720 THE METAMORPHOSIS OF FLIES. 
well as physiological relations from the pupa of the Muscide; 
it’does not become the imago, but it is nothing else from the start, i 
and undergoes only a slight completion, in order to slip out from 
the pupa-skin as a winged and sexually mature insect. A pupa- E 
sleep in the true sense it thus completely lacks, all functions of 
animal life in the pupa go on uninterruptedly, only the act of | 
taking food ceases. All the occurrences which fill out the interval :. 
of latent life in Musca, during which the blood no more circulates, ; 
every sensation and movement, as well as the taking of food—every 
act which may go under the head of ‘formation of the pupa” fall 
in Corethra into the larval period, and the pupal stage is to be i 
compared with the two last days of the Muscid pupa; when still 
in this respect the almost fully formed insect stands near its final 
perfection, since it, if artificially freed from its tun-shaped case, | 
is more or less movable and lively. RN 
Finally, we may distinguish two diametrically opposed forms of : 
insect metamorphosis; the one represented by Corethra stands 4 
nearest to development without metamorphosis; the other repre- 
sented by Musca is farthest removed from the ametabolic develop- 
ment and is the most extreme form of metamorphosis. Expressed 
in a very general way, the difference between the two consists 1n 
this, that at one time a continuous, at another a discontinuous de 
velopment occurs, in the sense, namely, that the parts of the body 
and organs of one stage of development originate directly from 
the similar parts of the foregoing stage ; or if such is not the cast, 
rather the parts of the body and internal organs of the later stages 
of development are substantially new formations. 
e may briefly characterize the two modes somewhat thu aie 
Type Corethra. The larval segments are converted directly into 
the corresponding divisions of the imago; the appendages of the. 
head into the corresponding ones of the head of the imago; those ; 
the thorax arise after the last moult of the larva, as outgrowths of 
BiT 
indicated even in the egg. The genital glands date p 
bryo, and are gradually developed; all the other systems of orga” 
