714 THE METAMORPHOSIS OF FLIES. 
- companying the process, which cannot be brought under this head. 
The nature of the moulting process I see in this, that the layer of 
cells, which we regard as the hypodermis, and which appears as a 
skin split apart from the chitinous skin, after certain changes of 
form, develops a new chitinous skin, and that this takes place with- 
out any solution of continuity of the same. The change of form 
may arise merely through simple growth, accompanied by a more 
or less considerable remodelling ; or it may be due to the forma- 
tion of new, or to the disappearance of parts already present. 
The latter occurs through shrinkage, the former to a growing out 
of the hypodermis. It is the hypodermis which generally imparts 
to the insect its form, from which the formation of the appendages 
of the segments proceeds, the typical as well as the accessory. 
The mere remodelling of the parts of the body already present we | 
saw fully illustrated in the Corethra larva; thus the head gradually 
takes on a different form, the mandibles, in the young larva beset 
with long, slender bristles, afterward..become thick, massive, rap- 
torial jaws, with sharp teeth, etc. But most instructive are these 
relations in the antennz, where indeed a less change of form, rather 
than a considerable increase in size may be noticed at each moult. 
This is accompanied, as we have seen, by a complete pushing 1D-, 
wards of the hypodermis, whereby is afforded in the only possible 
way an important space for the increase in size. After the split- 
ting off of the old chitinous skin the stretched hypodermis-layet 
again turns back throughout its length and breadth. No more 
direct proof can be given for the correctness of the view which 
regards the appendages of the segments of Arthropods as protru- 
sions of the skin. 
This occurrence is only important as being a preliminary stage 
for the rise of the antennz of the imago; still this arises through 
a simple, though still deep-reaching remodelling of the larval a 
tennæ, and thus stands in relation with the other typical cephalic 
appendages. No enlargement of the parts takes place, indeed the 
hypodermis is simply loosened and models itself anew, for the 
most part during a perceptible decrease in size, into the antenn® 
of the imago; so for example the mandibles. ` But should, on the 
contrary, an enlargement take place, as in the under lip, 
arises through a pushing up pf the hypodermis from the bone 
of a falling-in of the body wall, and here the difference a 
and form between the new and old organs is so considerable, t 
Flies Se, eee 
then this , a 
