Imago refers to the stage when an insect is fully developed, after it has changed its form. The imago of a dragonfly is its last lifecycle phase.
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Scientific Usage
Use 'imago' when discussing insects in a scientific or educational context, not casually. The entomologist explained the imago stage to the students.
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Metamorphosis Completion
'Imago' specifically marks the end of metamorphosis, the process insects undergo to mature. The caterpillar's metamorphosis ends with the imago, the butterfly.