There have been several speculations as to the origin and significance of the pupa. One that has had considerable vogue is based on the interpretation, by Lubbock (1883), Berlese (1913) and others, of the larva as a free-living embryo prematurely hatched before it has attained the form characteristic of the adult. A nymph, according to this theory, has completed its embryonic development in the egg where it passed through stages corresponding to those of the larva and attained the form of the adult before hatching. This concept assumes, therefore, that the pupal stage is the equivalent of all of the nymphal instars compressed into a single instar. There are many objections to this theory of which the following may be given. Development from the larva to the adult is not continuous. There is evidence that larval and adult structures are determined independently in the embryo, and the development of the one and the suppression of the other are controlled by the endocrine system. Furthermore, changes in hormone balance which take place in the last nymphal instar of the Exopterygota result in the development of the adult, while similar changes in the last larval instar of the Endopterygota bring about the development of the pupa. For this and other reasons the pupa must be regarded as an adult instar and not as a larval or nymphal instar.