The Two-winged Flies 
3 l 7 
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 
away and presumably drowned before they can get their wings unfolded 
and themselves clear of the torrent. It is an extraordinary life-history that 
Fig. 436.—Primary venation of wing of net-winged midge, Bibiocephala comstocki. 
R v R 2 , etc., branches of the radial vein. (Much enlarged.) 
these flies have, and the great danger attending the transformation to the 
adult stage probably partly explains why the species are so few. It is an 
unsuccessful type of insect life; the family is probably becoming extinguished. 
Because the few living species are so widely distributed over the world— 
o 
Fi g- 437 - Fig. 438. 
Fig. 437. Diagram of cross-section of head through compound eyes of net-winged 
midge, Blepharocera capitata, female, o, ocelli; hr., brain; o.l., optic lobes; large 
facets; 5./., small facets. 
Fig. 438.—Mouth-parts of larva of net-winged midge, Bibiocephala doanei. md., man¬ 
dible; wx., maxilla; l.ep., labrum-epipharynx; li. } labium; hyp., hypopharynx. 
(Much enlarged.) 
they occur in North America, South America, and Europe—entomologists 
believe that in past ages the family was much larger than it now is. 
The flies (Fig. 433) themselves can be distinguished when in hand by 
the curious secondary or pseudo net-veining of the wings. These faint cross 
