The Little and Large Show

Dragonfly or Damselfly?Side by Side The Little and Large Show The Shell Game Going to Extremes Side by Side answer Going Zen The Rule Acknowledgement of sources

Author Jeff Melvaine ◦

Quiz 2: The Little and Large Show

One of these species is a dragonfly and one is a damselfly; which is which?

Red-tipped Shadefly (Argiocnemis rubescens) pair, Bungawalbin Valley NSW.

Australian Emperor (Anax papuensis) pair, Wynnum North QLD.

What strikes me at first glance in this comparison is that the Emperor is much more strongly built. It’s also much bigger in all dimensions, although the camera can lie about this unless both the subjects to be compared are in the same image. Another obvious difference is that the Emperor holds its wings in the spread position when not actually flying; the Shadefly closes them over or near its abdomen when fully at rest. And the eyes of the Emperor are very large, forward facing, and meet at the top of the head, whereas the smaller eyes of the Shadefly are widely separated and directed outwards at the sides of the head, like the eyes of a Hammerhead Shark. The shark’s eyes, less useful in the gloom of deep water where a good sense of smell is more relevant, are proportionately much smaller than those of the Shadefly, but positioning the eyes at extreme positions on the side of the head allows a wider angle of view in both cases.

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Quiz 2 SPOILER ALERT

The mature male Emperor, when not in contact with a female, spends almost all his waking time airborne, gliding when simply observing, and accelerating to catch prey or chase away any challengers to his territorial dominance; the Shadefly male occupies a territory by perching most of the time. And in this case the conventional wisdom works; the Australian Emperor is the dragonfly in this comparison. But some Odonate species have found different solutions to their lifestyle issues.

Less immediately obvious in these pictures is that the forewings are much less similar to the hindwings in the Emperor. Here is a collation from the original wing diagrams for both species posted by John Tann on Wikipedia, converted to a common scale to give an indication of the relative size of each. There is a diagnostic feature here for separating dragonflies and damselflies that I’ll come to shortly; it is fundamental, but you need a good clear view of the wings to apply it in practice.

Particularly noticeable is the much greater width of the hindwings, particularly at the base, in the Emperor. This correlates closely with holding the wings in the spread position when perched; rotating the wings between the horizontal and vertical positions, particularly in a hurried takeoff, might cause a broad wing base to scrape on the ground, and result in damage, which would waste energy in flight for the rest of the insect’s life. The broader hindwing may have many advantages for an energetic flier, such as reducing turbulence by presenting a more nearly continuous surface to the airflow, particularly when gliding.

By contrast, the Shadefly wing is notable for the narrow stalk at the very base. Only one family of damselflies (the Demoiselles, Calopterygidae) lacks this feature, but the one Australian sighting from this family, of the Papuan Demoiselle a.k.a. Paradise Metalwing (Neurobasis australis), is unconfirmed. Here is a Papuan sighting of that species:

• https://inaturalist.ala.org.au/observations/72638345

Another point of detail relates to the position of the nodus. This is the small kink in the leading edge of the wing, which provides flexibility and reinforcement where the wing needs it most. For dragonflies, this is typically about half way along and reinforced, while for damselflies it is rather closer to the base of the wing, at about a third of the wing length in general. The dragonfly version correlates well with higher performance in flight. But, as we will see, a few species seem to have inverted some of these aspects of the dragonfly / damselfly comparison in their lifestyle adaptations.

The important structural feature mentioned above is the discoidal cell, close to the base of each wing. I have borrowed (and colourised) some of the Wikipedia wing diagrams posted by John Tann to illustrate what this looks like, in the style of the first stage of the key in the CSIRO published guide to the identification of Australian Odonata (Theischinger and Endersby 2009), which splits the dragonflies from the damselflies as follows:

Theischinger and Endersby characterise this as “Discoidal cell a simple quadrilateral, sometimes traversed by crossveins, occasionally open at base”.

Theischinger and Endersby characterise this as “Discoidal cell divided into hypertriangle and triangle, often differing in shape in fore and hind wing, and often traversed by crossveins”.

Note that the concept of a triangle is a bit approximate in some cases (typically in smaller dragonflies with a less dense vein pattern where you could imagine a triangle with at least some of the corners chopped off and/or the odd wobbly line.) And if you ever manage to find a Neurobasis australis on Australian territory, you will see that its discoidal cell is a normal quadrilateral, but buried in a snowstorm of crossveins:

• Neurobasis_australis_female_wings (file 34787356776.jpg) – Wikipedia
• Neurobasis_australis_male_wings (file 34664190892.jpg) – Wikipedia

I chose the photos of pairs in particular to illustrate this section because they show something important: the way that male Odonates grip their partners when pairing. You can see here that the male Emperor attaches his tail claspers to the top of the female’s head, whereas the male Shadefly attaches them to the female’s prothorax (that’s the first thoracic segment, with the small front pair of legs, and no flight muscles, which is why it is so much less massive than the second and third thoracic segments).

