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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Introduction to the Ephemeroptera (Mayflies) of Michigan

Mayflies are most familar to people for their brief adult life stage, usually 1-4 days, although for some species it is very brief (e.g., Ephoron ca. 1 hour!). Ephemeroptera is an ancient paleopteran insect family, being older than the Odonata, with fossils appearing in Devonian rocks but appears more abundant in the Carboniferous and especially the Permian rocks. Today, although probably past their heyday in terms of species (now about > 2000 species), mayflies are still important in aquatic ecosystems, especially for fisheries in terms of periphyton and detrital processing, and are often abundant.

Mayflies are typical hemimetabolous insects (egg -> nymph -> adult), with the exception of a unique (among insects), winged, non-sexual stage (subimago, or "dun" among fly-fishers) prior to the sexually mature winged stage (imago, or "spinner") (nymph -> subimago ->imago). Adults are recognized by 2 pairs of wings held vertically on the back, although the hind pair is reduced or absent in some species. Eyes may be sexually dimorphic (many species), with males having well-developed sight (sometimes 360°) for mating with females on the wing. Mouthparts and digestion tract are degenerate in the adult stage.

The nymph stage usually lasts from 2 months to 2 years, with about 20-30 instars, depending on species and usually temperature. The last nymphal instar sees a quiescent degeneration with a breakdown of the digestion tract. The nymph-subimago transition occurs in water, and is very vulnerable to predators, which is why mayflies are such good fish bait. Wing pads must rapidly be inflated with hemolymph (about 2-3 minutes). In the Ephemerellidae, a common trout stream mayfly, hydrofuge hairs keep water away from the subimago body, permitting partial inflating as it rises to the water's surface. For some very shortlived adult stages, the subimago molt is incomplete and the whitish subimago cuticle remains with the adult body. The subimago looks like an adult, but usually has shorter legs and cerci, smokier wings due to 2 cuticle layers scattering light, less colorful bodies, and having sexual organs not fully ready. This "pre-adult" stage is short, usually lasting overnight. The adult "spinner" stage see a swarming mating ritual.

Adults are shortlived, mating once and dying. Swarming functions for mates to find each other. Males congregate around a characteristic structure, such as a tree, flying up and down in a characteristic, specieis-specific pattern.

Adults (adapted from Edmunds and Waltz 1996)

