664 THE METAMORPHOSIS OF FLIES. 
tiation into the perfected animal, but rather distinct accumulations 
of cells take place and in the course of development give rise to 
the new “individual.” 
But in the flies there is another fact which compels us to look 
upon the larva and pupa as a single individual, however slight 
may be the community in organs and external form between the 
two developmental stages. It is this circumstance that the same 
mass of organized substance constitutes the body of the growing 
larva, as of the fly. During the metamorphosis no growth takes 
place. The larval skin is thrown off, the insect forms a case 
around itself beneath whose shelter it remains and builds up the 
final form of the perfect insect. It takes in or gives out products 
of combustion of the still unceasing process of respiration. We 
have ina manner a second egg-development, and as we consider 
egg and larva as a single individual, so must the pupa, though not 
containing the undeveloped larva as a yolk, be considered as one 
and the same individual. But it does not happen that there is no 
stage in the development of the pupa in which the larval organs 
are not present; though the organs of flight are indeed newly 
formed, the exclusion of the larval body does not take place sud- 
denly, but very gradually it grows parallel with a whole series of 
formations of new structures. Larva and fly overlap each other. 
There can be no doubt that they are one and the same individual ; 
that their development also is to be considered as a metamor- 
phosis. rae 
It is still the most complete metamorphosis conceivable which 
we find in the Muscide, far more complete—I do not profess to 
speak from more recent observations— than, for example, the met- 
amorphosis of the Lepidoptera. The destruction of the larv 
organs is in the Lepidoptera much less complete, as the lepidop- 
terous pupa has the power of moving its hind-body. The muscles 
of the larval segments concerned in the movements seem tor 
main, and the nervous power is not interrupted ; there is a poe 
munication between the nervous centre and the organism 5 = 
consciousness of the animal remains, it reacts on stimulation. A s 
cording to Herold the pulsation of the dorsal vessel goes 0M oul 
pupa in short ceases not for a moment to be a living being; 
the life of the Muscid is as latent as that of the fertilized e388 
Through the transfer of many organs of the larva into the 
there is less need of the rebuilding of parts. I find that even 
TSAR RNG SES Ear ee SS Ee ae foes x cor ees 
