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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- Aquatic Insects of Michigan
Elmidae (Coleoptera) of Michigan - Riffle Beetles - Identification
These are small beetles, generally about 2-3 in total length. Identification is sometimes difficult to work with, but the systematics of the group is well worked out. There are 20 species in 5 genera recorded in Michigan, with another 4 species and another genus likely to occur based on regional distribution. The herbivorous larvae and adults both are aquatic and often co-occur together in the same habitat. They are not swimmers, but slowing walk across their habitats, and thus are fairly easy to collect. Larvae are more mobile than adults, as their anal gills eject a bubble upon which the larva rides for bouyancy. Adults, after emerging from the terrestrial puparia, engage in a dispersal flight to reenter the water, from which they do not re-emerge. Larvae usually require 5-6 instars over a two or three year period.
Adults
(adapted from Hilsenhoff and Schmude 1992, White and Roughley 2008, and Schmude 1992)
| 1a | a. Hind coxae globular, about the same size as the other coxae | 2 |
| b. Posterioir margin of prosternal process almost as wide as head | ||
| 1b | a. Hind coxae transverse, larger than pro- and mesocoxae | 3 |
| b. Posterior margin of prosternal process much narrower than width of head | ||
| 2a(1a) | a. Unicolorous dark brown elytra, elytra with sublateral carinae | Macronychus glabratus Say |
| b. Antennae short, recurved under eye and with 7 segments, enlarged at apex | ||
| c. Pronotum without tranverse impressions | ||
| d. Tarsal claw without a basal tooth? | ||
| 2b | a. Elytra with conspicuous orange, crescent-shaped markings, pronotum and elytra without sublateral carinae | Ancyronyx variegatus (Germar) |
| b. Antennae with 11 segments, filaform | ||
| c. Pronotum with oblique transverse impressions at the apical third | ||
| d. Tarsal claw with a basal tooth | ||
| 3a(1b) | a. Surface of pronotum smooth, except for basal carinae and/or punctures | 4 |
| 3b | a. Surface of pronotum rought, with costae, granules, sulci and tubercles | 5 |
| 4a(3a) | a. Pronotal surface smooth, except for punctures | Dubiraphia, 6 |
| b. Lateral margin of pronotum smooth | ||
| c. Elongate beetles | ||
| also: Tomentum present on protibia | ||
| 4b | a. Pronotal surface with basal carinae | Optioservus, 10 |
| b. Lateral margin of pronotum weakly serrated | ||
| c. Ovoid beetles | ||
| also: Tomentum present on protibia | ||
| 5a(3b) | a. Larger beetles, >2.5 mm in length | Stenelmis, 11 |
| b. Tomentum absent from protibia | ||
| 5b | a. Smaller beetles, 1.8-2.3 mm in length | Microcylloepus pusillus (LeConte) |
| b. Tomentum present on protibia | ||
| 6a(4a) | a. Large, pronotum+elytra length 2.85-3.19 mm | Dubiraphia bivittata (LeConte) |
| b. Vittae broad, not constricted in the basal third | ||
| c. Male penis 524-582µm long, and at mid-length >2x as wide as parameres | ||
| 6b | a. Smaller, pronotum+elytra length <2.75 mm | 7 |
| b. Vittae often constricted in the basal third | ||
| c. Male penis shorter, <520µm long, and at mid-length <2x as wide as parameres | ||
| 7a(6b) | a. Usually quadrinotate, if vittate, vittae darkened at basal third, except on thrid strial interval | Dubiraphia quadrinotata (Say) |
| b. Penis thin, sword-shaped for its entire length and gradually curved ventrally, 402-470 µm long | ||
| c. Pronotum+elytra length of males 2.06-2.75 mm, of females > 2.25 mm | ||
| 7b | a. Vittate, if vittae clouded at basal third, then pronotum+elytra length <2.18 mm | 8 |
| b. Male penis < 375 µm long, wider in basal third, not distinctly curved ventrally | ||
| c. Length variable | ||
| 8a(7b) | a. Very small, pronotum+elytra length 1.77-2.17 mm | Dubiraphia minima Hilsenhoff |
| b. Vittae at basal third narrow and sometimes obscure, conspicuously widened near middle to include intervals 3-7 | ||
| c. Male penis narrower than parameres and nearly parallel-sided in apical two-thirds, 316-371 μm long | ||
| 8b | a. Larger, pronotum+elytra length in 98% of females > 2.20 mm | 9 |
| b. Vittae at basal third not substantially narrower than in posterior two thirds | ||
| c. Male penis at mid-length distinctly wider than parameres and tapered in apical third | ||
| 9a(8b) | a. Robust species, width of pronotum > 0.67 mm in males and > 0. 70 mm in females | Dubiraphia robusta Hilsenhoff |
