The Two-winged Flies 



317 



swift water works great havoc among the weak, soft-bodied emerging creatures. 

 I have watched many flies issuing, and a large proportion of them get swept 

 awav and presumably drowned before they can get their wings unfolded 

 and' themselves clear of the torrent. It is an extraordinary life-h.story that 



Fig 4,6.-Primarv venation of wing of net-winged midge, Bihiocephala comslocki. 

 J?,, R^, etc., branches of the radial vein. (Much enlarged.) 



these flies have, and the great danger attending the transformation to the 

 adult stage probablv partly explains why the species are so few. It is an 

 unsuccessful tvpe of insect life; the family is probably becoming extinguished. 

 Because the few living species are so widely distributed over the world- 



Fio. 438. . 



p,Q 437.— Diagram of cross-section of head through compound eves of not-winged 

 midge, Blepharocera capitala, female. 0, ocelli; ft'., brain; «./., optic lobes; /./., large 

 facets; s./., small facets. 



Pic 438.— Mouth-parts of lar\'a of net-winged midge, Bihiorcpkala doaiia. md., man- 

 dible; mx., maxilla; i.cp., labrum-epipharynx; //., labium; hyp., hypopharynx. 

 (Much enlarged.) 



they occur in North America, South .\merica, and Europe— entomologists 

 lielieve that in past ages the family was much larger than it now is. 



The llies (Fig. 433) themselves can be distinguished when in hand by 

 the curious secondary or pseudo net-veining of the wings. These faint cross 



