It's the term for an adult insect after metamorphosis
[DOC] [TLE] Imago | Word Imago at Open Dictionary of English by LearnThat ... [PAR] An adult insect produced after metamorphosis; (psychoanalysis) an ... the imago 
is the last stage an insect attains during its metamorphosis, its process of growth
... [DOC] [TLE] imago - definition and meaning - Wordnik [PAR] imago: An insect in its sexually mature adult stage after metamorphosis.