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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Ischnura (Coenagrionidae) (Forktails) of Michigan - Identification

A genus of world-wide distribution, four species have been collected in Michigan. Closely resembling nymphs of Enallagma and Coenagrion, mature specimens of Ischnura can be distinguished from the former two genera by the distinctive banded pattern of the eyes (which may be faint or lost in preserved specimens), and the pattern of abdominal spines along the lateral carinae of the abdominal segments. Many texts have utilized the character of seven distinct antennal segments to differentiate Ischnura and Coenagrion from Enallagma, however this is not reliable. I have seen specimens of Ischnura, particular immatures, with only 6 complete antennal segments, and the last antennal segment of some mature Enallagma individuals may bare a faint annulus that may make the segment appear as two.

Nymphs are principally lentic, most frequently found in wetlands, lakes and ponds with plentify aquatic vegetation. Specimens are also encountered in slow-moving or backwater areas of streams and rivers that likewise have abundant aquatic vegetation. I. posita tends to favor spring-fed brooks with slight current and ponds (Walker 1953). I. kellicotti, known only from a couple of southern Michigan counties, appears to be associated with lillypad ponds (Nuphar and Nymphaea), where nymphs cling to the undersides of these floating plants (Westfall and May 1996). I. hastata has recently been recorded in the Lower Peninsula. I. verticalis, by far the most common of the species of Ischnura in Michigan, and is almost ubiquitous in any permanent or temporary lentic environment. One of the earliest odonates to emerge (perhaps our earliest damselfly in flight), adults begin to appear in early May. Ponds on the UM-Dearborn campus have nearly mature nymphs as of April 15 (EB, pers.obs.). Populations can be multivoltine, and adults are seen on flight as late as late September. Only sphagnum bogs and muskegs tend not to support large numbers (Walker 1953). I (EB) have collected nymphs in conditions that appear quite eutrophic and exclusive of most other odonates. One record of I. perparva, a western species, was incorrectly identified and has been removed from our state list (O'Brien 1997).

Adults

1a a. Accessory genitalia on Ab2 venter Males, 2
b. End of abdomen without ovipositor, relatively undifferentiated
1b a. Ab2 venter morphologically undifferentiated from adjoining segments Females, 5
b. Prominent ovipositor under the last abdominal segment
 
2a(1a) a. Pterostimga of forewing separated from costa Ischnura hastata (Say)
b. Ab10 with a spine-like dorsoapical prominence ca. 0.5x as long as Ab9
c. Abdomen mostly yellow dorsally
2b a. Pterostimga of forewing bordered anteriorly by the costa 3
b. Ab10 with a spine-like dorsoapical prominence shorter than above, not spine-like
c. Abdomen not yellow dorsally
 
3a(2b) a. Dorsoapical prominence on Ab10 very low and barely bifid Ischnura kellicotti Williamson
b. Ab10 usually extensively blue dorsally
c. Ab7 with an apical dorsal blue spot
3b a. Dorsoapical prominence on Ab10 prominent and distinctly bifid 4
b. Ab10 entirely black
c. Ab7 entirely black
 
4a(3b) a. Ab8-9 entirely black Ischnura posita (Hagen)
b. Paraprocts with the posteriorly directed projection squared off, and margined with fine denticles at the apex
c. 2 postquadrangular antenodal cells in the hindwing
4b a. Ab8-9 entirely blue dorsally Ischnura verticalis (Say)
b. Paraprocts tapering to a blunt point, slightly hooked but without denticles
c. 3 postquadrangular antenodal cells in the hindwing
 
5a(1b) a. Mesostigmal plates each with a distinct posterior or oblique ridge or flange-like expansion that clearly extends above the general surface of the thorax, at least in the lateral half Ischnura verticalis (Say)
5b Mesostigmal plates with elevations low and not clearly extending above the general surface of the thorax 6
 
6a(5b) a. Postocular spots very large, subtriangular, and seldom obscured Ischnura kellicotti Williamson
b. Humeral and antehumeral stripes both complete, and of approximately the same width
c. Dorsum of Ab2 black with a bluish or orange spot apically
d. Vulvar spine present on Ab8, although sometimes small
6b a. Postocular spots small, more or less circular and often obscured in older specimens 7
b. Humeral and antehumeral stripes not as above
c. Dorsum of Ab2 entirely black, or at least with a complete longitudinal black stripe, or almost entirely orange
d. Vulvar spine present or absent
 
7a(6b) a. No vulvar spine on Ab8 Ischnura posita (Hagen)
b. Humeral stripe always present, antehumeral strip usually divided into two parts so as to resemble an exclamation mark, or at least constricted at about 1/3th the distance from the posterior end, the pattern often obscured with age, but visible if the thorax is wetted with alcohol or acetone
c. Postquadragular antenodal cells usually 2
7b a. Vulvar spine present on Ab8, often large Ischnura hastata (Say)
b. Either with a dark humeral stripe absent, or with the antehumeral stripe complete
c. Postquadragular antenodal cells 3
Also: Mesostigmal plates with the medial borders nearly straight and strongly convergent throughout its length; middorsal thoracic carina ending anteriorly in an obtuse bifurcation slightly behind the mesostigmal plates; hindwing length 15 mm or shorter
 

Mature Nymphs

1a a. Gills without a distinct nodus, stiff setae extending almost the entire length of both the dorsal and ventral margins Ischnura kellicotti Williamson
b. Associated with lily pads
1b Gills with a more or less distinct nodus, stiff setae extending not more than about 2/3 of their length (picture) 2
 
2a(1b) a. Metafemur usually < 2.3 mm Ischnura hastata (Say)
b. Head width usually < 2.9 mm c. Gills usually without drak crossbands
Also: Male cerci in posterior view with the medial margin strongly convergent downward, not concave; female cerci in lateral view with the doral margin nearly straight, and in posterior view with the medial margins diverging widely downward
2b a. Metafemur usually > 2.3 mm 3
b. Head width > 2.9 mm, or if shorter, then gills usually with dark crossbands
 
3a(2b) a. Gills usually with 3-8 dark crossbands at and beyond the nodus, although sometimes difficult (especially preserved specimens) to discern Ischnura posita (Hagen)
b. Gills often stalked, that part proximal to the nodus somewhat narrowed
c. Head with < 2.9 mm
d. Palpal setae almost always 5
e. Male cerci with the medial margins straight or concave, nearly parallel or divergent downward
2b a. Gills generally with no more than 2-3 dark crossbands Ischnura verticalis (Say)
b. Gills always widening gradully from their base
c. Head width usually 2.9 mm or more
d. Palpal setae usually with 6 setae, at least on one palpus
e. Male cerci with dorsomedial surface usually planar in the apical 1/3 to 1/2

 

Page created: 23 July 1998 - Last updated: May 17, 2016 (EB)