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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Enallagma (Coenagrionidae), and Coenagrion nymphs of Michigan - Identification

A widespread genus of coenagrionids, this is Michigan's most species-rich and common group of damselflies with 17 recorded species. Nymphs are diagnostically a difficult group of damselflies, and generally only mature specimens with intact caudal gills can be identified to species (unless associated with adults and/or molecular data). Because of the close resemblance of nymphs of our two species of Coenagrion with other Enallagma, these are included in the key below.

Most species inhabit lakes, ponds and wetlands as well as slower portions of streams and rivers, usually with abundant aquatic vegetation. E. anna, E. aspersum, E. basidens, E. divagans, and E. traviatum are known only from the LP. Both E. anna and E. basidens appear to be expanding their ranges throughout North America, being found in sheltered coves and edges of large lakes, small ponds and slow portions of streams, as well as human-impacted habitats such as impoundments and quarry ponds (Cannings 1989, O'Brien and Pratt 1998). E. antennatum, E. exsulans, E. geminatum, E. signatum and E. vesperum are known in the UP only from one record, but are widespread in the LP. The other species (E. boreale, E. carunculatum, E. civile, the circumpolar E. cyathigerum, E. ebrium, and E. hageni) are widespread in both peninsulas.

Population cohorts for several species have been studied, though little generic-level concensus has been reached. Baker and Clifford (1982) studied a population of E. boreale in a boreal forest pond in Alberta, Canada, and concluded that low temperatures during egg development strongly influenced subsequent growth of larvae, ultimately determining if there were one or two cohorts per year. Existence of mixed cohorts (bivoltine and uni- or multivoltine) has also been observed for E. aspersum in North Carolina (Ingram and Jenner 1976) and E. cyathigerum in Britain (Macan 1964). The latter species also was found to be univoltine at another site in Britain (Johannsson 1978), and Kormondy and Gower (1965) found E. ebrium to be univoltine in Pennsylvania as well. Pearlstone (1973) studied the feeding behavior of E. boreale in a small lake in southwestern British Columbia. Cladocerans and larval chironomids were the principal prey items, although nymphs appeared opportunistic with prey choice as other prey items were consumed (e.g., mayflies, stoneflies, water mites, copepods, other damselflies, etc.). Prey choice also did not marked change among the various instars of mature instars.

(Taxonomic References: Paulson 2011, Walker 1953, Westfall and May 2006)

Adults

1a a. Ventral portion of Ab2-3 modified as secondary genitalia Males, 2
b. Terminal end of Ab10 with a pair of flanking claspers (caudal appendages)
1b a. Ab2 venter morphologically undifferentiated from adjoining segments Females, 19
b. Distal portion of Ab8 sternum with a genital aperature, with a well-developed ovipositor underneath Ab9 and beyond
 
2a(1a) a. Black area on Ab2 dorsum occupying 1/2 the length of the segment, or less, confined to the apical half, or isolated near the center as anarrow transverse band (ventrolateral black stripes may extend further basad) 3
2b a. Black area on Ab2 dorsum occupying 2/3 to the entire length of the segment 12
 
3a(2a) a. Paraprocts shorter than, or subequal in length, to the cerci 4
3b a. Paraprocts distinctly longer than the cerci 8
 
4a(3a) a. Cerci, in lateral view, distinctly bifid, the arms subequal, appendage 1/3-1/2 the lateral length of Ab10 Enallagma ebrium (Hagen), in part
4b a. Cerci, in lateral view, not so, and, if bifid, then the superior arms are longer than the inferior and the appendage is about 2/3x the length of Ab10 5
 
5a(4b) a. Cerci, in ventrolateral view, broadly emarginate and at least 2/3x the lateral length of Ab10 6
5b a. Cerci, in ventrolateral view, not broadly and deeply emarginate, and usually about 1/2x the lateral length of Ab10 (except E. civile 2/3x or more) 7
 
6a(5a) a. Dorsum of Ab7 blue on apical 1/3-1/2 Enallagma aspersum (Hagen)
b. Tips of cerci, in lateral view, aslant posteroventrally
6b a. Dorsum of Ab7 black apically Enallagma anna Williamson
b. Tips of cerci, in lateral view, straight or rounded
also: Ventral arm of cerci, in dorsomedial view, black at base, directed ventromeddially and with a wide, apical, pale tubercle
 