The point of attachment of the male claspers to the female, and the actual number of claspers, is a reliable indicator to separate dragonflies from damselflies in Australia. Male damselflies have two superior appendages (cerci) and two inferior appendages (paraprocts), male dragonflies have two superior appendages (cerci) and one inferior appendage (epiproct). [In some non-Australian clubtail species (family Gomphidae), the situation is, or looks, somewhat different: the epiproct is degenerate and the cerci clasp the female prothorax, or the grip is on the head, but the epiproct is forked and superficially similar to two separate appendages.

To expand a little on those grips: The orthodox dragonfly grip places the epiproct against the top of the female head (between the eyes), and the two cerci are long enough and slender enough to fit behind the head and sometimes around the neck; in so doing they may lock into place against the front of the prothorax. The orthodox damselfly grip places the paraprocts in front of the cerci on top of the prothorax, and in some cases the cerci may lock into place against the front of the synthorax. Working out the exact configuration may involve taking electron micrographs of all participating surfaces and feeding that data into a computer simulation, so do not feel too humbled if you don’t quite get it just looking at enthusiast photos (including mine). These appendages can vary greatly in size and visibility, from one species to another, and between the upper and lower appendages of the same species. Because these variations help to ensure that Odonates choose partners of the correct species (and thus avoid waste of energy producing unviable eggs), they are usually very helpful indicators of species, but observing them from photographs or even captive individuals can be difficult; a preserved specimen usually gives a much better view than a photograph or even a nervous captive. (In defence of photographic records, the colour in a specimen may deteriorate badly, whereas photographs, particularly the digital kind, are rather more stable.)

So what benefit is there for male dragonflies in clasping the top of the female’s head instead of the top of the prothorax? A major factor in this example is the size and shape of the dragonfly head; for the Emperor to reach down over the female’s head to the top of the prothorax would be a much longer stretch than for the Shadefly. Also, the Emperor’s thorax is more upright than the Shadefly’s, so there isn’t as much room for the male’s abdomen to fit into the gap above the tiny prothorax, although the cerci certainly can. This thoracic profile may assist with balance; the Emperor’s head and thorax would be a good deal heavier proportionately, so the ideal balance point for the base of the wings would be further forward. The Emperor’s head (rounded) is more streamlined that that of the Shadefly, but the Shadefly has the exposed front of its synthorax (the combination of the larger second and third thoracic segments) raked backwards more towards the horizontal, a more streamlined profile. In alternative structures that each work well in their own way, there are generally advantages that compensate for the disadvantages.

Another consideration of that kind could be a trade off between having huge eyes that give better vision under all light conditions, and having a sleeker profile that can hide more easily in vegetation. But with so many different families of Odonates, there is no guarantee that they will all arrive at the same kind of balance between all these factors.

Meanwhile, you have probably noticed that the size of the dragonfly eye relative to that of the damselfly eye is a decisive difference between the examples above, both in absolute size, and in the proportion of the head area that it covers. But is the difference always so clear cut? The next group of examples will go some way to answering that.

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Quiz 2 TLDR Summary

The Australian Emperor is a typical dragonfly; big, muscular, a frequent and energetic flyer, perching with the wings open, and with eyes at the front of the head and meeting along the midline.

The Red-tipped Shadefly is a typical damselfly; small, slender, spending a lot of time perched with the wings closed, and with well separated eyes at the side of the head .
(That was an easy quiz. They get harder.)

Male dragonflies pair by clasping the top of the female’s head; they have two upper claspers and one lower clasper at the tip of the tail. (Some non-Australian clubtails do things a little differently.)

Male damselflies pair by clasping the tiny first segment of the female’s thorax; they have two upper and two lower claspers at the tip of the tail.

The diagnostic feature to tell a dragonfly from a damselfly in Australia is a feature of the wing vein pattern called the discoidal cell, best explained by the diagram in the main text above. In dragonflies, it’s a triangle (or in a few of the smaller dragonflies with simpler venation, a triangle with at least one corner snipped off), underneath two very prominent curved veins that are often joined at the base; in damselflies, it’s a long skinny quadrilateral with the long sides parallel (occasionally open at the base).

Dragonfly or Damselfly?Side by Side The Little and Large Show The Shell Game Going to Extremes Side by Side answer Going Zen The Rule Acknowledgement of sources

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