As far as winged insects go, the Odonata are among the oldest and most basal. They are split in two suborders: the Zygoptera (damselflies) and Epiprocta/Anisoptera (dragonflies) (Epiprocta was only erected to accomodate the Anisozygoptera, a suborder comprising of a single extant genus, Epiophlebia, and numerous extinct Jurassic fossils that turned out to be nothing more than stem-group anisopterans). The damselflies are the more diverse suborder.
I could not work this fluidly into the post, but for anyone interested in the history of science, Baron Edmund de Selys-Longchamps, a Belgian entomologist of the late 1800s, is known as the father of odanotology and the one who first introduced the damselfly/dragonfly split (although it must be said that while he was a pioneer in terms of odonate classification, his system was intuitive, not based on rigid evolutionary principles). As general info, the damselflies are the ones who hold their wings parallel to their bodies when resting, whereas the dragonflies are the ones who keep them spread out.
Given how pretty they can be, they are among the most concentrated-on group of insects among conservationists (what I call the Charisma Effect). This is not in vain though, as they are all rabidly insectivorous and therefore can play a very large role in agricultural pest control. Their behaviour, both as larvae and as adults, has therefore also been extensively studied.
Odonates are known not only for their beauty, but also for their flying ability and strength: not only can they fly with large chunks of their wings missing, they can also fly carrying a load weighing twice as much as their body. The odonates are generally split into two types of fliers: the perchers and the fliers. The perchers, as their name suggests, tend to make short flights from a single vantage point; all damselflies are perchers, as are libellulids and gomphids. The rest of the dragonflies are fliers. A third category has also been opened for those libellulids with a modified hindwing that allows them to glide (however the species here can just as easily be included as perchers). The differences between them are not simply ethological but also morphological and physiological. The fliers have air sacs around their thoracic flight muscles, allowing them to regulate the body’s temperature – without them, it only takes a few minutes for the insect to terminally overheat. To make room for all these air sacs, they have reduced the amount of muscle compared to the perchers. The perchers do not have these air sacs and tend to fly at maximum capacity, but for very short periods of time – they are sprinters, whereas fliers are long-distance runners.
Their flight musculature is also itself unique and the reason why odonates are considered to have the most advanced form of flight among the insects. All insects have muscles attaching to the wing base, however only the odonates (and the blattoids) use these muscles to directly power the stroke of the wing; in other insects, thoracic flexing contributes the most to wing flapping. This allows the odonates to powerfully thrust forward as well as hover. Their flight innovations are also enabled by various exclusive changes to the wing’s structure.
The picture below shows how complex wing venation in odonates can get and illustrates why I will not bother to go into any of these schemes in detail.
I will not go into all the details of these changes (mostly because it is still a rather controversial and not well-settled issue), but some generalities can be pointed out. The wings of the best odonate fliers are the most rigid – by comparing fossil wings to more advanced wings from recent taxa, we see an increase in structural rigidity. These improve things by reducing the amount of vibration (which in turn increases speed), for example. Also seen in the wing are four apomorphies that define the odonates (these are wing venation patterns which I will not summarise). These characters allow the wing to rotate (pivot may be a better verb) more. These changes all led to a fortification of the odonate wing through the clade’s evolution and are prerequisites for the odonates’ remarkable flying abilities. What must be remembered is that these trends can only be checked and confirmed using fossils: without them, we would have no idea how the ancestral odonates even looked like! In this respect, the most revolutionary fossil is Kennedya mirabilis, a Permian fossil described in 1925 that overthrew the then-predominant view of odonate ancestry, as well as allowing proper attempts at homologing wing veins between odonates and other insect orders (one of the major goals in insect phylogenetics is to reconstruct the ancestral wing venation pattern).
It must also be said that higher-level odonate phylogeny depends almost completely on wing venation patterns, nothing else. This again stresses how the whole odonate system revolves around flight (although at levels lower than the family, other characters must be brought into the picture). Of course, the fact that they have quite a good fossil record (arguably the best among the isnects) also plays a big factor (most fossils are just wings). Some of the earliest flying insects are odonates from the early Upper Carboniferous.