7a(5b) a. Dorsal arm of cerci more prominent than ventral arm, pale tubercle not surpassing tip of dorsal arm Enallagma civile (Hagen)
b. Medial surface of cerci with a strong, pointed recurved tooth at the base of tubercle
c. Black areas of Ab3-5 usuallly much less than 1/2x the length of their segments
7b a. Dorsal and ventral arms of cerci nearly equal in prominence, pale tubercle extending beyond tips of the dorsal arm Enallagma carunculatum Morse
b. Medial surface of cerci without a sharp recurved tooth
c. Black areas of Ab3-4 usually > 1/2x the length of their segments
also: Black on dorsum of Ab7 not reaching extreme base of Ab7, except sometimes as a hairline
 
8a(3b) a. Black humeral stripe usually much narrower than pale antehumeral for most of its length 9
b. Paraprocts generally >1/2x the length of Ab10
c. Dorsal black mark on Ab7 usually not reach extreme base of that segment
 
8b a. Black humeral stripe nearly equal to, or wider than, pale antehumeral for most of its length 11
b. Paraprocts generally >1/2x the length of Ab10
c. Dorsal black mark on Ab7 always reaching extreme base
 
 
9a(8a) a. Cerci, in lateral view, not upturned at apex Enallagma boreale Selys
b. Pale tubercle of the cerci located on the medial margin near the apex
9b a. Cerci, in lateral view, upturned at apex 10
b. Pale tubercle located at extreme tip (best seen in dorsal view)
 
10a(9b) a. Each cercus, in dorsal view, with a ridge running diagonally forward from distal black tooth across the dorsoapical surface, then recurving anteromedially to, or nearly to, the medial margin of cercus, and without a distinct anterior tooth Enallagma annexum (Hagen)
10b a. Each cercus, in dorsal view, with a similar curved ridge ending in a distinct anterior tooth near the medial margin of cercus Enallagma vernale Gloyd
 
11a(8b) a. Cerci, in lateral view, distinctly bifid it apex Enallagma ebrium (Hagen), in part
also: Cerci at least 1/3x the length of Ab10; Ab8 entirely blue except for a very inconspicuous, lateral, apical black spot
11b a. Cerci, in lateral view, not bifid in apically, rather the tips slightly upturned Enallagma hageni (Walsh)
also: Black marking on dorsum of Ab5 usually < 1/2x length of that segment; tips of cerci only slightly upturned
 
12a(2b) a. Ventrolateral black markings extending the entire length of Ab8-9, and nearly the entire length of Ab2 Enallagma geminatum Kellicott
12b a. Ab2, 8 and 9 without ventrolateral black markings 13
 
13a(12b) a. Black humeral stripe at midlength 1/5-1/2 the width of the blue antehumeral stripe Enallagma traviatum Selys
also: Cerci nearly as long as Ab10 laterally, longer than paraprocts, in dorsal view with inferior arms directed medially toward each other, their tips nearly touching; Ab3 dorsum with black marking reaching base, Ab10 half to wholly black
13b a. Black humeral stripe at midlength much > 1/2 the width of the antehumeral stripe, or, if only 1/4 as wide, then antehumeral stripe orange or yellow (E. vesperum) 14
 
14a(13b) a. Cerci, in lateral view, bifid, or at least the apex emarginate to produce dorsal and ventral arms or lobes, and distinctly shorter than Ab10 laterally 15
b. Legs with distinct black markings
c. Pale color of mature specimens mostly blue, violet, or green
14b a. Cerci neither bifid or emarginate apically as above, and nearly as long or longer than Ab10 18
b. Legs practically without black markings, except sometimes in distal 1/2 of femora
c. Pale color of mature specimens orange or red
 
15a(14a) a. Vein M2 arising near fourth postnodal crossvein in forewing, near third in hindwing Enallagma basidens Calvert
b. Ab4-6 with dorsal pale blue marking on the basal 1/3
c. Black humeral stripe divided along mesopleural suture by a pale longitudinal stripe
15b a. Vein M2 arising nearer the fifth postnodal crossvein in forewing, near fourth in hindwing 16
b. Ab4-6 with dorsal pale blue marking on the basal 1/10
c. Black humeral stripe entire
 
