1 8 AN A ceo UNT OF BRITISH FLIES. 



In the Orthorrhapha the pupse 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 Cydorrhapha the pupae 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 larvae, 

 " 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 antennae, wings and legs. 



He divides the Nemocera into two : (i.) the True Nemocera, in- 

 cluding the Cecidoniyidce, Mycetoph'didce, Culicida;, Chiro7iomidce, 

 Tipulidce, Fsychodidce, and possibly the Dixidce, may form another 

 family in this group. The remaining families he calls (ii.) Nemocera 

 anomala, these being archaic forms, and having a peculiar geogra- 

 phical distribution.! They also have a different structure to the 

 True Nemocera. 



The following are the chief characteristics of the True Nemocera : 

 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 

 Nemocera anomala. 

 iii. Antennae 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 Tipulidce. 

 v. Generally more slender and lighter than the Nemocera ano- 

 mala (compare Tipula and Bibid) 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 Eni. Mo. Mag., Feb., 1S91. 



t The Bihionida, for instance, were at their maximum in tertiary times. Giard 

 has given an account of this family in his " Note sur les Bibionides P'ossiles." Bull. 

 So. Dtp. Nord., tome i., 1S78. 