| b. Male penis 305-332 μm long, nearly parallel-sided in basal half, except at base | ||
| c. Pronotum+elytra length 2.26-2.65 mm | ||
| 9b | a. Species usually smaller and narrower, width of pronotum infrequently > 0.67 mm in males or > 0. 70 mm in females | Dubiraphia vittata (Melsheimer) |
| b. Male penis 244-300 μm long, gradually tapered from base to near apex | ||
| c. Pronotum+elytra length 2.00-2.44 mm | ||
| 10a(4b) | a. Smaller species, pronotum+elytra length 1.90-2.40 mm | Optioservus trivittatus (Brown) |
| b. One vitta on each elytron with a third vitta overlapping the elytral suture | ||
| 10b | a. Larger species, pronotum+elytra length 2.90-3.50 mm | Optioservus fastiditus (LeConte) |
| b. Each elytron vittate or bimaculate, with no vitta on elytra suture | ||
| 11a(5a) | Testaceous elytral marks overlap umbone | 12 |
| 11b | Testaceous elytral marks, if present, do not overlap umbone | 15 |
| 12a(11a) | a. At least the basal portion of palpomeres brown to black | Stenelmis musgravei Sanderson (in part) |
| b. At least portions of tarsomeres rufous to piceous | ||
| c. Antennae testaceous, or apical 1-2 antennomeres dusky (uncommonly more antennomeres dusky or darker) | ||
| also: Elytra maculate to vittita; pronotal+elytral length 2.28-2.80 mm | ||
| 12b | a. Palpi, tarsi, and antennae entirely flavous to falvotestaceous | 13 |
| 13a(12b) | a. Apicoventral margin of tarsomere 5 produced into a pointed process | Stenelmis sandersoni Musgrave |
| b. maximum width of elytra 1.21-1.56 mm | ||
| c. Pronotum highly sculptured, with prominent median longitudinal costae and lateral tubercles, deep median longitudinal sulcus, and costate posterolateral tubercle | ||
| also: Pronotum most often wider than long, ratio of pronotal width:length 0.95-1.17; tibiae most often testaceous medially; pronotal+elytral length 3.05-3.84 mm | ||
| 13b | a. Apicoventral margin of tarsomere 5 nearly straight | 14 |
| b. Maximum width of elytra 0.84-1.31 mm | ||
| c. Pronotum not as highly sculptured | ||
| 14a(13b) | a. Pronotal posterolateral tubercle elongate and costae | Stenelmis mera Sanderson |
| also: Legs bicolored, femora (except apices) dark gray, tibiae testaceous; pronotal granules numerous and of mixed sizes; elytral marks often extend mesad past interval 3 anteriorly and/or medially; pronotal+elytral length 2.31-3.13 mm | ||
| 14b | Pronotal posterolateral tubercle at most slightly elongate, not noticeably costate | Stenelmis cheryl Brown |
| also: Each elytron univittate, vitta between striae 3-5, uniform in width, the overlapping portion of the umbone equal in width to that of a vittate; median longitudinal sulcus distinct between posterolateral tubercles; pronotal+elytral length 2.75-3.51 mm | ||
| 15a(11b) | a. At least the basal portions of the palpomeres brown to black | 16 |
| b. Portion of tarsomere 5 rufous to piceous | ||
| c. Antennae testaceous or with a variable number of apical antennomeres dusky to piceous | ||
| 15b | a. Palpi, tarsi, and antennae flavous to falvotestaceous | 18 |
| 16a(15a) | a. Apical 5-8 antennomeres piceous | Stenelmis antennalis Sanderson |
| b. Palpi testaceous | ||
| c. Basal half of tarsomere 5 darker than the apical half | ||
| d. pronotal+elytral length 2.35-3.34 | ||
| 16b | a. Antennae testaceous, or apical 1-4 (rarely more) antennomeres dusky to fuscous | 17 |
| b. Palpi brown to black | ||
| c. Base and apex of tarsomere 5 darker than the medial portion | ||
| 17a(16b) | a. Pronotal+elytral length 2.34-2.96 mm | Stenelmis musgravei Sanderson (in part) |
| also: lotic | ||
| 17b | a. Pronotal+elytral length 2.69-3.34 mm | Stenelmis quadrimaculata Horn |
| also: lentic or lotic | ||
| 18a(15b) | Each elytron trivittate, mesal and/or lateral vitta may be somewhat obscured | 19 |
| 18b | Each elytron univittate, bimaculate, or immaculate | 20 |
| 19a(18a) | a. Pronotal and femoral granules numerous, conspicuous | Stenelmis sexlineata Sanderson |
| b. Tarsomere 5 of type 1, 0.04 mm shorter to 0.01 mm longer than the preceding four tarsomeres combined | ||
| c. Pronotal+elytral length 2.93-4.03 mm | ||
| 19b | a. Pronotal and femoral granules sparse, inconspicuous | Stenelmis douglasensis Sanderson |
| b. Tarsomere 5 type 2, 0.05-0.13 mm longer than the preceding four tarsomeres combined | ||
| c. Pronotal+elytral length 3.21-3.71 mm | ||
| 20a(18b) | a. Pronotum highly sculptuered, with prominent median longitudinal costae and lateral tubercles, deep median longitudinal sulcus and oblique lateral depressions, and posterolateral tubercle elongate and costate | Stenelmis crenata (Say) |
