946 FRESH-WATER BIOLOGY 



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



upon the abdomen, serving for attachment to stones in 

 rapid streams; abdomen broadened posteriorly. 



Family SIMULIIDAE. 



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 . . 61 



6 1 (62) Last antennal segment much longer than the one preceding; dorsal 



areas striated like 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 STRATIOMYITDAE. 



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 lid. 



Suborder Cyclorhapha. 



This group includes many of the higher aquatic Diptera 

 (SYRPHIDAE SCIOMYZIDAE, etc.) still too insufficiently 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. O. A. Johannsen. 



IMPORTANT REFERENCES ON NORTH AMERICAN 

 AQUATIC INSECTS. 



COMSTOCK, J. H. 1917. Manual for the Study of Insects. i4th Ed. Ithaca. 



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



FOLSOM, J. W. 1913. Entomology, with Reference to its Biological and 



Economic Aspects. 2d Ed. Philadelphia. 

 HOWARD, L. 0. 1901. The Insect Book. New York. 

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



