18 AN ACCOUNT OF BRITISH FLIES. 
In the Orthorrhapha the pup escape by a T-shaped rent in the 
larval skin. The larva has in this division a more or less perfectly 
developed chitinous head. In the Cyclorrhapha the pupe escape 
from the larval skin by a circular opening, and the larva has no 
chitinous first segment, but is quite acephalous. 
Quite recently (1891)* Osten-Sacken has adopted a new method 
of classifying the first of Brauer’s sections of the Diptera; namely, 
the Orthorrhapha, as he considers the attempts made by Schiner 
and Brauer at dividing this sub-division of the Diptera into smaller 
groups as unsatisfactory. The latter authors based their groups on 
characters of subordinate value, taken from the wings and larve, 
“‘ without sufficient regard for the organization and affinities of the 
imagos.” 
Osten-Sacken considers that the study of the organs necessary for 
the functions of external life will lead to a natural arrangement, such 
as the eyes and antennze, wings and legs. 
He divides the WVemocera into two: (i.) the Zrue Nemocera, in- 
cluding the Cecidomyide, Mycetophilide, Culicide, Chironomide, 
Tipulide, Psychodide, and possibly the Dixide, may form another 
family in this group. The remaining families he calls (ii.) Vemocera 
anomata, these being archaic forms, and having a peculiar geogra- 
phical distribution.t| They also have a different siructure to the 
True Nemocera. 
The following are the chief characteristics of the Zrue Memocera : 
i, Head never holoptic, and thus the difference in size between 
the male and female head and the difference in the size of 
the eyes is very small or absent. 
ii, The eyes are round, oval, or lunate. They may meet, but 
never in such a way as seen in the holoptic head of the 
LVemocera anomala. 
iii. Antenne very large in proportion to the small head. 
iv. Legs very often long, generally thin and weak, and not fitted 
for walking, as seen in the Zipulide. 
. Generally more slender and lighter than the Nemocera ano- 
mala (compare Z7#u/a and 4zbdio) in general appearance. 
vi. Geographical distribution world-wide ; most inhabiting damp 
and shady places, and prefer the latter part of the day for 
flight. 
< 
* Vide Ent. Mo. Mag., Feb., 1891. 
+ The Azbconide, for instance, were at their maximum in tertiary times. Giard 
has given an account of this family in his ‘* Note sur les Bibionides Fossiles.” Bull. 
Sc. Dép. Nord., tome i., 1878. 