    1a a. Forewing venation greatly reduced, apparently only three or four longitudinal veins behind R1 Oligoneuriidae, Homoeoneuria ammophila (Spieth)
    b. Body dark
    1b a. Forewing venation complete or moderately reduced, numerous longitudinal veins present behind R1 2
    b. Body color variable
    2a a. Penes of male longer than forceps Behningiidae, Dolania americana Edmunds and Traver
    b. Antennae of female inserted on prominent anterolateral projections
    c. Intercalary behind CuA of forewing subparallel to and almost as long as CuA, attached to CuA by seven or more unbranched crossveins
    2b a. Penes of male shorter than forceps 3
    b. Antennae of female not inserted as above
    c. Intercalaries behind CuA variable; intercalary behind CuA much shorter, with few or no crossveins attaching it to CuA, or crossveins anastomosed, especially near wing margin
    3a a. In forewings, base of veins MP2 and CuA strongly divergent from base of vein MP1 4
    b. MP2 strongly bent twoards CuA basally and sometimes fused at base with CuA
    c. hind wings with numerous veins and crossveins; vein MA of hindwings unforked
    3b a. In forewings, base of veins MP2 and CuA little divergent from vein MP1 (vein MP2 only may diverge from MP1), fork of MP usually more symmetrical 7
    b. Hindwings variable, may be reduced or absent
    c. If hind wings present, vein MA variable
    4a a. Costal angulation of hindwings acute or at a right angle Neoephemeridae, Neoephemera bicolor McDunnough
    b. Forewing vein A1 unforked
    c. Costal crossveins basal of forewing bullae weak or atrophied
    4b a. Costal angulation of hindwings usually rounded, but if nearly acute or at right angles, froewing vein A1 forked near margin 5
    b. Costal crossveins basal of forewing bullae well developed
    5a(4b) a. Middle and hind legs of male and all legs of female feeble and nonfunctional Polymitarcyidae, Ephoron Williamson
    b. Color usually pale
    c. Wings often somewhat translucent and colorless or with gray or purplish gray shading
    also: Outer margin of forewing with dense network of reticulate veinlets
    5b a. All legs of both sexes well-developed and functional 6
    b. Color variable
    6a(5b) a. Forewing vein A1 forked near wing margin Potamanthidae, Anthopotamus McCafferty and Bae
    b. Abdomen usually yellowish, in some species with reddish lateral stripes or spots on terga
    6b a. Forewing vein A1 unforked, attached to hind margin by three or more veinlets Ephemeridae
    b. Abdomen of most species with striking dark pattern on terga and sterna
    7a(3b) Cubital intercalaries of forewing consist of a series of veinlets, often forking or sinuate, attaching vein CuA to hind margin 8
    7b Cubital intercalaries of forewing variable, but not as above, or sometimes absent 11
    8a(7a) a. Remnants of gill tufts (often purplish colored) present at sides of vestigial maxillae and bases of fore coxae Isonychiidae, Isonychia Eaton
    b. Forelegs largely or entirely dark and middle and hind legs pale
    c. Vein MP of hind wing forked near margin
    d. Terminal filament vestigial
    8b a. No gill tuft remnants on vestigial maxillae and fore coxae 9
    b. Leg color not as above
    c. Vein MP of hindwing forked near base to mid-length
    d. Terminal filament variable
    9a(8b) a. Three caudal filaments Acanthametropodidae, Acanthametropus pecatonica (Burks)
    b. Terminal filament distinctly longer than tergum 10
    c. Hind wing 0.5x or more as long as forewings
    Habitat: rare, large rivers
    9b a. Two apparent caudal filaments 10
    b. Terminal filament vestigial
    c. Hind wings < 1/2 long as forewings
    10a(9b) a. Claws of each pair dissimilar (1 sharp, 1 blunt) Ameletidae, Ameletus Eaton
    b. Costal projection of hindwings acute
