THE METAMORPHOSIS OF FLIES. 605 
These collections of cells form flat disk-like bodies which are 
enclosed in a structureless membrane and may be termed the im- 
aginal disks. In each imaginal disk of the thorax arises a quarter 
of a segment with the appendage belonging to it; the two imag- 
inal disks of the head, the appendages of the brain, unite them- 
selves into a hinder division, the eye-disks ; and an anterior which 
forms the germ of the antenne and remaining portion of the head. 
The pupation in Sarcophaga occurs eight or ten days after the 
exclusion of the larva from the egg. It is accompanied by a 
marked contraction of the whole body, with an infolding of the 
first segments. 
Under the hardened, barrel-shaped, larva skin [pupariam], the 
true pupa is formed, i. e., the body of the fly enclosed by a special 
membrane, the pupa-sheath. The process of formation of the 
body of the fly, while thus enclosed, lasts for four days after the 
pupation. Then it reaches that stage which in the development 
of the butterfly is shown by a stripping off of the larva skin, and 
Sormation of the pupa is ended; then begins the development 
of the same. This consists in the building up of the external 
form of the body, and in establishing the position and develop- 
ment of the internal organs. This period can be divided into 
two sections, which are here described chronologically. The first 
division consists of the more delicate modelling of the outer form. 
Hitherto the insect has appeared only in its crudest shape, the 
‘ppendages of the thorax and head are but rudimentary, neither 
attaining their full size, nor their definitive form. All these parts 
are now entirely formed, and are matured in from two to seven 
days, and already covered with colorless hairs and bristles. 
The second division covers the period of the eighth to the 
twentieth day, during which time the inner organs are completed, 
the outer surface of the body assumes its peculiar colors. 
The first period, that of the formation of the pupa, lasting from 
one to four days, begins with the destruction of the four anterior , 
segments of the larva. The hypodermis which gives it its form 
is loosened, the muscles of the body-walls, as also of the pharynx, 
e cellular walls of the pharynx itself, the anterior part of the 
“sophagus, with the sucking stomach, follow next. During this 
= le thoracic pieces are developed from the imaginal disks ; 
stg Sive origin to the appendages, which are indeed very short, 
Still each joint can be distinguished, and are nothing but a 
