Aquatic Insects of Michigan

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

Aeshnidae (Darners) of Michigan - Identification

Introduction

Aeshnids are some of our largest dragonflies, often beautifully patterned on the body. There are 21 species in 8 genera recorded in Michigan, the majority of which are denizens of lentic habitats with ample aquatic vegetation.

Adults are strong fliers, conspicuous for their hemispherical heads with large eyes that meet between the vertex and occiput, stout thorax, usually long legs used for perching vertically or "hanging" on vegetation, long and narrow, often colorful, abdomens, and large, usually clear, wings. Females are noticeable for their long ovipositors, which are used to insert eggs into vegetation in or along waterbodies. Nymphs 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).

Taxonomic references: Needham et al. 2014, Paulson 2011, Tennessen 2007, Walker 1958

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 green
	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	6
	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	7
6a(5a)	a. Ab1 with a low, ventral tubercle covered with very small spines	Rhionaeschna Foerster
	b. Fork of vein Rs nearly symmetrical at the base
6b	a. Ab1 without such a tubercle	Aeshna Fabricius
	b. Fork of vein Rs distinctly asymmetrical at the base
7a(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
7b	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	7
	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
6a(5b)	a. Prementum wider, its maximum width 0.8x its length (not including the hinge), and about 2x its basal width	Rhionaeschna Foerster
6b	a. Prementum narrower, its maximum width <0.75x its length (not including the hinge), usually distinctly less, and usually <2x its basal width	Aeshna Fabricius

References

Corbet PS. 1962. A biology of dragonflies. E. W. Classey Ltd.: Oxon, England. xvi + 247 pp.
Needham JG, Westfall MJ, May ML. 2014. Dragonflies of North America, Third Edition. Scientific Publishers: Gainesville, Florida. xvi + 657 pp.
Paulson D. 2011. Dragonflies and damselflies of the East. Princeton Field Guides. Princeton University Press, Pinceton, New Jersey, USA. 538 p.
Tennessen KJ. 2006. Odonata, pp. 237-294, in An Introduction to the Aquatic Insects of North America, 4th Ed. Merritt RW, Cummins KW, Berg MB. (eds.). Kendell/ Hunt Publishing Company: Dubuque, Iowa, USA.
Walker EM. 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: Tuesday, February 28, 2017 (EB)