Aquatic Insects of Michigan

by Ethan Bright, Museum of Zoology Insect Division and School of Natural Resources and Environment
University of Michigan

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Coleoptera - Beetles

(under construction)

(adapted from Hilsenhoff 1995, Short and Roughley 2019)

1a a. Head produced anteriorly as a rostrum Curculionidae
b. Tarsal formula 4-4-4
c. Antenna geniculate
1b a. Head not produced as a rostrum 2
b. Tarsal formula variable
c. Antenna not geniculate


2a(1b) a. Compound eyes divided into separate dorsal and ventral segments Gyrinidae
b. Ant2 elongate and scoop-shaped
c. Meso- and metathoracic legs short, flattened
d. Tarsi folding fan-wise
2b a. Compound eyes undivided
b. Ant2 variable but without a scoop-shaped Ant2
c. Meso- and metathoracic legs not extremely flattened
d. Tarsi not folding fan-wise

3a(2b) a. Elytra truncate, exposing at least 2 entire abdominal tergites Staphylinidae
also: Riparian, semi- or periaquatic
3b a. Elytra covering the entire abdomen, or only part of the last abdominal tergite is exposed 4
4a(3b) a. Hind coxae expanded as broad, flattened plates covering covering 2 or 3 basal abdominal segments and concealing all but apices of hind femora Haliplidae
4b a. Hind coxae sometimes extending posteriorly along midline, but never as broad plates 5
5a(4b) a. Hind coxae with medial portion extending posteriorly to divide 1st abdominal sternite into lateral sclerites 6
b. Prothorax with distinct notopleural sutures
5b a. Hind coxae not extending posteriorly to divide 1st abdominal sternite 10
b. Notopleural sutures almost always absent
6a(4a) a. Hind tibiae and tarsi cylindrical or subcylindrical in cross section, without long, stiff swimming bristles Carabidae, terrestrial, accidental to aquatic environments, not keyed further
also: Elytra bearing several long, slender, erect sensory hairs
6b a. Hind tarsi and usually tibiae flattened, streamlined, and bearing long, stiff swimming bristles 7
7a(6b) a. Ventral surface of body in lateral aspect with anterior of prosternum greatly depressed and much more dorsal than its postcoxal process and meso- and metasternum Dytiscidae (in part), Hydroporinae
b. Pro- and mesotarsi appear to be 4-segmented because segment 4 is very small, concealed between lobes of segment 3, and much shorter than segments 3 or 5 (except Bidessonotus)
7b a. Ventral surface of body in lateral aspect with anterior of prosternum, its postcoxal process (prosternal process), and meso- and metasterrnum in same plane 8
b. Pro- and mesotarsi distinctly 5-segmented, segment 4 as long as segment 3
8a(7b) a. Prosternal process spear-shaped, usually pointed apically Dytiscidae (in part)
b. Protibia without a large, curved, apical spur
8b a. Prostemal process greatly widened apically and nearly truncate Noteridae
b. Protibia with a large, curved spur at apex
10a(5b) a. Antennae with terminal segment no longer than combined length of 2 preceding segments 11
also: Terminal segments may be fused into a globular or elongate club
10b a. Antennae with terminal segment as long as combined length of 3-4 preceding segments 12
11a(10a) a. Antennae terminating in abrupt, globular or elongate club 12
also: Antennae with 2nd segment attached apically on 1st; antennal club consisting of 2-5 articulated segments
11b a. Antennae slender, elongate or very short, thick, with basal segment enlarged 16
12a(10a, 11a) b. Antennal club with 5 segments Hydraenidae
a. Abdomen with 6-7 visible sternites
12b a. Abdomen with 5-6 visible sternites 13
b. Antennal club with 3 segments


13a(12b) a. Pronotum projected forward, totally or partly covering the head Georissidae, Georissus pusillus LeConte
b. Tarsomere1 of proleg very small (tarsi pseudotetramerous), foretarsi appearing as having 4 segments
c. Metacoxae widely separated, intercoxal process broadly truncate
d. Very small beetles, < 2.2 mm, body strongly convex
also: Semiterrestrial, in damp soil along water edges
13b a. Pronotum not, or only slightly, projected forward, not or only slightly covering the head 14
b. Tarsomere1 of proleg not markedly reduced in size, foretarsi clearly with 5 segments
b. Metacoxae nearly contiguous, intercoxal process narrow
c. Shape variable


14a(13b) a. Pronotum bearing five well-developed longitudinal grooves Helophoridae, Helophorus
14b a. Pronotum lacking strongly impressed longitudinal grooves 15


