Middle English, also dorre, "buzzing or whirring insect," from Old English dora, a word of unknown origin, perhaps imitative.
"lice infestation," 1809, with -osis + Latin pediculus, diminutive of pedis "a louse," said in some sources to be akin to pedere "to break wind" (see petard) on notion of "foul-smelling insect" [Watkins]. But de Vaan traces it to a PIE *pesd- "annoying insect" and compares Avestan pazdu- "beetle, maggot." Pedicule "louse" is attested in Middle English (early 15c.).
"final or adult stage of an insect," 1797, from Latin imago "an image, a likeness," from stem of imitari "to copy, imitate" (from PIE root *aim- "to copy"). "The name is due to the fact that such an insect, having passed through its larval stages, and having, as it were, cast off its mask or disguise, has become a true representation or image of its species." [Century Dictionary]