Monday, May 9, 2011

Identification: Dragonflies versus Damselflies

I’ll be posting periodically on odonate identification and right at the top—dragonflies versus damselflies, is a natural place to begin. As usual, I’ll be using the term “dragonfly” (and the plural “dragonflies”) in the strict sense meaning only odonates in the suborder Anisoptera, excluding damselflies which are in the suborder Zygoptera. For a refresher on these terms, see my post Is that a Dragonfly or an Odonate?

Generally speaking dragonflies and damselflies really aren’t all that similar, but it’s useful to have a good fundamental understanding of the features that differentiate them. At the very least you can impress your friends, right? So below are the features that will tell you whether you’re looking at a dragonfly or a damselfly in roughly the order of the bigger, more obvious to the itty parts that require closer scrutiny. I mostly cover adults since that’s what most people see, but I’ll talk about nymphs a bit at the end.


I primarily have the odonate fauna of the US and Canada in mind as I write this post, but these differences work for much of the rest of the world. There are some groups in other regions which don’t fit one or more of these criteria, but I’m going to keep this simple and not worry about them at this time.

Overall Build: Dragonflies are more robust and chunkier than damselflies. This is especially noticeable in the abdomen which is very thin on damsels relative to their overall length. Also notice the ratio of the abdomen length to thorax length which is greater on damsels. There’s a reason why John Acorn’s book Damselflies of Alberta was subtitled Flying Neon Toothpicks in the Grass—damselflies are thin.
On the left a dragonfly—Twelve-spotted Skimmer (Libellula pulchella), and on the right a damselfly—Boreal Bluet (Enallagma boreale). Not to scale. Compare their build and wing positions.

Wing Position: Dragonflies perch with their wings open, either straight out to the sides or drooping to some degree. A common exception to this is right after a dragonfly has emerged—check out the final images of my post A Dragonfly Emergence Sequence with a series of shots of an emerging dragonfly. Damselflies most often perch with their wings closed above the abdomen, or partially open. Even when damsels perch with the wings open, as some do on a regular basis, the wings are not open to the same degree that dragonflies open their wings.

Wing Shape: The fore- and hindwings of damselflies are essentially identical in shape, while on dragonflies the hindwing is noticeably more broad, particularly near the base.
On the left a pair of dragonfly wings, and on the right a pair of damselfly wings. Forewings above; hindwings below. Not to scale. Note that the dragonfly's hind wing is clearly more broad than its forewing; the damselfly's wings are virtually identical in shape. 

Head/Eye Shape: Damselflies are hammer-headed—the eyes cap the ends of the relatively wide, almost dumbell-shaped head. One of my first posts, Damselflies have a good head on their shoulders, was about how the shape of the damselfly head can be beneficial. Dragonfly eyes are more variable—some have a gap between the eyes, on some the eyes meet at a point on top of the head, and on some the eyes meet at a seam over the top of the head. Even when there is a space between the eyes on top of the head of a dragonfly, that gap is no wider than the diameter of either eye while the gap is more than the diameter of either eye on damsels.

On the left two dragonfly heads, and on the right a damselfly head with the eyes colored green. Not to scale. The views are from the top. In particular compare the gap between the eyes on the lower dragonfly and the damselfly.

Clasping Point: By this I mean where the male holds on to the female while in tandem and copulating. Female dragonflies are clasped at the back of the head between the eyes; female damselflies are clasped near the front of the thorax
On the left a female dragonfly (Red-veined Meadowhawk, Sympetrum madidum) is clasped by a male at the back of the head. On the right a female damselfly (Spotted Spreadwing, Lestes congener) is clasped near the front of the thorax. Not to scale.

Male Abdominal Appendages: Copulation between male and female odonates can only occur when males clasp females with their abdominal appendages. The difference between male dragonflies and damselflies is in which structures are developed into clasping equipment. In both cases, a pair of structures called cerci (singular = cercus) are used. On dragonflies, the epiproct is also developed for this purpose, while on damselflies it is a pair of paraprocts. The result is that dragonflies have a total of three abdominal appendages—two upper and a single lower (two cerci + one epiproct); damselflies have a total of four abdominal appendages—two upper and two lower (two cerci + two paraprocts).

The nymph of a dragonfly, Eight-spotted Skimmer (Libellula 
forensis). Note the short, spiky appendages at the end of the
abdomen (on the left).
Nymphs: The nymphs (or larvae if you prefer) of dragonflies and damselflies are pretty different. Very much like the adults, dragonfly nymphs are more robust, chunky beasts while damselfly nymphs are more slender and delicate looking. Besides overall shape, the primary structural difference to look for is at the rear end—dragonfly nymphs have simple, short, spiky appendages (technically, the cerci, epiproct, and paraprocts) while damselfly nymphs possess three relatively long, leaf-like external gills (technically, the middle gill is a strongly modified epiproct and the lateral gills are strongly modified paraprocts; they have cerci too, but these are tiny compared to the gills and require close scrutiny to see).

Dragonfly nymph gills are located within the abdomen and respiration is accomplished by pumping water in and out rectally (leading to jokes about bad breath). This also provides a mode of predator evasion in the form of “jet propulsion” when they forcefully expel water from the abdomen. Since the gills of damselflies are largely located externally on the leaf-like appendages, they don’t have jet propulsion. But they do use their gills like flippers to propel them fish-like through water.

Damselfly nymphs actually look more like some other aquatic insects than they do dragonfly nymphs. The Dragonfly Woman discussed how to differentiate damselfly nymphs from mayfly and stonefly nymphs. Above is an image of a dragonfly nymph; I don’t have any decent shots of damselfly nymphs yet, but check out The Dragonfly Woman’s link above for one.

2 comments:

  1. I love this post. You have made everything so clear! We just moved to Vancouver Island from the arctic, and one of the most pleasant pastimes is to sit at the edge of our meadow and observe the incredible swarms of dragonflies/damselflies. I can't wait for tomorrow to Bevin to identify them with the new information you've given me. Than you!
    And best wishes,
    MaryAnn Hardy

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  2. Thank you so much! I have to study entomology for a school club, and this makes it so much easier. :) Great information!

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