changes, as are re- 
quired for the changed mode of life of the animal; thus the dor- 
sal vessel passes wholly unchanged over into the fly, and except 
the slight shortening of the cesophageal ring, the nervous system 
also. Other organs are completed through increase in size in 
some, through concentration and disappearance in other places, as 
for example in the digestive canal, and the sexual organs which 
had long previous been completely formed. Only parts which 
are single become entirely superfluous and disappear and never ar- 
rive at a complete, new structure, independent of the systems of 
organs already present. 
It is wholly otherwise in Musca, where all the systems of organs 
of the larva disappear, and are formed anew from new building 
materials, whether they return to molecules, which mingle with 
the blood, as the hypodermis of the anterior larval segments, as 
all the larval muscles, many of the traches, the anterior part of 
the digestive canal; or whether they pass through that interesting 
process which I havé termed histolysis, and whose nature consists 
in a destruction of the histological elements, without giving UP 
the general form of the organ, and in a succeeding new formation 
from the ruins of the tissues. 
But most remarkable and pregnant is the difference in the pro- 
cesses of growth within the body of the pupa, since in Corethra 
the fat bodies play throughout a very subordinate and scarcely 
perceptible part, while in the Muscide a building up of the inner 
organs without the intervention of this important part of the body 
were not possible. There is need only for the colossal mass of ie 
fat body in the muscid larva; and seeing the thick whitish pap-like 
substance, with the product due to its destruction filling the body- 
cavity of the pupa, we can estimate the true significance of the fat 
body in the development of the Muscide. And it would appear 
that the fat body in these insects is not only a depot of nutritive 
material, but that out of the products arising from its destruction, 