Odonates perform what is probably the most drastic metamorphosis among the insects, with the musculature, exoskeleton and respiratory system all getting restructured, and these changes continue to take place even in the young adult. The end result is the unique morphology and anatomy of the odonates; their different look isn’t just superficial, there are entire muscle groups missing in adult odonates that are supposedly characteristic for all the pterygotes. This is partly the reason why winged insect phylogeny is not easy to work out.
As far as diversity goes, dragonflies are a rather small order. They are cosmopolitan (except Antarctica), reaching their peak diversity in tropical and subtropical climates. Due to the Ice Ages in the Pleistocene, the lowest diversity of dragonflies is seen in the western Palaearctic: with the various mountain chains and the Mediterranean Sea, there was no way for the dragonflies (and the rest of the odonates!) to retreat back to Africa, where the climate was suitable, and so many extinction took place.
As is the norm in entomology, the odonates themselves are monophyletic, as are the dragonflies and as are many of their classically-recognised constituent (super-)families, including the libelluloids (or Cavilabiata, as they are sometimes fashionably called – I will stick to libelluloids for the sake of any french readers). Their main apomorphy is the presence of a supporting anal loop in the hindwing – this is a boot-shaped pattern. More advanced lineages of libelluloids have a progressively more ‘complete’ anal loop. The tree shown below is acceptable (of course, it may contain many errors, as all cladograms are hypotheses; but for our purposes, it demonstrates the relationships as they are colloquially known and it is beyond the scope to deconstruct them here).
As for the families within the Libelluloidea, the libellulids are definitely monophyletic; the other clades proposed over the years (e.g. Synthemistidae, Macromiidae, Corduliidae) have not been conclusively confirmed as being mono- or paraphyletic – more (proper) research is needed. The picture below shows a selection of different hypotheses for the in-group libelluloid phylogenies suggested so far. Just to show that this is really an open question. For clarity though, they are generally divided into two categories: the libellulids and the cordulids; there are no clearly distinguishing characters though. There are also other families, also with unclear affinities: the Chlorogomphidae, Macromiidae and Synthemistidae; they are generally placed close to the cordulids, but as I said, nothing is certain yet!
As for their fossil record, the stem group is known from the Mid- to Late Jurassic (the earliest anisopterans are of Triassic age), with an extensive Cretaceous radiation resulting in the formation of the true libelluloids replacing the older Mesozoic forms (the first true libelluloid fossil is ~90 Ma). The picture below shows a cool Lower Cretaceous one.
Within the libelluloids, the libellulid family is (as the name probably suggests) the most known and diverse, comprised of over 1000 species in 13 subfamilies. They contain all the dragonflies you commonly see at a pond or stream and are a monophyletic group. Again, I will not go into wing venational patterns (the easiest way to tell them apart); but when in the field, you can look for sexual dimorphism, a flattened abdomen and most usually a red or blue colour. More reliably, they never have tubercles at the margins of the eyes. As for their larvae, just look for the frontal plate of the frons (hint: it’s extremely hard to see). Also, the labial palps have long, poorly-developed setae. All in all though, it’s easier to collect them and let grow them, as it is hard to characterise libellulid larvae specifically. The earliest fossils are also from the Upper Cretaceous of Kazakhstan.
And since I know this complaint is going to come: pretty odonate pictures come a dime a dozen. Go find your own.
These also count as further reading – any entomology book has a section on the phylogeny and evolution of the odonates. If not, Grimaldi & Engel is always the best reference!
Fig. 2: Fleck, G., Ullrich, B., Brenk, M., Wallnisch, C., Orland, M., Bleidissel, S. & Misof, B. 2008. A phylogeny of anisopterous dragonflies (Insecta, Odonata) using mtRNA genes and mixed nucleotide/doublet models. Journal of Zoological Systematics and Evolutionary Research 46, 310-322.
Fig. 3: Ware, J., May, M. & Kjer, K. 2007. Phylogeny of the higher Libelluloidea (Anisoptera: Odonata): An exploration of the most speciose superfamily of dragonflies. Molecular Phylogenetics and Evolution 45, 289-310.
Fig. 4: Bechly, G. 1998. New fossil dragonflies from the Lower Cretaceous Crato Formation of north-east Brazil (Insecta: Odonata). Stuttgarter Beiträge zur Naturkunde Serie B (Geologie und Paläontologie) 264, 1-66.