Odonata

The largest living odonate is the giant Central American helicopter damselfly Megaloprepus coerulatus (Zygoptera: Pseudostigmatidae) with a wing span of 191 mm (7.5 in). The heaviest living odonates are Tetracanthagyna plagiata (Anisoptera: Aeshnidae) with a wing span of 165 mm (6.5 in), and Petalura ingentissima (Anisoptera: Petaluridae) with a body length of 117 mm (4.6 in) (some sources 125 mm (4.9 in)) and wing span of 160 mm (6.3 in). The longest extant odonate is the Neotropical helicopter damselfly Mecistogaster linearis (Zygoptera: Pseudostigmatidae) with a body length of 135 mm (5.3 in). Sometimes the giant Hawaiian darner Anax strenuus (Anisoptera: Aeshnidae) is claimed to be the largest living odonate with an alleged wing span of 190 mm (7.5 in), but this seems to be a myth as only 152 mm (6.0 in) wing spans are scientifically documented.

The smallest living dragonfly is Nannophya pygmaea (Anisoptera: Libellulidae) from east Asia, with a body length of 15 mm (0.59 in) and a wing span of 20 mm (0.79 in). The smallest damselflies (and also the smallest odonates) are species of the genus Agriocnemis (Zygoptera: Coenagrionidae) with a wing span of only 17–18 mm (0.67–0.71 in).

Male blue ringtail (Austrolestes annulosus), a damselfly (Zygoptera: Lestidae)

Dragonfly (top) and damselfly (bottom) wing shape and venation

These insects characteristically have large rounded heads covered mostly by well-developed, compound eyes, which provide good vision, legs that facilitate catching prey (other insects) in flight, two pairs of long, transparent wings that move independently, and elongated abdomens. They have three ocelli and short antennae. The mouthparts are on the underside of the head and include simple chewing mandibles in the adult.[6]

Flight in the Odonata is direct, with flight muscles attaching directly to the wings; rather than indirect, with flight muscles attaching to the thorax, as is found in the Neoptera. This allows active control of the amplitude, frequency, angle of attack, camber and twist of each of the four wings entirely independently.[7]

In most families there is a structure on the leading edge near the tip of the wing called the pterostigma. This is a thickened, hemolymph–filled and often colorful area bounded by veins. The functions of the pterostigma are not fully known, but it most probably has an aerodynamic effect and may also have a visual function. More mass at the end of the wing may also reduce the energy needed to move the wings up and down. The right combination of wing stiffness and wing mass could reduce the energy consumption of flying. A pterostigma is also found among other insects, such as bees.[8]

The nymphs have stockier, shorter, bodies than the adults. In addition to lacking wings, their eyes are smaller, their antennae longer, and their heads are less mobile than in the adult. Their mouthparts are modified, with the labium being adapted into a unique prehensile organ called a labial mask for grasping prey.[9] Damselfly nymphs breathe through external gills on the abdomen, while dragonfly nymphs respire through an organ in their rectum.[6]

The giant Upper Carboniferous dragonfly ancestor, Meganeura monyi, had a wingspan of about 680 mm (27 in),[10]

Members of the Odonata first appeared during the Triassic, though members of their total group, Odonatoptera, first appeared in the Late Carboniferous, making them one of the earliest groups of winged insects.[11] The fossils of odonates and their cousins, including Paleozoic "giant dragonflies" like Meganeuropsis permiana from the Permian of North America, reached wing spans of up to 71 cm (28 in) and a body length of 43 cm (17 in), making it the largest insect of all time.[12] This insect belonged to the order Meganisoptera, the griffinflies, related to odonates but not part of the modern order Odonata in the restricted sense. They have one of the most complete fossil records going back 319 million years.[13]

The Odonata is closely related to mayflies and several extinct orders in a group called the Palaeoptera, but this grouping might be paraphyletic. What they do share with mayflies is the nature of how the wings are articulated and held in rest.

Tarsophlebiidae is a prehistoric family of Odonatoptera that can be considered either a basal lineage of Odonata or their immediate sister taxon.

The phylogenetic tree of the orders and suborders of odonates according to Bybee et al 2021:[14]

In some treatments,[15] the Odonata are understood in an expanded sense, essentially synonymous with the superorder Odonatoptera, but not including the prehistoric Protodonata. In this approach, instead of Odonatoptera, the term Odonatoidea is used. The systematics of the "Palaeoptera" are by no means resolved; what can be said however is that regardless of whether they are called "Odonatoidea" or "Odonatoptera", the Odonata and their extinct relatives do form a clade.[16]

The Anisoptera was long treated as a suborder, with a third suborder, the Anisozygoptera (ancient dragonflies). However, the combined suborder Epiprocta (in which Anisoptera is an infraorder) was proposed when it was thought that the "Anisozygoptera" was paraphyletic, composed of mostly extinct offshoots of dragonfly evolution. The four living species placed in that group are (in this treatment) in the infraorder Epiophlebioptera, whereas the fossil taxa that were formerly there are now dispersed about the Odonatoptera (or Odonata sensu lato).[17] World Odonata List considers Anisoptera as a suborder along with Zygoptera and Anisozygoptera as well-understood and widely preferred terms.[18][19][20]