Libellulidae: On the Wing

Dragonflies of the order Odonata are unquestionably one of the more familiar groups of insects to the general public. They are large, visible and eye-catching, and may be quite colourful. Some have even taken to 'twitching' dragonflies in the same manner as bird species, identifying species observed on the wing and keeping a tally of how many they have seen.

And at the top of many people's list: the wandering glider Pantala flavescens, copyright Jeevan Jose, the world's most widespread dragonfly species.

Ecologically, in contrast, dragonflies may be called a relatively conservative group. All begin their lives as aquatic predators before emerging with adulthood as fast-moving aerial predators. All are generalists, feeding on whatever other insects may be unfortunate enough to fall into their grasp. All dragonflies conform to a fairly similar overall bauplan when compared to the diversity of forms that may be found in many other insect orders (for instance, there are no flightless dragonflies). Classification of dragonflies has often focused heavily on features of the wing venation, tracing its lines in their criss-crossing network.

Hind wing of a libellulid with the anal loop highlighted, from here.

The largest of the generally recognised families of dragonflies is the Libellulidae, containing over 1000 of the approximately 6000 known species of Odonata (Pilgrim & von Dohlen 2008). Characteristic features of the Libellulidae include the presence of the 'anal loop', an arrangement of veins in the hind wing forming what has been described as a boot shape. In the case of the genus Libellula, at least, the shape of the anal loop rather reminds me of one of the legs on the Manx flag. Members of the Libellulidae are commonly known as perchers or skimmers in reference to their hunting behaviours; others have similarly composed names such as darters or pondhawks. A number of members of the family have strikingly banded or coloured wings, leading to vernacular labels such as amberwings or pennants. Members of the genus Tramea are commonly known as saddlebags in reference to the dark patches at the base of their hind wings.

Common picturewing Rhyothemis variegata, copyright Tarique Sani.

Members of the Libellulidae have been divided between about a dozen subfamilies, again primarily defined on the basis of wing venation. However, distinctions between the subfamilies have always been vague with many subfamilies recognised by particular combinations of characters rather than characters unique to each subfamily alone. This vagueness has been underlined by recent molecular studies which have identified most subfamilies as polyphyletic. It seems likely that the defining features of these subfamilies are convergences related to similar ecologies. The 'Sympetrinae' include species with a preference for open watery habitats such as ponds and marshes where they spend a lot of time perched on exposed vegetation (Pilgrim & von Dohlen 2008). The 'Tetrathemistinae', with narrow wings with somewhat reduced venation, are found along forest streams (Fleck et al. 2008). The genera Tramea and Pantala, falsely united in the subfamily Trameinae by broadened bases on the hind wings, are specialised for long-distance flights spending extended periods on the wing (Pilgrim & von Dohlen 2008). Indeed, the wandering glider Pantala flavescens is the world's most widespread dragonfly species, being found in warmer regions of the entire globe and seemingly capable of migrations between separate continents.

The slightly freakish-looking larva of Orionothemis felixorioni, from Fleck et al. (2009).

So if we're going to have a stable classification for libellulids, we need to look past their wings. Intriguingly, larval features may prove more useful in this regard than adult characters. Fleck et al. (2008) examined a group of genera previously classified in the Tetrathemistinae but whose larvae were more similar to those found among members of the Libellulinae. A molecular phylogeny showed that, whereas the Tetrathemistinae as a whole were polyphyletic, these genera were indeed associated with the Libellulinae as their larvae indicated. With further research, we find that libellulid classification need not be all in vein.

REFERENCES

Fleck, G., M. Brenk & B. Misof. 2008. Larval and molecular characters help to solve phylogenetic puzzles in the highly diverse dragonfly family Libellulidae (Insecta: Odonata: Anisoptera): the Tetrathemistinae are a polyphyletic group. Organisms, Diversity & Evolution 8: 1–16.

Pilgrim, E. M., & C. D. von Dohlen. 2008. Phylogeny of the Sympetrinae (Odonata: Libellulidae): further evidence of the homoplasious nature of wing venation. Systematic Entomology 33: 159–174.