WATER BEETLES (COLEOPTERA) OF OHIO1. Nine complete abdominal segments, 10th reduces but distinct. Urogomphi long, 3-segmented, integument noticeably chitinized. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Helophorus sp.
Eight complete abdominal segments, segment 9 and 10 reduced and forming a stigmatic atrium or cavity (atrium absent in Berosus). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
2. Antennae with points of insertion nearer anterolateral angles or head than are insertion points of mandiblea; labium and maxillae inserted in furrow beneath head. . . . . . Hydrochus scabratis
Antennae with points of insertion further from anterolateral angles than those of mandible; labium and maxillae inserted at anterior margin of ventral side of head. . . . . . . . . . . . . . . . . 3
3. Abdominal segments with broad or long lateral projections. . . . . . . . . . . . . . . . . . . . . . . . . 4
Lateral projections usually absent; if present, not nearly so prominent . . . . . . . . . . . . . . . . . 5
4. First 7 abdominal segments with long, lateral trachael gills; mandibles with 2 teeth the lower one of which is very small or not obvious . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Berosus
Abdominal segments and posterior thoracic segments wit broad lateral projections. Crenitis sp.
5. Antennae biramal, terminal articles accompanied by a fingerlike antennal appendage. . . . . . . 7
Antennae not biamal, fingerlike antennal appendages absent. . . . . . . . . . . . . . . . . . . . . . . .13
6. Frontal sutures parallel and not uniting to forman epicralial suture; ligula absent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Laccobius sp.
Frontal sutures not parallel; epicranial suture present or not; ligula present. . . . . . . . . . . . . . 7
7. Antenna short; epicraanial suture absent; legs reduced. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Antenna longer; epicranial suture present, but usually sort; legs fairly long, not reduced. . . . . 9
8. Frons truncate behind; clypeus like a fine 6-toothed saw; mandible with 2 inner teeth; ligula about as long as margin of pronotum without a fringe of stout setae; legs not visible from above. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Paracymus
Frons rounded behind; clypeou with 4 teeth; mandible with 3 inner teeth; ligula not as ong as palp, 1-segmented; anterior margin of prohtoum with a fringe of stout setae; legs barely visible from above. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anacaena sp.
9. Mandible asymmetrical, the left with 2 inner teeth, the right with only 1; abdomen with prolegs on segments 3-7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enochrus sp.
Mandibles symmetrical, each wit 2-3 teeth; abdomen without prolegs. . . . . . . . . . . . . . . . . 10
10. Clypeus with 5 distinct teeth through middle tooth may be small. . . . . . . . . . . . . . . . . . . . 17
Clypeus with 6 or more distinct teeth; mandible with 2 inner teeth. . . . . . . . . . . . . . . . . . . 11
11. Clypeus with 6 distinct teeth, placed in 2 groups, 2 on the left and 4 on the right. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Helochares maculicollis
Clypeus with more than 6 teeth, those toward right not as clearly defined and with several smaller teeth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
12. Clypeus with tooth on left side smaller than tooth on right side. . . . . . . . . . . Cymbiodyta sp.
Clypeus with left and right side teeth equal in size. . . . . . . . . . . . . . . . . Helocombus bifidus
13. Head subspherical; forntal sulei U-shaped; mandibles not symmetrical. . . . . . . . . . . . . . . . 14
Head subrectangular, frontal sulci V-shaped; mandibles symmetrical or not, pronotum entirely sclerotized. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
14. Right mandible thinner than left mandible, right mandible with bifid tooth; abdominal segment without lateral setae. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrophilus sp.
15. Lateral abdominal gills well developed and pubescent; mentum convex toward basal half, anterolateral angles less prominent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrochara sp.
Lateral abdominal gills rudimentary, but indicated by tubercular projections, each with several terminal setae; mentum with sides almost straight, anterolateral angles very prominent. . . . . 16
16. Mesonotal and metanotal scclerites much reduced, triangular, hind margiins very narrow, almost pedunculate; apex of ligula not bifid; lateral abdominal gills of 9th segment short. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tropisternus
17. Middle tooth on clypeus smaller than the others; prosternum entire. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sperchopis tesselatus
All teeth on clypeus subequal, outer left tooth usually a little distant from the rest; prosternum with a mesal fracture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrobius sp.
Key to Hydrophilidae Nymph Genera