944 FRESH-WATER BIOLOGY 



5 (6) Head imperfectly chitinized in the rear, and wholly retracted within 



the pro thorax; posterior spiracles situate upon a respiratory 

 disc Family Tipulidae. 



6 (s) Head fully developed and usually free 7 



7 (35) No prolegs developed upon the prothorax 8 



8 (11) Prolegs developed upon some of the anterior segments of the ab- 



domen , 9 



9 (10) The body terminates in a very long and conspicuous respiratory tube. 



Family Ptychoptekidae. 



10 (9) Without respiratory tube, body U-shaped in locomotion. 



Family Dixidae, 



11 (8) Body without prolegs 12 



12 (31) Thorax thickened, the outline of its constituent segments more or 



less confluent; a fin of swimming hairs developed beneath 

 the end of the abdomen, and often a respiratory tube on 

 the dorsal side Family Culicidae . . 13 



13 (26) The last abdominal segment with a single dorsal breathing tube, 



through which may be seen a pair of large tracheae. . . 14 



14 (15) Antennae fold back against head and terminate in two or three claws. 



Corethrella. 



15 (14) Antennae usually only with a few small erect bristles and one or two 



pointed processes, or pendent and raptorial 16 



16 (23) With brush of hairs projecting forward from the mouth, vibratile. 17 



17 (22) Antennae not pendent and raptorial 18 



18 (19) No ventral brush or rudder on last abdominal segment beyond air 



tube; small species, one of which is found in water in 

 pitcher plant Wyeomyia. 



19(18) Last segment with ventral brush. 20 



20 (21) Head with thick spines; with four blood gills; with stellate hairs on 



the abdomen. Small species Uranotaenia. 



21 (20) Head without stout spines in addition to the usual setae. 



Crdex (sens. lat.). 



22 (17) Antennae pendent and raptorial Mochlonyx, 



23 (16) Mouth brush folded outward, raptorial, not vibratile 24 



24 (25) A plate on the side of eighth abdominal segment. . . Megarhinus. 



25 (24) A patch of small scales on the side of the eighth abdominal segment. 



Psorophora. 



26 (13) Last segment without long breathing tube 27 



27 (30) Last segment dorsally with a flat area in which may be seen two 



spiracles 28 



28 (29) Large species with the anal segment bladderKke. Mandibles strongly 



developed Pdorempis. 



39 (28) Species of medium size with anal segment cylindrical. . . Anopheles, 



