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The name Palaeoptera has been traditionally applied to those primitive groups of winged insects (most of them extinct) that lacked the ability to fold the wings back over the abdomen as characterizes the Neoptera. While this may not sound like a fundamental distinction, the complexities of the wing-folding mechanism, as well as the mechanical operation of the wings in flight (indirect flight muscles), are such that it clearly indicates the Neoptera are a monophyletic lineage. The problem is that the absence of wing-folding does not necessarily mean the Paleoptera form a natural group - they may simply be what's left over after the Neoptera are removed. As it stands, the relationship of the two living Paleopteran groups (Ephemeroptera and Odonata) to the Neoptera has not been resolved yet; there are three competing hypotheses, and in two of these, the Paleoptera appear to be paraphyletic (and, therefore, an artificial grouping that should be abandoned). If the extinct lineages are taken into account, it seems likely that the concept of Paleoptera as a natural group will eventually be discarded.
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