16a(15b) a. Ab8 predominately blue, nearly always with a small, basal, black triangular spot Enallagma divagans Selys
b. Black humeral stripe < 2x width of the blue antehumeral stripe
16b a. Ab8 entirely metallic black dorsally 17
b. Black humeral strip 2x width of the blue antehumeral stripe
 
17a(16b) a. Superior and inferior arms of cerci long and widely divergent, the dorsal arm longer Enallagma antennatum (Say)
b. Face orange
c. Ab4 < 3x length of Ab2
17b a. Superior and inferior arms of cerci short and not widely divergent, the ventral arm longer Enallagma exsulans (Hagen)
b. Face blue
c. Ab4 nearly 4x length of Ab2
 
18a(14b) a. Black humeral strip much narrower than pale antehumeral stripe Enallagma vesperum Calvert
b. Middorsal stripe narrower than antehumeral
18b a. Black humeral stripe about as wide as pale antehumeral stripe Enallagma signatum (Hagen)
b. Middorsal stripe wider than antehumeral stripe
 
19a(1b)
females
a. Dark humeral stripe lacking or wanting,or partly black but interrupted or divided longitudinally by light brown marking, or entirely light brown 20
19b a. Dark humeral stripe entirely black and complete, no light brown marking present 25
 
20a(19a) a. Area between lateral ocelli mostly blue or blue-green, although a small black dot may be present Enallagma traviatum Selys
b. Middle lobe of pronotum without distinct dorsolateral pit
c. Dorsum of Ab9 mostly blue, although with small triangular basal dark markings
also: Ab8 with black extending at most 3/4x the distance from base to apex; mesostigmal plates with posterior margins strongly elevated in lateral half, so that the posterolateral corner is at least as high as the anterolateral corner; Abdomen no longer than 26 mm
20b a. Area between lateral ocelli entirely black or, if small pale spot present, middle prothoracic lobe with distinct dorsolateral pit on each side 21
b. Middle lobe of pronotum with distinct dorsolateral pit
c. Dorsum of Ab9 at least 1/3 black
 
21a(20b) a. Middle prothoracic lobe with a dorsolateral pit on each side Enallagma vesperum Calvert
b. Thoracic color usually predominately yellow-green, yellow or orange
21b a. Middle prothoracic lobe without dorsolateral pits 22
b. Thoracic color predominately grayish green, blue, or light brown
 
22a(21b) a. Forewing with 5-8 postnodal crossveins and M2 arising near fourth postnodal Enallagma basidens Calvert
b. Light brown line dividing black humeral stripe very distinct, full length, and less than 1/2 the width of the entire stripe
c. Pale spots anterior to lateral ocelli
22b a. Forewing usually with 9-12 postnodal crossveins and M2 arising near the fifth pronotal 23
b. Light brown line dividing black humeral stripe without sharply defined edges, sometimes incomplete, and varying in width from a hairline to nearly the full width of the stripe
c. Anterior anterior to lateral ocelli black
 
23a(22b) a. A broad, but distinct, swelling present on mesepisternum posterior to each mesostigmal plate Enallagma antennatum (Say), in part
b. Mesostigmal plates each with a small, yellow tubercle at the posteromedial corner
c. Black humeral stripe usually divided by brown spot only at the upper end
d. In dorsal view, Ab9 black but often with a median, pale spindle-shaped spot
23b a. No distinct swellings on mesepisternum posterior to mesostigmal plates 24
b. Mesostigmal plates usually without pale, posteromedial tubercles (E. divagans with distinct tubercles, usually black but rarely pale)
c. Black humeral stripe usually divided by brown stripe for most of its length
d. In dorsal view, Ab9 variable but never with a median, pale spindle-shaped spot
 
24a(23b) a. Mesostigmal plates each with distinct posteromedial tubercle Enallagma divagans Selys
b. Middorsal thoracic carina usually black
24b a. Mesostimgal plates without distinct posteromedial tubercles Enallagma exsulans (Hagen)
b. Middorsal thoracic carine pale
 
