PRATT: 1MAGHSTAL DISCS. 
247 
imaginal discs in its larva as in the nmscidian larva. And we do, in- 
ancl dsc. mt'thx., pro-, meso-, and metatlioracic imaginal discs. 
deed, find similar conditions in general; but there are some interest¬ 
ing and instructive differences. The larva is apodous and acepha¬ 
lous, like the muscidian, but in many ways it is less highly special- 
C D 
ized; indeed, it seems, in some respects, to occupy an intermediate 
jiosition between Corethra and Musca. In the position of the 
thoracic discs, for instance, it closely resem¬ 
bles Corethra. These discs are found just 
beneath the integument in two very regular 
rows, and not near the centre of the larva, 
as in the muscidian larva. The accompany¬ 
ing cuts represent frontal sections through 
the anterior end of an old larva (Fig. A being 
a ventral and Fig. B a dorsal section), show¬ 
ing the position of the thoracic discs. In struc¬ 
ture the meso- and metatlioracic discs stand 
exactly halfway between the same discs in Co¬ 
rethra and in Musca. In Corethra, according 
to Weismann (’66, p. 78), all the thoracic 
discs are of larval origin, and each is a 
double fold of the hypodermis, of which it 
remains a part, as is shown by Figure C. 
In Melophagus, on the other hand, these 
discs arise in the embryo; thev are also 
