THE METAMORPHOSIS OF FLIES. 607 
accompanied by an uninterrupted, strong contraction of the eight 
hinder segments of the larva still contained within. These last are 
shortened, and soon assume the form of the abdomen of the adult 
y. The head thus presses forwards out of the thorax, keeps 
pace in development with the thorax; the body of the pupa then 
lies as a whole contained within the puparium, and thus indicates 
the end of the first period. 
The process of development of the first four days is confined 
not wholly to the outer crust of the body, but also to the new re- 
modelling, or transformations of some of the inner organs. The 
nervous centres which had been separated in the larva here become 
United ; an infra-wsophageal ganglion separates (abschniirt) from 
the ventral cord, and the upper (supra-cesophageal ganglion) di- 
vides into two divisions, of which the outer may be considered the 
central organ of the sense of sight (ganglion opticum) and as the 
bulb of the compound eyes. 
All the anterior and middle portion of the alimentary canal 
sloughs off, and at the end of four days becomes renewed. This 
happens only to the cesophagus and chyle-stomach, while the pro- 
Yentricalus and cecal appendages of the stomach are not thus re- 
produced. They break up cell by cell; these cells are carried 
into the chyle-stomach forming there a compact mass, which is 
Surrounded with a peculiar covering, as if encysted. They do 
fill up the cavity, but swim in a honey-like liquid which by 
this time will have been secreted by the cells of the walls. Here 
the cells are preserved, though the organ is destroyed, thus the 
reconversion is effected in the walls of the chyle-stomach. c 
cell decays by fatty degeneration and in the place of the old cells 
‘rise new ones which rebuild the organ. The destruction of the 
cells is accompanied by a contraction of the muscular walls, and - 
thereby an important shortening of the organ is produced. Next 
these muscles as well as the traches decay, many branches of 
Mash are interwoven around the stomach of the larva; the 
mentary canal remains without air-vessels until the last day of 
the pupa state. As soon as the abdomen has formed, by the con- 
action of the subcutaneous muscles of the last eight larval seg- 
ments, the muscles disappear, and at the same time, namely, 
c ong the advance of the head, at which time also the nervous 
es become pushed forward ; they tear away also the degener- 
ated nervous branches, whose terminal threads likewise become 
