1. Apex of abdomen with broad, rounded ovipositor (females). . . . . . . . . . . . . . . . . . . . . 20
Apex of abdomen without an ovipositor but with a more or less complicated assemblage of parts (males). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Males
2. Ninth sternite produced into a flap covering most of the genitalia. . . . . . . . . . . . . . . . . . 3
Ninth sternite not produced inot a flap. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Terminal plate wide with apex divided inot tow large processes. . . . . . . . . . Sialis aequalis
Terminal plate narrowed at apex and not markedly incised. . . . . . . . . . . . . . . Sialis vagans
4. Terminal plate produced at apex inot a pair of long arms. . . . . . . . . . . . . . . . . . . . . . . . . 5
Terminal plate sometimes with an apical incision, or a pair of short "horns," but not a pair of long arms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Apical arms of termiinal plate separated at their base by a distance of almost equal to their length. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sialis hasta
Apical arms of terminal palte close together at base and converging at tip. . . . Sialis mohri
6. Legs with femora rufous and tibae the same color or blackish. Head punctured and dull (not smooth and polished). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sialis americana*
Legs black, or with tibiae slightly ligther than femora. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Genital hooks long and whiplike, extending forward between the lateral plates. . . . . . . . . . . 8
Genital hooks short, never longer than the plate from which they arise; lateral plates always closely appressed on meson. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8. Apex of lateral plates produced into large knobs. . . . . . . . . . . . . . . . . . . . . Sialis infumata
Apex of lateral plates not knoblike. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9. Basal lobe of genital hooks smaller; terminal plate as long as genital hooks are high. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sialis velata
Basal lobe of genital hooks markedly robust compared with distal lobe; termina plate shorter than genital hooks are high. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sialis itasca
10. Terminal plate with the apex narrow, tapered to a blunt point and not incised on meson. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sialis iola
Terminal plate with apex wider, and either markedly incised or almost truncate. . . Sialis joppa
Females (S. hasta not included in the Ross key for females, see bottom of key for discription)
11. Ninth sternite large, triangular and distinctly chlerotized. . . . . . . . . . . . . . . . . Sialis mohri
Ninth sternite apparently membranous and difficult to distinguish in un cleared specimens. . 12
12. Legs with femora rufous and tibiae the same color or darker than femora. Head punctured and dull (not smooth and polished). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sialis americana*
Legs eith entirely black or with the tibiae lighter than the femora. . . . . . . . . . . . . . . . . . . . 13
13. Eighth sternite narrow and divided inot two saparate lobes by a median desion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sialis aequalis, Sialis vagans
Eighth sternite either wider, or not divided so completely inot two separate parts. . . . . . . . 14
14. Eighth sternite rectangular and with the distal protions set out as distinct swellings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sialis infumata
Eighth sternite not as above, either with apex rounded out or not set off with such conspicuous lobes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
15. Eighth sternite flat, not tilted so that the apical margin is raised. Bursa copulatrix with a terminal fingerlike process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sialis itasca
Eighth sternite tilted so that the apical margin is markedly raised and basal margin depressed. Bursa copulatrix with terminus triangular. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
16. Species occuring only East of the Great Plains region. . . . . . . . . . . . . Sialis iola, Sialis joppa
* Sialis americana given for Ohio by Ross (1935) near Kent, but not by Canterbury (1978).
Sialis hasta female discription from Ross (1935): Female similar in color and general structure to male. Genitalia very similar to those of infumata, differing as follows: seventh sternite with only apical half sclerotized, apical margin broadly and shallowly emarginate, eighth sternite with antero-lateral portions slightly more rounded than [infumata].
Key to Sialis Adult Species