THIS HAS JUST STARTED
The Odonata are normally taken to be the sister group of the Neoptera, in a cladogram (Archaeognatha + (Monera + (Thysanura + (Ephemeroptera + (Odonata + (Neoptera)))))) - see for example Kristensen 1991 (in Insects of Australia).
The consensus tree arrived at by Wheeler, Cartwright and Hayashi for the 'arthropoda' and allied phyla (Wheeler, W.C., Cartwright, P., Hayashi, C.Y.
1993. Arthropod phylogeny: a combined approach. Cladistics 9: 1-39) viz:
(Annelida + (Onychophora + ((Trilobita + Chelicerata) + (Crustacea + (Myriapoda + Hexapoda))))) has real problems within Insecta; the Odonata are placed weirdly within the Neoptera, a cause to suspect the characters they used.
The Odonata are palaeopterous insects, they lack the hinge structures which allow neopterous insects to fold their wings alongside their bodies. The fossil record shows there was an enormous radiation of palaeopterous insects through the Carboniferous and into the Permian, however all but the Ephemeroptera (Mayflies) and Odonata died out. Thus within the pterygota the Odonata can be seen to be quite isolated. Structural evidence is good that the extinct Protodonata, Protozygoptera, and Protanisoptera form a natural group with the Odonata (sensu stricto); but there is considerable debate over relationships within and among these groupings. Carpenter regards the Protodonata and the Odonata as sister groups whereas members of the Moscow school treat these as one order. The fossil evidence is reviewed in Carpenter, F.M. 1992. Treatise on invertebrate paleontology, Part R: Arthropoda (4), vol. 3: Hexapoda. Geological Society of America.
Odonata
Predatory palaeopterous insects with well-developed eyes and four pairs of similar membranous wings. Wings with a dense and characteristic pattern of venation. Thoracic structure of adult stage highly skewed through development of hypertrophied mesanepisternum. Larva aquatic (some forms secondarily terrestrial).
Adult: distinguished from all other insects by the highly derived male secondary reproductive apparatus situated on the underside of abdominal segments 2 and 3.
Larva: distinguished from all other insects by the highly derived, prehensile labium used as a food capturing device
The extant Odonata are conventionally divided into three suborders: Zygoptera, Anisozygoptera and Anisoptera. Debate abounds both on relationships among the suborders and within the suborders. Extant Anisozygoptera (Epiophlebiidae) and fossil 'Anisozygoptera' form a heterogeneous assemblage. The extant Epiophlebiidae can be closely allied with the Anisoptera on the basis of larval morphology. Associated larvae are not available for the various fossil 'Anisozygoptera'.
Zygoptera
Adults: eyes separated by > dorsal diameter, fore- and hindwings closely alike, with narrow base, Dc simple quadrilateral in both wings, nodus in proximal half of costa, male with two pairs of anal appendages, the inferiors sited below the anus, male copulatory organ ligula (segment 2).
Larva: narrow-bodied, abdomen sub-cylindrical with three (rarely two) caudal lamellae.
Anisozygoptera (Heterophlebioidea)
Adult: eyes separated by < dorsal diameter, fore- and hindwings closely alike, with narrow base, Dc simple quadrilateral in forewing, divided into hypertriangle and triangle in hindwing, nodus near middle of costa, male with one pair of superior anal appendages and single inferior sited above the anus, male copulatory organ derived from hamuli posteriores (segment 2).
Larva: body more or less robust, at least slightly dorso-ventrally flattened, abdomen broader than thorax, rectum hypertrophied and containing lamellate gill structures internally, anal appendages short, pointed, forming 'anal pyramid'. Larva lacks ability to jet-propel.
Anisoptera
Adult: eyes separated by < dorsal diameter (usually touching or confluent), fore- and hindwings dissimilar in shape (hindwings broader at base). Dc divided into hypertriangle and triangle (often differing in shape between fore- and hindwing), nodus near middle of costa, male with pair of superior appendages and a single inferior sited above the anus, male copulatory organ derived from vesica spermalis (segment 3).
Larva: body more or less robust, at least slightly dorso-ventrally flattened, abdomen broader than thorax, rectum hypertrophied and containing lamellate gill structures internally, anal appendages short, pointed, forming 'anal pyramid'. After second instar larvae can convulsively expell water from rectum and jet-propel.