946 FRESH-WATER BIOLOGY 



57 (36) A single median proleg on the prothorax and a terminal sucking disc 



upon the abdomen, serving for attachment to stones in 

 rapid streams; abdomen broadened posteriorly. 



Family Simuludae. 



58 (2) Mandibles decurved, parallel, their motion vertical, or nearly so. 



Brachycera . . 59 



59 (64) Posterior spiracles placed together within a terminal cleft. ... 60 



60 (63) Terminal cleft vertical; head retractile. 



Family Tabanidae . . 6r 



61 (62) Last antennal segment much longer than the one preceding; dorsal 



areas striated Hke those of the abdomen. . . . Chrysops. 



62 (61) Last antennal segment not longer, usually much shorter, than the one 



preceding; dorsal areas smooth or striated, but those of the 

 thorax nearly or quite free from striae. . . . Tabanus. 



63 (60) Terminal cleft transverse; head not retractile. 



Family Stratiomyiidae, 



64 (59) Posterior spiracles separate Family Leptidae. 



65 (i) Head membranous, very imperfectly developed, often apparently 



wanting. Pupa formed in the hardened and contracted 

 larval skin (puparium), which opens by a circular cap or hd. 



Suborder Cyclorhapha. 

 This group includes many of the higher aquatic Diptera 

 (SYEPHnjAE SciOMYzroAE, etc.) still too insufiSciently known to 

 admit of the construction of a satisfactory key. 



Note: Acknowledgment is hereby made of help generously given in the preparation of the 

 foregoing keys in parts as follows: in Hemiptera by Mr. C. R. Plunkett; in Coleoptera by 

 Dr. J. C. Bradley; in Diptera by Dr. 0. A. Johannsen. 



IMPORTANT REFERENCES ON NORTH AMERICAN 

 AQUATIC INSECTS. 



CoMSTOCK, J. H. 1917. Manual for the Study of Insects. 14th Ed. Ithaca. 



Kellogg, V. L. 1905. American Insects. New York. 



FoLSOM, J. W. 19 13. Entomology, with Reference to its Biological aaf? 



Economic Aspects. 2d Ed. Philadelphia. 

 Howard, L. O. 1901. The Insect Book. New York. 

 Mlall, L. C. 1903. Natural History of Aquatic Insects. 



