Aquatic
Insects of Michigan
by
Ethan Bright, Museum of Zoology Insect Division and School of Natural
Resources and Environment
University of Michigan
Aeshnidae - Darners
Introduction
Aeshnid larvae are conspicuous, bare-skinned, thin-legged larvae 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). These larvae are easily distinguished from other anisopteran larvae 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 larvae 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
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 furcillata (Say) |
| 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 furcillata (Say) |
| 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 created: June 10, 2003 (EB) Last updated: Tuesday, September 12, 2006 (EB)