The relationships of the Odonatoidea (= Odonata plus Protodonata) to other pterygote insects continue to be much debated. All three possible relationships amongst Odonatoidea, Ephemeropteroidea and Neoptera have been proposed.
1. Mackerras (1970) placed Odonata as sister to the other pterygotes:
|===== Ephemeroptera |=====| |=====| |===== Neoptera | | =====| |=========== Palaeodictyopterodea | |================= Odonatoidea
Boudreaux (1979) supported a similar arrangement so far as the living lineages are concerned.
2. Hennig (1981), and Kukalova-Peck (1991) each argued for a sister-group relationship between Odonata and Ephemeroptera:
|===== Odonatoidea |=====| |=====| |===== Ephemeroptera | | =====| |=========== Palaeodictyopterodea | |================= Neoptera
3. Kristensen (1981, 1991), Wheeler (1989) and Bechly (1996) each favored a sister-group relationship between Odonata and Neoptera, but disagreed on placement of the other branches. The three possibilities are:
|===== Odonatoidea |=====| |=====| |===== Neoptera | | =====| |=========== Palaeodictyopterodea | |================= Ephemeroptera
or
|===== Odonatoidea |=====| |=====| |===== Neoptera | | =====| |============ Ephemeroptera | |================== Palaeodictyopterodea
or
|===== Odonatoidea |=====| | |===== Neoptera =====| | |===== Ephemeroptera |=====| |===== Palaeodictyopterodea
The greatest sources of difficulty in placing the Odonata in relation to the other orders are that authors disagree about which veins in the odonate wing relate to which veins in the wings of other insects, and about which states of wing vein characters are primitive and which are derived. A brief outline of the major proposals is given in the accompanying page on Odonate wing venation.