    10b a. Claws of each pair similar, sharp Siphlonuridae
    b. Costal projection of hindwings obtuse or weak
    11a(7b) Three well-developed caudal filaments and terminal filament present 12
    11b Two well-developed caudal filaments present, terminal filament rudimentary or absent 15
    12a(11a) a. Hind wings present and usually relatively large with one or more veins forked 13
    b. Costal projection shorter than wing width
    12b a. Hind wings small, with two or three simple veins only or absent 19
    b. If hind wing present, costal projection long (1.5-3.0x width of wing) and straight, or recurved
    13a(12a) a. Vein A-l of fore wings attached to hind margin by a series of veinlets Ametropodidae, Ametropus neavei McDunnough
    b. Two pairs of cubital intercalaries present, anterior pair long, posterior pair very short
    13b a. Vein A-I not attached to hind margin as above 14
    b. Cubital intercalaries not as above
    14a(13b) a. Short, basally detached marginal intercalaries present between veins along entire outer margin of wings Ephemerellidae
    b. Genital forceps of male with one short terminal segment
    14b a. No true basally detached marginal intercalaries in positions indicated above, usually absent along entire outer margin of wings Leptophlebiidae
    b. Genital forceps of male with two or three short terminal segments
    15a(11b) a. Hind wings with numerous. long, free marginal intercalaries Baetiscidae, Baetisca Walsh
    b. Cubital intercalaries absent in fore wings with vein A1, terminating in outer margin of wings
    15b a. Hind wings not as above, sometimes absent 16
    b. Cubital intercalaries present in fore wings with vein A1 terminating in hind margin of wings
    16a(15b) a. Short, basally detached, single or double marginal intercalaries present in each interspace of fore wings Baetidae
    b. Veins MA2 and MP2 detached basally from their respective sterns, hind wings small or absent
    c. Penes of male membranous
    d. Upper portion of eyes of male turbinate (raised on a stalk like portion)
    16b a. Marginal intercalaries attached basally to other veins 17
    b. MA2 and MP2 attached basally, hind wings relatively large
    c. Penes of male well developed
    d. Eyes of male not turbinate
    17a(16b) a. Hind tarsi distinctly five segmented Heptageniidae and Arthropleidae
    b. Tarsi shorter than tibiae
    c. Cubital intercalaries consist of two pairs similar to figure
    17b a. Hind tarsi apparently four segmented 18
    b. Basal segment fused or partially fused to tibiae, the fusion line being in basal half of tibiae plus tarsi
    c. Cubital intercalaries consist of one or two pair
    18a(17b) a. Eyes of male contiguous (touching) or nearly contiguous dorsally Metretopodidae
    b. Fore tarsi 3x length of fore tibiae
    c. Abdomen of female with apical and basal segments subequal to middle segments in length and width
    d. Subanal plate evenly convex
    18b a. Eyes of male separated dorsally by twice width of median ocellus Pseudironidae, Pseudiron centralis McDunnough
    b. Fore tarsi two times length of fore tibiae
    c. Abdomen of female long and slender, apical segments distinctly more elongate and slender than basal segments
    d. Subanal plate with medial emargination
    Habitat: large, sandy rivers, adults rarely collected
    19a(12b) a. Vein MA of fore wings forming a more or less symmetrical fork, and veins MP2, and IMP extend less than three-fourths of distance to base of vein MP Tricorythidae
    b. Genital forceps of male two or three segmented
    c. Thorax usually black or gray
    19b a. Vein MA of fore wing not as above, MA2 attached basally by a crossvein, wing veins MP2 and IMP almost as lone as vein MP, and extend nearly to base Caenidae
    b. Genital forceps of male one segmented
    c. Thorax usually brown