25a(19b) a. Middle lobe of prothorax with a pair of dorsal or dorsolateral pits, the edges of which are sharply defined 26
25b a. Middle lobe of prothorax without dorsoal or dorsolateral pits, although shallow depressions may be present 31
 
26a(25a) a. Mesostigmal plates more than 1/2x wide as long, the posterior margin strongly convex 27
26b a. Mesostimgal plates less than 1/2x wide as long, the posterior margin straight or nearly so 28
 
27a(26a) a. Posterolateral margins of mesostigmal plates in dorsolateral view raised above the general surface of the thoracic dorsum Enallagma hageni (Walsh)
27b a. Posterolateral margins of mesostimgal plates in dorsolateral view not raised above the general surface of the dorsum, although with a slight prominence in the anterolateral corner Enallagma ebrium (Hagen)
 
28a(26b) a. Dorsum of Ab1 pale on apical 1/3-1/2 29
b. Mesostigmal plates without a high, pale tubercle at the posteomedial corner, nor with high mesepisternal tubercles posterior to the mesostigmal plates
c. In dorsal view, Ab8 with large pale areas
28b a. Dorsum of Ab1 wholly black, except for a narrow, pale apical annulus 30
b. Mesostimgal plates each usually with a high, pale tubercle at the posteromedial corner, or, if the tubercles of mesostimgal plates are somewhat indistinct, then prominent mesepisternal tubercle present just posterior to each plate
c. In dorsal view, Ab8 nearly or entirely black
 
29a(28a) a. In dorsal view, Ab7 largely pale in basal 2/3 or more, except along midline Enallagma aspersum (Hagen)
b. Ab1 usually with a small, round, lateral, apical black spot
c. Ab8 with pair of large, blue, dorsolateral spots in the basal 1/2, usually separate or nearly so from the pale ventrolateral area
29b a. In dorsal view, Ab7 mostly black basally Enallagma geminatum Kellicott
b. Ab1 with a vertically elongate, lateral, apical black spot
c. Ab8 with basal blue spots bordered laterally by a black stripe, therefore not confluent with the ventrolateral pale area
 
30a(28b) a. Width of black humeral stripe much less than 1/2x the width of the pale antehumeral stripe Enallagma vesperum Calvert
30b a. Width of black humeral stripe > 1/2x the width of the pale antehumeral stripe Enallagma signatum (Hagen)
 
31a(30b) a. Hind lobe of pronotum with a pale, median tubercle bearing long setae Enallagma antennatum (Say), in part
b. A broad, but distinct, swelling present on mesepisternum posterior to each mesostigmal plate
c. Face usually orange (sometimes tan due to postmortem changes)
d. In dorsal view, Ab9 black but often with amedian, pale, spindle-shaped spot
31b a. Hind lobe of pronotum without a pale, median tubercle, or, if tubercle present, mesespisterna without distinct swellings 32
b. see a
c. Face green, grayish blue or tan
d. In dorsal view, Ab9 either mostly pale or black without a median pale spot
 
32a(31b) a. Hind lobe of pronotum with a pale, median tubercle Enallagma divagans Selys
b. Ab10 entirely pale or with a small, black dorsal spot at apex, Ab9 usually with a small, basal, bilobed black spot, only rarely extending full length of segment
32b a. Hind lobe of pronotum without a median tubercle 33
b. Ab10 usually with more extensive black markings, Ab9 usually black from base to apex
 
33a(32b) a. Posterior border or each mesostigmal plate well-defined by a narrow sulcus extending its entire length 34
33b a. Posterior border of each mesostigmal plate indistinct over part of its length, the sulcus lacking or incomplete 36
 
34a(33a) a. Mesostimgal plates with medial borders parallel or slightly divergent forward, with a shallow depression running diagonally from the anteromedidal to posterolateral portion of the plate Enallagma civile (Hagen)
also: Dorsum of Ab1 with black stripe almost reaching apex of segment and of uniform width, black stripe on dorsum of Ab8 usually not markedly constricted basally
34b a. Mesostigmal plates with medial borders usually at least slightly convergent forward, and each with a distinct, elongate depression confined to the anteromedial corner 35
 
