Odonata - Dragon- and Damselflies

The insect order Odonata is an old insect order having evolved in the Pennsylvannian and dominated in the Permian. Today, there are more than 5,000 species grouped into two extant suborders: Zygoptera (damselflies) and Anisoptera (dragonflies). A third suborder, Anisozygoptera, is now considered extinct and the two Asian species placed in that group are now considered as primitive Anisoptera (Tennessen 2003).

Distinguishing between the two suborders of Michigan is rather easy. Both nymphs and adults of all Odonata are carnivores. Nymphs of all our species are aquatic (some semi-terrestrial species exist in other parts of the world), respiring by means of gills (internal with Anisopera, external with Zygoptera, the later that may also respire cutaneously). The modified labium, used by all larval odonates to capture food, characterizes the larval stage of this order from from all other insects. It is held underneath the head, and, when prey is detected, thrusted out in a lightning-like strike. Prey is then seized with palpal lobes at the end of the labium, brought to the mouth and finally macerated with the larva's mandibles. Adults are large, agile fliers. The wings of anisopterans, which are dissimilarly shaped, are held horizontally (sometimes flexed below the body line) when at rest, whereas those of zygopterans, which are similarly shaped, are held together above the body (sometimes partly spread apart in Lestes and Chromagrion) (Westfall and Tennessen 1996). The eyes of zygopterans are generally situated further apart than with most anisopterans. Wing venation is commonly used to distinguish higher level taxa.

Adults (adapted from Needham et al. 2000 and Westfall and May 1996)

1a	a. Front and hind wings similar in shape and size, each with a quadrangle instead of a triangle and subtriangle	Zygoptera, 2
	b. Eyes separated by more than their own width
	c. Males with 2 inferior appendages (paracprocts)
	d. Females with a fully developed ovipositor bearing styli
	e. When perched, wings held together above abdomen or only partly spread
1b	a. Front and hind wings dissimilar in shape and size, the hind wing considerably wider at base than the front wing, each having a triangle and subtriangle	Anisoptera, 4
	b. Eyes meeting middorsally or not separated by a space grater than their own width
	c. Males with a single inferior appendage (epiproct), which my be bifid
	d. Females with or without an ovipositor
	e. When perched, wings usually held horizontally
2a	a. Antenodal cross veins numerous	Calopterygidae
	b. Postnodal crossveins not in line with the veins below them
	c. Anal vein at its base separate from posterior border of wing
	d. Quadrangle with several crossveins
2b	a. Only 2 antenodal crossveins present	3
	b. Postnodal crossveins in line with the veins below them
	c. Anal vein joined with wing margin for a distance from the wing base
	d. Quadrangle never with crossveins
3a(2b)	Veins M3 and Rs arising nearer the arculus than the nodus	Lestidae
3b	Veins M3 and Rs arising nearer the nodus than the arculus	Coenagrionidae
	also: Anal vein and Cu2 present, quadrangle distinctly trapezoidal
4a(1b)	a. Triangles of forewings < 2x as far from arculus as those of hind wings	5
	b. Triangles of both wings similar in shape, generally elongated in long axis of wings
	c. The 2 antenodal crossveins thickened, most of the other antenodals of the 1st series not aligned with those of the 2nd series
4b	a. Triangles of forewings at least > 2x as far from arculus as those of hind wings	8
	b. Triangles of forewings generally elongated transversely, those of hind wings elongated longitudinally
	c. No thickened antenodal crossveins, those of the 1st series aligned with those of the 2nd series
5a(4a)	a. Brace crossvein absent	Cordulegastridae
	b. Eyes barely contiguous dorsally or very slightly separated, by distance much less than distance between lateral ocelli
	c. In ventral view, prementum with large anteromedian cleft
	d. Ovipositor of female with gonapophyses extending well beyond posterior margin of Ab10 and without styli
5b	a. Brace crossvein present near proximal end of pterostigma	6
	b. Eyes either in contact for some distance dorsally (Aeshnidae), or widely separated (Gomphidae, Petaluridae), distance about equal to distnace between lateral ocelli
	c. prementum variable
	d. ovipositor variable
6a(5b)	a. Eyes in contact for some distance dorsally, forming a distinct eye seam	Aeshnidae
	b. In ventral view, anterior margin of prementum with median cleft small, sometimes nearly obsolete
	c. Male epiproct usually triangular (except Gomphaeschna and Anax)
	d. Female ovipositor with well-developed, bladelike gonapophyses, the outer pair each bearing a stylus
6b	a. Eyes widely separated, distance about equal to distance between lateral ocelli	7
	b. Male epiproct not triangular
	c. Prementum and ovipositor variable
7a(6b)	a. In ventral view, prementum with large anteromedian cleft	Petaluridae, Tachopteryx thoreyi (Hagen) in Selys
	b. Pterostigmata each at least 0.25x the distance from the nodus to the end of vein R1, narrow, slightly concace along posterior side
	c. Ovipositor with well-developed, blade-like gonapophyses, the outer pair each bearing a stylus
	Note: very rare in Michigan, currently known from around areas of groundwater seeps at only two localities in SW Michigan
7b	a. In ventral view, prementum entire	Gomphidae
	b. Pterostigmata each much less than 0.25x the distance from the nodus to the end of vein R1, slightly widened at midlength, and thus not concave posteriorly
	c. Ovipositor reduced to small subgenital plate
8a(4b)	a. Triangle of each hindwing about 0.5x as far from arculus as that of each forewing	Macromiidae
	b. Anal loop short and wide, without midrib
	c. Abdomen without lateral carinae
8b	a. Triangle of each hindwing much less than 0.5x as far from arculus as that of each forewing	9
	b. Anal loop elongate, divided lengthwise by distinct midrib into 2 rows of cells (absent in Nannothemis)
	c. Lateral carinae on Ab3, 4, or 5 to Ab8 or 9
9a(8b)	a. Anal loop slightly expanded distally but without distinct "toe"	Corduliidae
	b. In lateral view, posterolateral margin of each eye distinctly sinuate
	c. Anal border of male hind wing with an anal triangle and the anal margin angulate
	d. Males: anal border of each hindwing of angulate, auricles present on Ab2, and keels present on metatibiae
9b	a. Anal loop usually markedly expanded distally, with distinct "toe" (absent in Nannothemis)	Libellulidae
	b. In lateral view, posteriorlateral margin of each eye hardly sinuate
	c. Anal border of each hindwing of both sexes rounded, and males without an anal triangle
	d. Males: without auricles on Ab2, and without metatibial keels

