606 THE METAMORPHOSIS OF FLIES. 
projection outwards of these same thoracic pieces. However 
here—as we had until now thought — the appendages of the imago 
do not result from a simple thrusting out of the larval hypodermis, 
as would seem to be correct in a morphological view, which would 
consider the appendages of the insectean body as projections of 
the skin, during their development not aborted, but persi 
They are in fact still, however, projections outwards of the skin, 
though they are formed at a time when the skin has not grown 
into closed segments. Still we find at the end of the second day 
the formative disks of the thorax appended to their pedicels 
(nerves, tracheze) in the form of swollen transparent vesicles, and 
within them the thoracic pieces to which they are closely related, 
with its appendage, can be easily observed. In three days they 
have attained their perfection, the skin peels off a falls away, 
and they now become three completed rings, the thoracic p 
ments. At the same time the tracheæ of the larva are thrown 0; — 
and then begins the formation of a peculiar tracheary system, 
which performs its functions only during the pupal period. we 
trunks and larger branches it resembles the larval system, but in 
the terminal rings is unlike anything else. In this respect thelt 
structure is very peculiar, in that all the e 4 
the liquids of the body, and nowhere, as before, do they i : 
branches to the different organs. The filling of the new gam 
T 
of air vessels with air does not go on during the remaining e 
j aE: 
iii cannot be completely seen during the life 
through the cross division of the proximal tubes (intima 
in a determined place of the stem near the anterior stige ae a 
At the third day the three segments of the thorax unite to pe 
: a small ring which posteriorly coalesces with the edge of wane 
larval segment; but the anterior edges are puffed up and a parts 
In the opening lie loose the chitinous parts of the mouth 
„the apparatus of hooks. The head of the fly is not yet to ‘sonst : 
but the rudiments of the same are still visible within _ silt : 
In the two formative disks of the body, which we would = a 
as appendages to the brain, develop into a vesicle contain 
esophagal ganglion, the head-vesicle, on which the pee et : 
tenn are already indicated, and from under who : 
the proboscis grows out. On the fourth day the head, W s pë is 4 
advanced forwards from within the thorax, comes to igh ae 
