Definition of Insect Instars in Terms of ‘Apolysis’ or ‘Ecdysis’2

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Abstract
Evidence is given that supports Wigglesworth's definition of an instar as begun and ending at ecdysis and does not support Hinton's definition as beginning and ending at apolysis or retraction of the epidermis from the old cuticle. The Diptera Cyclorrhapha are used as the primary examples because they play an important role in the arguments of both Wigglesworth and Hinton. The points raised here result from extensive studies of fly development, tracheal system molting, and epidermis development at light and electron microscope levels.