THE METAMORPHOSIS OF PLIES. 721 
pass with little or no alteration into the imago. Fatty body none or 
inconsiderable. Pupa state short and active. 
Type Musca. ‘Thorax and head of the imago arising independ- 
ently from the corresponding divisions of the hypodermis of the 
larva; only the abdomen directly through the transformation of the 
eight hinder larval segnients. Thorax and head with their append- 
ages develop from imaginal disks, which are of embryonal origin 
and are attached within the cavity of the body to nerves or trachea. 
Immediately after the formation of a tun-shaped pupa case from the 
chitinous skeleton of the larva, the imaginal disks grow together into 
the thorax and head. Destruction of all the systems of larval or- 
gans, either total or through histolysis. New formation of the same 
by means of the masses of nuclei arisingout of the destruction of 
the fatty body. Genital glands indicated in the embryo have a con- 
tinuous further development. Pupa state lasting for a long time 
and with a latent life. 
The two types are most sharply distinguished from each other 
by the presence or absence of true imaginal disks, and I might for 
this reason suggest dividing those insects having a a 
into Insecta discota and adiscota.* 
A subsequent memoir remains to be prepared, in which an effec- 
tive division is to be made showing what families belong to one 
and what to the other group. It may be here previously observed, 
that the two groups do not stand completely opposed, without 
transitions, but that there are intermediate forms; indeed whole 
families, perhaps whole orders of insects may on account of the 
want of imaginal disks be regarded as Insecta adiscota, while their 
development in other respects may closely join them to the discota. 
Tio itali Sa Oe ee Jatin th th 
AMER. NATURALIST, VOL. VIII. 46 