35a(33b) a. Medial ridges of mesostigmal plates usually concave or sinuate and moderately convergent anteriorly, rarely very straight and strongly convergent Enallagma annexum (Hagen)
35b a. Medial ridges of mesostigmal plates very straight and strongly convergent Enallagma vernale Gloyd
 
36a(33b) a. Mesostigmal plates each with medial ridge raised to form an oval or elongate medial prominence, and with the posterior border indistinct (lacking sulcus) in medial 1/2 only Enallagma boreale Selys
b. Dorsum of Ab8 usually with blue area relatively more extensive than on Ab7, sometimes entirely blue
36b a. Mesostigmal plates each without medial prominence, and with the posterior border either indistinct in lateral 1/2 only or along entire length, or along middle section 37
b. Dorsum of Ab8 usually with blue area about the same extent as on Ab7, never entirely blue
 
37a(36b) a. Mesostigmal plates with medial ridges usually convergent forward, each with a broad depression running from anteromedial corner to the middle of the posterior border, and with posterior border usually indistinct only in the lateral 1/2, and straight Enallagma anna Williamson
37b a. Mesostigmal plates with medial ridges usually divergent forward, each with a ridge running from the anterolateral to the posteromedial corner, thus delimiting a small, transversely oval depression near the anteromedial corner, and with posterior border usually indistinct along most of its length, and more or less sinuate Enallagma carunculatum Morse
 
 

Mature Nymphs
(Note: Currently, unless nymphs are associated with adult stage and/or molecular information, E. annexum and E. vernal are not distinguishable between each other, and females of E. ebrium and E. hageni not distinguishable between each other)

1a a. Ant2 distinctly longer than Ant1, Ant3 2x or more longer than Ant1 2
b. Dorsal antenodal stout setae of medial gill, if present, forming a complete series, although sometimes proximally very minute
c. Gills with the pigment usually restricted to the tracheae, sometimes with 1-3 indistinct transverse tripes and/or an axial stripe
d. Eyes usually unpatterned
1b a. Ant2 not longer than Ant1, Ant3 < 2x long as Ant1 17
b. Dorsal antenodal stout setae of medial gill partly or wholly reduced to slender hairs, or confined to the vicinity of the nodus
c. Gills generally more extensively pigmented
d. Eyes usually pattered with dark stripes or spots
 
2a(1a) a. Gills with patches of closely branched and deeply pigmented tracheae, the intervening areas with mostly unpigmented and much less branched tracheae 3
2b a. Gills with tracheae uniformly and usually much less closely branched, pigmentation of the tracheae general although often interrupted by clear areas 4
 
3a(2a) a. Cerci of male in dorsal view bluntly angular, the outer edge nearly straight and with an oblique ridge, in lateral view bluntly rounded Enallagma ebrium (Hagen)
3b a. Cerci of male in dorsal view with an evenly curved margin and without a ridge, in lateral view bluntly angular Enallagma hageni (Walsh)
Note: females of E. ebrium and E. hageni nymphs apparently undistinguishable, unless associated with adults/molecular information
4a(2b) a. Dorsal antenodal setae of median gill < 20 Enallagma geminatum Kellicott
b. Cerci of male, in dorsolateral view, with a deep dorsal concavity, sometimes even with dorsal margin appearing concave, in strict lateral view, and subacute, in dorsal view
c. Median gill usually < 4.5mm
4b a. Dorsal antenodal setae >20, usually at least 25 5
b. Cerci of male with concavity, if any, medial or slightly dorsomedial, not usually visible in dorsolateral view and, if visible, cerci in dorsal view appear more or less globular and wider than long
c. Medial gill usually > 4.5 mm
 
5a(4b) a. Dorsal and ventral series of stout antenodal setae of median gill nearly equal in length, and consisting of rather closely set setae 6
5b a. Dorsal antenodal series of setae of the median gill nearly 2x long as ventral series, with the ventral series consisting of comparatively small and widely spaced setae 10
 
6a(5a) a. Main tracheal branches forming angle of about 45° with central axis 7
b. Ant3 usually 2.5-3.0x longer than Ant1
6b a. Main tracheal branches forming angle distinctly less than 45° with central axis 8
b. Ant3 shorter, usually about 2x as long as Ant1
 
