Ephemeroptera - Mayflies

Introduction

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	Leptohyphidae, Tricorythodes Ulmer
	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

References

Waltz RD, Burian SK. 2007. Ephemeroptera, pp. 181-236 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 created: June 14, 2003 (EB) - Last updated: October 19, 2007 (EB)