15a(14b) a. Pronotum distinctly narrower than the elytral base Hydrochidae, Hydrochus
b. Overall body form narrow and elongate
c. Antennae before club with 4 glabrous antennomeres
d. Meso- and metatibiae lacking natatory setae
e. Elytra with strongly impressed sculpture, often encrusted with dirt
f. Small beetles, length < 5 mm
15b a. Pronotum not distinctly narrower than the elytral base Hydrophilidae
b. Body form generally not as above
c. Antennae before club with 5-6 glabrous antennomeres, if only 4, then meso- and metatibiae bearing long natatory setae
d. See c
e. Elytra generally smooth or with moderate sculpture
f. Size variable
16a(11b) a. Tarsal formula 4-4-4 Chrysomelidae
also: Antennae thickened apically, longer than head and thorax; mandibles smalI, directed ventralIy
16b a. Tarsal formula 5-5-5 17
17(16b) a. Abdomen with at least 8 visible sternites Lampyridae
17b a. Abdomen with 5-7 visible sternites 18
18a(17b) a. Prosternum expanded anteriorly as prominent lobe beneath head, head usually contracted into thorax concealing antennae and eyes 19
18b a. Prosternum not markedly expanded anteriorly beneath bead, antennae clearly visible 22
19a(18a) a. Antennae usually thick, with enlarged basal segment, about as long as head 20
19b a. Antennae filiform or serrate, much longer than head 21
20a(19a) a. Antennae with 10 or fewer segments Lutrochidae, Lutrochus laticopes Casey
b. Hind coxae contiguous
20b a. Antennae with 11 segments Dryopidae (in part)
b. Hind coxae separated
21a(19b) a. Anterior coxae transverse with trochantin visible Dryopidae (in part)
b. Antennae (if visible) very short, thick, with enlarged basal segment
21b a. Anterior coxae round, trochantin concealed Elmidae (in part)
b. Antennae slender, filiform
22a(18b) a. Tarsi with 4th segment deeply bilobed Scritidae
22b a. Tarsi usually filiform, 4th segment not bilobed 23


23a(22b) a. Antennae filiform or clavate, partly concealed within prosternum Elmidae (in part)
23b a. Antennae never concealed Psephenidae

(adapted from Hilsenhoff 1995, Short and White 2019)

1a a. Legs present, sometimes small, but always with 3-6 clearly defined segments 2
1b a. Legs absent 8
2a(1a) a. Legs (excluding claws) with 5 segments 3
b. Tarsi with 2 claws (Fig. 20A) (exception: Haliplidae with single claw)
2b a. Legs with 3-4 segments 8
b. Tarsi with single claw


3a(2a) a. Abdomen with 2 pairs of stout, terminal hooks on Ab10 Gyrinidae
b. Ab1-9 bearing lateral gills
3b a. Abdomen without hooks on terminal segment 4
b. Adominal segments usually without lateral gills, occasionally with ventral gills


4a(3b) a. Abdomen with 9 or 10 segments 5
4b a. Abdomen with 8 segments 6


5a(4a) a. Tarsus with single claw Haliplidae
b. Mandibles grooved internally
c. At least last larval instar with erect, dorsal projections from thoracic and abdominal tergites
5b a. Tarsi with 2 claws Carabidae, terrestrial, accidental to aquatic environments, not keyed further
b. Mandibles not grooved
c. Tergites without projections
6a(4b) a. Urogomphi slender, longer (usually much longer) than Ab1 Dytiscidae (in part)
6b a. Urogomphi stout, shorter than Ab1 or rudimentary or absent 7
7a(6b) a. Legs short, stout, adapted for digging Noteridae
b. Mandibles with enlarged molar portion
7b a. Legs long, slender, adapted for swimming Dytiscidae (in part)
b. Mandibles falcate (sickle-shaped), without enlarged molar portion
8a(1b, 2b) a. Labrum not represented as separate sclerite (the ventral labium may be visible dorsally) 9
8b a. Labrum separated from clypeus by a distinct suture 14
9a(8a) a. Body dorsoventrally flattened, with large, transverse thoracic and abdominal tergites Lampyridae
b. Pronotum expanded anteriorly, usually concealing head from above
c. Urogomphi indistinct
9b a. Body round or subcylindrical in cross section 10
b. Head projecting anteriorly from prothorax and visible from above
c. Movable urogomphi often visible


