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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Aeshnidae (Darners) of Michigan - Identification

Introduction

Aeshnid immatures are conspicuous, bare-skinned, thin-legged nymphs that are widespread in freshwater, found clinging upon aquatic vegetation, stems and other trashy material along and near the waters' edge. Their laterally positioned, well-developed eyes are amongst the largest of all odonates, and are well suited to their clinging habits and aggressive predatory techniques (see Corbet 1962). Immatures are easily distinguished from other anisopteran nymphs by the long, flat mentum, a prementum that is at most slightly cleft (usually not), antennae with at least 6 to 7 segments, and generally smooth abdomen that (except for Nasiaeschna) have no mid-dorsal prominences or hooks. Except for two genera not found in Michigan, the labial palps are without raptorial setae. The abdomen is widest in the middle segments, and appears somewhat triangular or hemispherical in cross-section. Like that of Zygoptera, eggs of adults are laid endophytically (in plant tissue), and the developing genitalia of maturing female nymph are often helpful species-level diagnostic characters. Some aeshnids have direct development, others have significant periods of egg diapause (see Walker 1958, Corbet 1962).

Key to the Adults

    1a a. Midbasal (median) space with more than one crossvein Boyeria McLachlan
    b. Sides of thorax with 2 rounded, pale spots
    1b a. Midbasal space with not more than 1 crossvein 2
    b. Sides of thorax variously marked but never with only 2 rounded pale spots
     
    2a(1b) a. Sectors of arculus arising from its upper end Anax Leach
    b. Thorax uniform greenAnax junius - copyright 2006 - Keith Derosiers - used with permission
    c. Anal border of hindwing rounded in both sexes
    d. Auricles absent
    2b a. Sectors of arculus arising near its middle 3
    b. Thorax usually brown, marked with blue, green, or yellow
    c. Male with anal border of hindwing angulate
    d. Auricles on Ab2
     
    3a(2b) Vein Rs not forked 4
    3b Vein Rs forked 5
     
    4a(3a) a. Two cubito-anal crossveins on both fore- and hindwings Gomphaeschna Selys, 1871
    b. Pterostigma surmounting 1 crossvein, not counting brace vein
    c. Supertriangle without crossveins
    d. Epiproct of male deeply forked
    4b a. Three or more cubito-anal crossveins Basiaeschna janata (Say)
    b. Pterostigma surmounting 2 or more crossveins
    c. Supertriangle with crossveins
    d. Epiproct of male triangular
    also: Anal triangle of male 2-celled; base of wings with large brown spot; one row of cells between Cu1 and Cu2 beginning at the triangle
     
    5a(3b) Stalk of Rs bending forward to an asymmetrical fork Aeshna Fabricius
    also: vein Rs forked proximal to the pterostigma; supertriangle as long as, or shorter than, midbasal space
    5b Stalk of Rs straight and fork symmetrical 6
     
    6a(5b) Radial planate subtending one row of cells Nasiaeschna pentacantha (Rambur)
    in lateral view, frons prominent and face flat
    Male cerci about as long as Ab9, female cerci much shorter
    Male anterior lamina without a spine
    6b Radial planate subtending more than one row of cells Epiaeschna heros (Fabricius)
    In lateral view, frons not noticeably prominent and face slightly convex
    Male cerci about as long as Ab9+10, female cerci somewhat longer
    Male anterior lamina bearing a curved spine on each side

Mature Nymphs

    1a a. Hind angles of head decidedly angulate 2
    b. Ab5-9 with well-developed lateral spines
    1b a. Hind angles of head rounded 5
    c. Ab6-9 or Ab7-9 with well-developed lateral spines
    note: Aeshna eremita, which has hind angles of head bluntly angular, has only minute lateral spines on Ab5
     
    2a(1a) Abdomen broadly rounded 3
    2b Abdomen with distinct middorsal ridge 4
     
    3a(2a) a. Distal margin of palpal lobes truncate Boyeria McLachlan
    b. Often with evident pale, middorsal spot on Ab8
    c. Paraprocts short, about equal to middorsal length of Ab9+1
    3b a. Distal margin of palpal lobes ending in curved tips Basiaeschna janata (Say)
    b. No evident spot on Ab8
    c. Paraprocts longer, length greater than middorsal length of Ab9+10
     
    4a(2b) a. Low median dorsal ridge with blunt hooks on Ab7-9 Nasiaeschna pentacantha (Rambur)
    b. Palpal lobe rounded at tip
    4b a. Low median ridge on dorsum of abdomen without blunt hooks on Ab7-9 Epiaeschna heros (Fabricius)
    b. Palpal lobe truncate at tip
     
    5a(1b) a. Antennae longer than distance from their base to rear of head Gomphaeschna Selys, 1871
    b. Distal margin of ligula deeply bilobed, with a V-shaped notch
      a. Antennae about half as long as distance from their base to rear of head 6
    b. Distal margin of ligula obtuseangulate, at most very slightly bilobed, with the notch close
     
    6a(5b) a. Truncated blade of lateral lobe with prominent end hook Anax Leach
    b. Compound eyes as long as their greatest width
    c. Length of mentum 2x or more long as width at base
    d. Paraprocts about equal to Ab8+9
    6b a. End hook of lateral lobe not prominent Aeshna Fabricius
    b. Compound eyes much shorter than their greatest width
    c. Mentum > 1.5x as long as width at base
    d. Paraprocts shorter than above, about equal to Ab9+10

References

    Corbet, P. S. 1962. A biology of dragonflies. E. W. Classey Ltd.: Oxon, England. xvi + 247 pp.
    Walker, E. M. 1958. The Odonata of Canada and Alaska, Vol. 2. University of Toronto Press: Toronto. xii + 318.
    Westfall MJ, Tennessen KJ. 1996. Odonata, pp. 164-211, in An Introduction to the Aquatic Insects of North America, 3rd Ed. Merritt RW, Cummins KW (eds.). Kendell/ Hunt Publishing Company: Dubuque, Iowa, USA.

Page last updated: Sunday, May 22, 2016 (EB)