Mature Nymphs

    1a Thoracic notum enlarged to form a shield extended to Ab6, gills enclosed beneath sheath
    Baetiscidae, Baetisca Walsh
    1b Thoracic notum not enlarged, anterior abdominal gills exposed 2
    2a(1b) a. Gills on Ab2-7 forked and elongate-lanceolate, with margins fringed 3
    b. Usually with mandibular tusks projected forward and visible from above the head
    c. If tusks absent, head and thorax with pads of long spines
    2b a. Gills on Ab2-7 variable 6
    b. If gills forked and elongate-lancelotae, margins not fringed
    c. Tusks rarely present on mandibles
    3a(2a) a. Head and prothorax with dorsal pad pad of long spines on each side Behningiidae, Dolania americana Edmunds and Traver
    b. Without mandibular tusks
    c. Gills ventral
    3b a. Head and prothorax without pads of spines 4
    b. Mandibular tusks present
    c. Gills lateral or dorsal
    4a(3b) a. Legs unmodified, not expanded Potamanthidae, Anthopotamus McCafferty and Bae
    b. Abdominal gills held laterally
    4b a. Foretibiae fossorial, more or less modibied, either expanded or with tubercles 5
    b. Abdominal gills held dorsally
    5a(4b) a. In lateral view, mandibular tusks curved upright Ephemeridae
    b. Ventral apex of hind tibiae projected into distinct acute point
    5b a. In lateral view, mandibular tusks not curved upright Polymitarcyidae, Ephoron Williamson
    b. Ventral apex of hind tibiae rounded
    6a(2b) Double row of long setae on inner margins of femora and tibiae of forelegs 7
    6b Long setae absent on forelegs, or not arranged as above 8
    7a(6a) a. Gills on Ab1 ventral Oligoneuriidae, Homoeoneuria ammophila (Spieth)
    b. Fibrils longer than gill plate
    note: rare
    7b a. Gills on Ab1 dorsolateral, similar in position and structure to other gills Isonychiidae, Isonychia Eaton
    b. Fibrils shorter than gill plates
    note: Nymphs minnowlike, strong swimmers; common
    8a(7b) Gills on Ab2 operculate or semioperculate (covering succeeding pairs) 9
    8b Gills on Ab2 neither operculate or semioperculate, either similar to those on succeeding segments, or absent 11
    9a(8a) a. Gills on Ab2 triangular or subtriangular, do not meet medially Leptohyphidae, Tricorythodes Ulmer
    b. Gill lamellae on Ab3-6 simple, without fringed margins
    also: Forefemora with transverse row of long setae
    9b a. Gills on Ab2 quadrate, meeting or almost meeting medially 10
    b. Gill lamellae on Ab3-6 with fringed margins
    10a(9b) a. Mesonotum with distinct rounded lobe on anteriolateral corners Neoephemeridae, Neoephemera bicolor McDunnough
    b. Operculate gills fused medially
    c. Developing hind wingpads present
    10b a. Mesonotum without anterolateral lobes Caenidae
    b. Operculate gills not fused medially
    c. Without developing hind wingpads
    11a(8b) a. Gills absent from Ab2, rudimentary or absent on Ab1, and present or absent on Ab3 Ephemerellidae
    b. Gills on Ab3-7 or 4-7 consist of anterior (dorsal) oval lamella and posterior (ventral) lamella with numerous lobes
    c. Paired tubercles often present on abdominal terga
    11b a. Gills present on Ab2, either Ab1-5, 1-7, or 2-7 12
    b. Paired tubercles rearely present on abdominal terga
    12a(11b) a. Body distinctly flattened dorsoventrally 13
    b. Head flattened
    c. Eyes and antennae dorsal
    d. Mandibles not visible in dorsal view
    12b a. Body not flattened, or if flattened, mandibles visible and forming part of the flattened dorsal surface of head 14
    13a(12a) a. Claws as long or longer than tarsi Pseudironidae, Pseudiron centralis McDunnough
    b. Tibiae and tarsi bowed
    Habitat: large sandy rivers
    13b a. Claws much shorter than tarsi Heptageniidae and Arthropleidae
    b. Tibiae and tarsi straight
    14a(12b) a. Claws of forelegs differ in structure from those on middle- and hindlegs 15
    b. Claws of middle and hindlegs long and slender, about as long as tibiae
    14b a. Claws of all legs similar in structure, usually sharply pointed, rarely spatulate 16
    b. Claws variable in length, if those of middle and hind legs long and slender, then usually shorter than tibiae
    15a(14a) a. Claws on forelegs simple, with long slender denticles Ametropodidae, Ametropus neavei McDunnough
    b. Spinous pad present on forecoxae
    15b a. Claws on forelegs bifid Metretopodidae
    b. Without spinous pad on forecoxae
    16a(14b) a. Gills on Ab2-7 either forked, with all margins fringed, or with double lamellae terminated in filaments or points Leptophlebiidae
    b. Apicolateral margin of maxillae with a dense brush of hairs
    16b a. Abdominal gills not as above, but gills either more or less oval or heart-shaped, and lamellae either single, double, or triple folded, fringed on inner margin in one rare genus, never terminating in filaments or points 17
    b. Apicolateral margin of maxillae variable, but never with a dense brush of hairs
    17a(16b) a. Labrum with a median notch on distal margin Baetidae
    b. Terminal filament variable, may be shorter than tergum 10 or subequal to cerci
    c. Antennae long, 2-3x or more width of head, or antennae shorter than 2x width of head but with labrum with notched distal margin
    17b a. Labrum with a median notch on distal margin 18
    b. Terminal filament subequal to cerci
    c. Antennae shorter than 2x width of head
    18a(17b) a. Tibiae and tarsi bowed Acanthametropodidae, Acanthametropus pecatonica (Burks)
    b. Claws very long and slender, claws of hindlegs about as long as tarsi
    also: Dorsum of abdomen with median hook-like tubercles; hook-like tubercles also present on each thoracic sternum
    Habitat: rare, in large rivers
    18b a. Tibiae and tarsi not bowed 19
    b. Claws usually not long and slender
    19a(18b) a. Maxillae with a crown of pectinate spines Ameletidae, Ameletus Eaton
    b. Gills with a single lamellae, more or less oval with a sclerotized band along the lateral margin and usually with a similar sclerotized band on or near the mesal margin
    19b a. Maxillae without pectinate spines Siphlonuridae
    b. Gills variable, not as above


    Edmunds GF, Waltz RD. 1996. Ephemeroptera, pp. 126-163 in Merritt RW, Cummins KW (editors), An Introduction to the Aquatic Insects of North America, 3rd Edition. Kendall/Hunt Publishing Company: Dubuque, Iowa.

Page created: June 14, 2003 (EB); Last edited: July 29, 2003 (EB)