1877. ] Recent Literature. 427 
cells of the thoracie segments of the larva. In butterflies and beetles, 
the part corresponding to the exoderm of the imaginal disc is derived 
from the epithelial sac of the leg of the larva; the mesoderm likewise 
is a new formation. It is very probable that a further inquiry into the 
processes of the post-embryonal development of other Diptera, and 
also of insects of other orders, principally of such groups as have so- 
called apodal larvæ, will disclose some transitional forms of post-embry- 
onal processes leading towards the extreme and well-characterized type 
of development of the Muscide. Thus it is now well known that 
Miastor} is a connecting link between Muscide and Tipulide, as re- 
gards the history of the development of its head. The dorsal portion of 
the head of Miastor is developed from the cephalic discs, the position of 
which is similar to that of the cephalic discs of the Muscide ; the ventral 
and lateral portions of the head of the imago of Miastor are developed 
with the participation of the teguments of the larval cephalic segment. 
The development of the organs of the mouth of Miastor is much nearer 
to that of Corethra. 
“The musculature of the head of the larva of Miastor, according to 
Zalensky, a without change into the muscles of the head of the 
imago. Itis to be regretted that we do not find in the work of Za- 
lensky any mention of destructive processes in the cephalic segment of 
the larva of Miastor. If the destruction of the exoderm of the cephalic 
segment of the larva is not recognized, at least in its dorsal region, it is 
difficult to understand how the newly formed part of the head can as- 
sume its normal relative position towards the old larval exoderm of the 
cephalic segment. 
“Some morphological importance must also be attributed to the inter- 
esting fact, that in the larva of Miastor, the formation which is homol- 
ogous to the pair of cephalic discs of the larve of Muscide appears in 
the shape of a single (not paired) organ, although judging by the nerves 
with which it is connected, it corresponds to the pair of cephalic discs of 
the Muscide. 
“The principle adopted by Weismann for the division of all insects into 
two sharply defined types, according to their post-embryonal develop- 
ment, and which depends upon the presence or absence of histolytic _ 
Processes, and also on the mode of formation of the histological ele- 
ments of the tissues of the i imago,— in one case from the Kérnchenku- 
geln, in the other with the participation of the elements of the tissues of 
the larva, — after all that has been said above, must lose its scientific 
value. It seems to me that the principles which must guide us in the 
grouping of insects with regard to their post-embryonal Lee 
Are the number and quality of the destructive processes, the 
Modes of the building up of the organs of the imago from the newly 
-1 Article by Mr, sped in the Proceedings of the Third Meeting of the Rus- 
sian Naturalists in Kie 