10a(9b) a. Maxilla with palpifer appearing as part of stipes Staphylinidae (in part), larvae not aquatic, incidental to aquatic systems, not keyed further
b. Spiracles annular (ring-shaped)
also: Various marginal aquatic or riparian habitats
10b a. Maxilla with palpifer appearing as a segment of palpus 11
b. Spiracles biforous (having 2 openings)
11a(10b) a. Abdomen with 9 complete segments, Ab10 terminal, small but distinct with no spiracular atrium or cavity present 12
11b a. Abdomen with 8 complete segments, Ab9-10 reduced and modified into a spiracular atrium or cavity (atrium absent in Berosus) 13
12a(11a) a. Legs short, 3-segmented Georissidae, Georissus pusillus LeConte
also: Riparian species
12b a. Legs long, 5-segmented Helophoridae, Helophorus
also: Urigomphi long, 3-segmented; integument noticeably chitinized
13a(11b) a. Antennae with points of insertion nearer the anterolateral angles of the head than are the insertion points of the mandibles Hydrochidae, Hydrochus
b. Labium and maxillae inserted in a furrow beneath the head
c. Lacinia present but small
13b a. Antennae with points of insertion further from the anterolateral angles of the head than those of the mandibles Hydrophilidae
b. Labium and maxillae inserted at the anterior margin of the ventral side of the head
c. Lacinia absent
14a(8b) a. Thorax and abdomen short, obese, without distinct sclerites 15
b. Legs reduced or absent
14b a. Thorax and abdomen cylindrical, flattened, or fusiform (spindle-shaped), but not markedly obese 16
b. Thoracic and abdominal tergites clearly defined
c. Legs adapted for walking
15a(14a) a. Legs very small but complete and visible Chrysomelidae
b. Spiracles on Ab8 forming large, sclerotized dorsal hooks
15b a. Legs entirely absent Curculionidae
b. Spiracles sometimes set on tubercles, but Ab8 never with sclerotized dorsal hooks
16a(14b) a. Abdomen with 10 segments 17
b. Ab9 bearing articulated, 1- or 2 jointed urogomphi
16b a. Abdomen with 9 segments 18
b. Ab8 or Ab9 sometimes bearing immovable urogomphi, but articulated urogomphi never present
15a(14a) a. Mandibles with large, asperate (roughened) molar lobe Hydraenidae
also: Ab10 with pair of recurved ventral hooks; urogomphi with 2 segments
15b a. Mandibles falcate (sickle-shaped), without molar lobe Staphylinidae (in part), larvae not aquatic, incidental to aquatic systems, not keyed further
also: Various marginal habitats including intertidal
18a(16b) a. Antennae much longer than head, multi-articulate (many jointed) Scritidae
18b a. Antennae short 19
19a(18b) a. Body extremely flattened, with thoracic and abdominal tergites expanded laterally as thin laminae concealing head and legs from above Psephenidae
19b a. Body cylindrical, subcylindrical, or fusifOlm; head and legs visible in dorsal aspect 20
20a(19b) a. Ab9 without operculum Ptilodactylidae
b. Ab1-8 sternites sometimes bearing fasciculate (clustered) gills
also: Abdomen with distinct tufts of gills, either restricted to anal region
20b a. Ab9 with a lidlike operculum covering the anal region ventrally 21
Ab1-8 stemites 1-8 never bearing gills
21a(20b) a. Terminal abdominal segment bifid or slightly emarginate (notched) posteriorly and with lateral ridges Elmidae
b. Head capsule with groups of 5 lateral stemmata
21b a. Terminal abdominal segment rounded posteriorly 22
b. Head capsule with groups of 6 stemmata (ocelli), 5 lateral and 1 ventral, or stemmata (eyes) absent
22a(21b) Opercular chamber containing 2 retractile hooks and 3 tufts of retractile gills Lutrochidae, Lutrochus laticopes Casey
mandibles with prostheca
21b Opercular chamber without hooks or gills Dryopidae
mandibles without prostheca


Hilsenhoff WL. 1995. Aquatic Insects of Wisconsin. Keys to Wisconsin Genera and Notes on Biology, Habitat, Distribution and Species. Natural History Museums Council, University of Wisconsin-Madison. Publication Number 3, 79 pages.
Short AEZ, White DS. 2019. Chapter 21: Aquatic Coleoptera, pp. 791-908 in Merritt RW, Cummins KW, Berg MB, An Introduction to the Aquatic Insects of North America, 5th Edition. Kendall Hunt Publishing Company, Dubuque, Iowa. xviii + 1480 pp.

Page last updated: May 31, 2020 (EB)