Nymphs (adapted from Westfall and Needham 1996 and Walker and Corbet 1975)

1a	a. Body slender, head wider than thorax and abdomen	Zygoptera, 2
	b. Abdomen terminating in three caudal lamellae
1b	a. Body stout, head usually narrower than thorax and abdomen	Anisoptera, 4
	b. Abdomen terminating in 5 short, stiff, pointed appendages
2a	a. First antennal segment elongate, greater than segments 2-5 combined	Calopterygidae
	b. Prementum with deep median cleft
2b	a. First antennal segment not so elongate, less than segments 2-5 combined	3
	b. Prementum with at most a tiny median cleft
3a	Basal half of labium greatly narrowed and elongate, folded labium extends back to mesocoxae or beyond	Lestidae
3b	Basal half of labium not greatly narrowed, folded labium extends back only to procoxae	Coenagrionidae
4a	Mentum flat or nearly so, without dorsal premental setae	5
4b	Prementum and palpal lobes forming spoon-shaped structure	7
5a	a. Antennae 4-segmented, third segment often enlarged	Gomphidae
	b. Pro- and metatarsi 2-segmented
	c. Ligula without a median cleft
5b	a. Antennae 6- and 7-segmented	6
	b. Pro- and metatarsi tarsi 3-segmented
	c. Ligula with a median cleft
6a	a. Antennae segments short, thick, and hairy	Petaluridae, Tachopteryx thoreyi (Hagen) in Selys
	b. Prementum with sides sub-parallel in distal three-fifths, abruptly narrowed near base
	c. A pair of lateral-dorsal abdominal hair tuft present
	Note: rare, in groundwater seeps, known from only 2 localities in SW LP Michigan
6b	a. Antennal segments slender and bristle-like	Aeshnidae
	b. Prementum widest in distal half, then much narrower in basal half or more
	c. Abdomen without lateral-dorsal abdominal hair tufts
7a	a. Distal edge of lateral lobe with large, irregular teeth without associated setae	Cordulegastridae
	b. Ligula with a median tooth-like cleft
7b	a. Distal edge of lateral lobe entire, or with even-sized dentations, with associated setae	8
	b. Ligula not as above
8a	a. Head with thick, erect frontal horn postitioned between antennae, and metafemur very long, reaching at least to apex of Ab8	Macromiidae
	b. Metasternum with broad, median tubercle
8b	a. Head without frontal horn positioned between antennae, and metafemur not reaching apex of Ab8	9
	b. Metasternum without median tubercle
Note: No one character can reliably separate the families Corduliidae and Libellulidae, hence one may have to run through the keys of both families to reliably identify a specimen to genus, particularly if the key is used for earlier instars. Particular difficulties may arise when keying out nymphs of Perithemis tenera, which is a libelluid whose nymphs look very much like a corduliid.]
9a	a. Distal edge of lateral lobe of labium generally with crenations less than one-fourth as wide, or obsolete, or cerci usually less than one-half as long as epiproct	Libellulidae
	b. Abdomen distally more tapered
9b	a. Distal edge of lateral lobe of labium with prominent crenations, usually at least as one-fourth deep as wide, and cerci usually at least one-half as long as epiproct	10
	b. Abdomen generally ends abruptly
10a	Lateral spine of Ab8, when present, shorter than middorsal length of Ab9	Corduliidae
10b	Lateral spine of Ab8 as long as middorsal length of Ab9, or longer	Libellulidae, genus Pantala

References

Needham JG, Westfall MJ, May ML. 2000. Dragonflies of North America, Revised Edition. Scientific Publishers: Gainesville, Florida. xv + 940 pp.
Tennessen KJ. 2003. Odonata (Dragonflies, Damselflies), pp. 814-823 in Resh VH, Cardé RT, Encyclopedia of Insects. Academic Press, San Diego, California, USA.
Walker EM, Corbet PS. 1975. The Odonata of Canada and Alaska, Vol. 3. University of Toronto Press: Toronto. xvi + 308 pp.
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.

Last updated: August 6, 2008 (EB)