7a(6a) a. Cerci in caudal view more rounded distally, in the male shaped like clenched boxing gloves seen end-on, the distal projection more than half as wide (from dorsomedial to ventral face) as long (from dorsolateral to ventromedial face), in the female with distal projection usually higher than wide, set off from basal portion by a dorsal fold Enallagma boreale Selys
7b a. Cerci in caudal view less rounded distally, in the male with the distal projection forming a distinct, transverse ridge that is less than 1/2 as wide as long, in female with distal projection usually not higher than wide, set off from basal portion by dorsal and ventral folds Enallagma annexum (Hagen) and Enallagma vernale Gloyd
 
8a(6b) a. Ant2 distinctly shorter than Ant3, and distinctly less than 2x as long as Ant1 Enallagma annexum (Hagen) and Enallagma vernale Gloyd
b. Antennae usually with 6 segments
c. Median gill <5x long as wide
d. Cerci of male, in lateral view, more or less rounded distally
8b a. Either Ant2 nearly equal in length to Ant3, and nearly2x as long as Ant1, or median gill >5x long as wide, or both 9, link to Coenagrion, or use next couplet
b. Antennae with 6-7 segments
c. see a
d. Cerci of male, in lateral view, more or less angulate distally
 
9a(8b) a. Median gill no more than 5x as long as wide, and usually slightly acuminate apically Coenagrion resolutum (Hagen)
b. Cerci of male without a dorsomedial concavity
c. Ovipositor rudiments of female extending >0.6 mm beyond the posterior row of denticles on Ab9 sternum
d. Metafemur usually >2.9 mm
9b a. Median gill at least 5x as long as wide, and usually not acuminate apically Coenagrion interrogatum (Hagen)
b. Cerci of male with a distinct dorsomedial concavity
c. Ovipositor rudiments of female extending <0.6 mm beyond the posterior row of denticles on Ab9 sternum
d. Metafemur usually <2.9 mm
 
10a(5b) a. Ovipositor reduments absent Males, 11
10b a. Ovipositor rediments present Females, 14
 
11a(10a) a. Ventral surface of cerci strongly concave in ventrolateral view 12
11b a. Ventral surface of cerci broadly convex in ventrolateral view 13
 
12a(11a) a. Cerci very large and robust, > 0.38 mm long in dorsal view Enallagma anna Williamson
b. Dorsal antenodal series of setae of median gill usually extending at least 2/3 the length of the gill
12b a. Cerci more slender, <0.35 mm long in dorsal view Enallagma aspersum (Hagen)
b. Dorsal antenodal series of setae of median gills and ventral antenodal series of lateral gills usually extending < 1/2 length of the gill
 
13a(11b) a. Cerci, in posterior view, with maximum distance from dorsomedial to ventrolateral margins greater than maximum distance from dorsolateral to ventromedial margins Enallagma carunculatum Morse
b. Cerci usually < 0.3 mm long in dorsal view
13b a. Cerci, in posterior view, with maximum distance from dorsomedial to ventrolateral margins less than maximum dorsolateral to ventromedial distance Enallagma civile (Hagen)
b. Cerci usually > 0.3 mm long in dorsal view
 
14a(10b) a. Length of cerci, in dorsal view, 0.29 mm or more 15
14b a. Length of cerci, in dorsal view, 0.28 mm or less 16
 
15a(14a) a. Cerci, in ventrolateral view, with medial margin distinctly concave Enallagma civile (Hagen)
15b a. Cerci, in ventrolateral view, with medial margin straight or barely concave Enallagma anna Williamson
 
16a(14b) a. Cerci, in ventrolateral view, with medial margin slightly concave Enallagma aspersum (Hagen)
b. Axial tracheae of gills often appearing distinctly coiled around each other basally
16b a. Cerci, in ventrolateral view, with medial margin usually straight or convex, rarely very slightly concave Enallagma carunculatum Morse
b. Axial tracheae of gills usually not appearing distinctly coiled around each other basally
 