| b. Ratio of pronotal width:length 0.91-1.13, most often > 1.00 | ||
| c. Discal costae on elytra distinct and convergent basally | ||
| d. ratio of elytral length:width 1.55-1.96 e. pronotal+elytral length 2.69-3.76 mm | ||
| 20b | Combination of characters not as above | 21 |
| 21a(20b) | a. Pronotal granules most often numerous and moderately large, uniform in size, with no pronotal dark areas, with females with a basomesal cluster of pronotal granules | Stenelmis decorata Sanderson |
| b. Male genitalia Type I, penis without medial swelling | ||
| also: Tarsomere 5 on pro- and mesothoracic leg are 0.06-0.14 mm longer than the preceding four tarsomeres combined, while on the metathoracic leg it is 0.01-0.09 mm longer; Pronotal + elytral length 2.56-3.49 mm | ||
| 21b | a. Pronotal granules inconspicuous, or small and scattered, or very large and often with dark basomesal triangles on the pronotum | 22 |
| b. Male genitalia Type II | ||
| 22(21b) | a. Combination of either of these characteristics: i) Pronotal and femoral granules absent, or small and scattered, pronotum moderately to highly sculptured, posterolateral tubercle elongate and costate, and elytra widely maculate to vittate; or ii) Pronotal granules absent or microgranulate, and femoral granules tiny and inconspicuous, pronotum less sculptured with posterolateral tubercles slightly elongate but not costate, and elytra most often widely vittate | Stenelmis grossa Sanderson |
| b. Male with lateral flange on penis acutely subangular basally, giving an arrow-shaped appearance | ||
| also: Pronotum+elytral length 2.70-4.06 mm | ||
| 22b | a. Combination of characters not as above | 23 |
| b. Male with lateral flange on penis evenly arculate and gradually narrowed basally | ||
| 23a(22b) | a. Elytra most often vittate, each vitta between striae 3-5 and slightly narrowed medially, infrequently maculate | Stenelmis bicarinata LeConte |
| b. Robust, elytra length:width ratio 1.64-1.94 | ||
| c. Pronotum fairly convex with shallow median longitudinal sulcus and oblique lateral depressions, low and broad median longitudinal costae and lateral tubercles, and often with dark basomesal triangles and dark median longitudinal sulcus | ||
| also: Pronotal granules large; Pronotum+elyral length 2.94-3.65 mm | ||
| 23b | a. Combination of characters not as above | Stenelmis fuscata Blatchley |
| also: Ratio of elytra length:width 1.72-1.95; Elytra immaculate to narrowly maculate or rarely vittate; Pronotum relatively convex with broad, low costae and tubercles, small pronotal granules, and often as wide or wider at base; Males with a mesotibial ridge | ||
Larvae
(adpated from Hilsenhoff and Schmude 1992, White and Roughley 2008)
1a a. Prothorax with a posterior sternum 2 1b a. Prothorax without a posterior sternum 4 2a(1a) a. Posteriolateral angles of anterior abdominal segments produced Ancyronyx variegatus (Germar) 2b a. Posterolateral angles of abdominal segments not produced 3 3a(2b) a. Abdominal terga covered with granules
Stenelmis b. Anterior margin of head with a distinct tooth mesad of each antenna 3b a. Abdominal terga with granules in longitudinal rows Microcylloepus pusillus (LeConte)
b. Anterior margin of head without a distinct tooth mesad of each antenna 4a(1b) a. Last abdominal segment elongated, about 5x longer than wide Dubiraphia 4b a. Last abdominal segment shorter, <3x longer than wide 5 5a(4b) a. Mesopleuron divided, composed of 2 parts Macronychus glabratus Say 5b a. Mesopleuron undivided, composed of 1 part Optioservus, 6 6a(5b) a. Final instar length of last abdominal tergum < 0.75 mm
Optioservus trivittatus (Brown) 6b a. Final instar length of last abdominal tergum > 0.90 mm Optioservus fastiditus (LeConte)
References
-
Hilsenhoff WL, Schmude KL. 1992. Riffle beetles of Wisconsin (Coleoptera: Dryopidae, Elmidae, Lutrochidae, Psephenidae) with notes on distribution, habitat, and identification. The Great Lakes Entomologist 25(3):191-213.
Schmude KL. 1992. Revision of the riffle beetle genus Stenelmis (Coleoptera: Elmidae) in North America, with notes on bionomics. Ph.D dissertation, University of Wisconsin-Madison. v + 388 p.
White DS, Roughley RE. 2008. Aquatic Coleoptera, pp. 571-671 in Merritt RW, Cummins KW, Berg MB (eds.), An Introduction to the Aquatic Insects of North America, Fourth Edition. Kendall/Hunt Publishing Company, Dubuque, Iowa, USA.
Page last edited: January 28, 2015 (EB)