Which unique feature distinguishes the extinct order Palaeodictyoptera from all extant insect groups?

• A. Three pairs of wings.
• B. Viviparous reproduction.
• C. Absence of metamorphosis.
• D. Winglets on the prothorax.

Answer: (D)

Wing placement on the body is a key clue to how insect wings evolved. In the extinct Palaeodictyoptera, fossils show small wing-like projections on the first thoracic segment, the prothorax, in addition to the usual wing-bearing segments behind it. These prothoracic winglets are a distinctive feature not found in any living insect group, where wings, when present, arise on the middle and often the hind thoracic segments but not on the prothorax. That unique prothoracic wing arrangement makes this order stand out from all extant insects. Other options describe traits that aren’t uniquely tied to this group or aren’t exclusive to extinct lineages.