17a(1b) a. Posterolateral margins of head almost always producted into prominent lobes, the distance from the base of the antennae to the hindmost part of the margin nearly 1/2 the width of the head across the eyes 18
b. Lateral carinae of abdomen prominent, but not projecting abruptly from the posterior end of of each segment, and generally bearing many slender hairs but few or no stout setae (except E. basidens)
c. Gills usually with patches of closely branched and deeply pigmented tracheae
17b a. Posterolateral margins of head rounded but not especially prominent, the distance from the base of the antennae to the hindmost part of the margin distinctly less than 1/2 the width of the head across the eyes 22
b. lateral carinae of abdomen projectly abruptly from the poasteior end of each segment but not prominent in front, with a group of stout setae on the prominence but otherwise nearly bare
c. Gills without patches of closely branched and deeply pigmented tracheae
 
18a(17a) a. Median gill with 2-4 apical, dark, transverse bands often confluent along axis, lateral gills usually similar to median gill, although sometimes dark pigment nearly fills preapical area Enallagma basidens Calvert
b. Lateral carinae of abdomen projecting rather adruptly from the posterior part of the end of each segment, with a scattered group of stout setae on the prominence
c. Body length exclusive of gills and antennae usually 11 mm or less
18b a. Median gill without such dark apical bands, although an irregular and diffuse dark area may be present 19
b. Lateral carinae of abdomen prominent but not projecting abruptly from end of each segment, and generally bearing many slender hairs but few stout setae
c. Body length exclusive of gills and antennae usually >11 mm
 
19a(18b) a. Gills without a distinct nodus or abrupt color change beyond midlength, but with 4-5 widely spaced dark bands formed by patches of closely branched and darkly pigmented tracheae Enallagma divagans Selys
b. Gills narrow, at least 6x long as wide
19b a. Gills with a distinct nodus, or at least an abrupt color change about 1/3-1/2 of the distance from the apex 20
b. Gills wider than above, usually 4-6x long as wide
 
20a(19b) a. Median gill with stiff dorsal setae absent or extremely tiny and almost invisible Enallagma traviatum Selys
b. Gills usually uniform dark brown from base to nodus
20b a. Median gill usually with distinct striff dorsal setae proximal to nodus (except E. antennatum) 21
b. Gills proximal to nodus usually less deeply pigmented or with pigment restricted to basal and axial areas
 
21a(20b) a. Median gill without stiff, drosal antenodal setae, and without a definite nodus Enallagma antennatum (Say)
b. Lateral gills narrow, 6-8x long as wide
21b a. Median gill with 8-12 stiff, dorsal antenodal setae, and with a distinct nodus located about 1/2 the distance from base to apex Enallagma exsulans (Hagen)
b. Lateral gills wider than above, 4-5x long as wide
 
22a(17b) a. Dorsal margin of median gill without prominent stout setae Enallagma vesperum Calvert
b. Median gill at most 3x as long as wide at its greatest width, the distal 2/3 greatly expanded (in lateral view) with the dorsal margin strongly convex
c. Tracheae of gills widely spaced, usually directly opposite and perpendicular to the axis
22b a. Dorsal margin of median gill with at least a few prominent setae just proximal to the nodus Enallagma signatum (Hagen)
b. Median gill at least 3x as long as wide, usually longer, and widening graduallly from the base with the dorsal margin not so strongly convex
c. Tracheae of gills much more numerous and closely spaced, making an acute angle with the axis
 
 

References

Baker RL, Clifford HF. 1982. Life cycle of an Enallagma boreale Selys population from the boreal forest of Alberta, Canada (Zygoptera: Coenagrionidae). Odonatologica 11(4):317-322.
Cannings RA. 1989. Enallagma basidens Calvert, a dragonfly new to Canada, with notes on the expansion of its range in North America (Zygoptera: Coenagrionidae). Notulae Odonatologicae 3(4):53-55.
Ingham BR, Jenner CE. 1976. Life histories of Enallagma hageni (Walsh) and E. aspersum (Hagen) (Zygoptera: Coenagrionidae). Odonatologica 5:331-345.
Johannsson OE. 1978. Co-existence of larval Zygoptera (Odonata) common to the Norfolk Broads (U.K.). Oecologia 32:303-321.
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Page created: July 17, 1998 - Last updated:  May 22, 2020 (EB)