A semi-quantitative biological assay for the eclosion hormone is described. 2. In the pharate moth hormonal activity is confined to the brain and the corporacardiaca (CC). 3. During the life history of the moth, activity is practically absent from the brain and CC of larval and pupal stages and appears only in preparation for adult emergence. 4. The titer of eclosion hormone in the brain and CC of Pernyi moths was followed through adult development. In both structures activity first appeared on about day 7. Titers then increased for the next 3 to 5 days followed by a plateau. On the morning of the day of emergence, the titer in the brain fell and that in the CC correspondingly reached its highest level. At the time of eclosion in the evening, the CC titer then dropped and hormone appeared in the blood. 5. Tests on brains and CC from cockroaches and bugs failed to show activity. 6. It was concluded that the eclosion hormone is used only for the pupal-adult ecdysis and, thus, is probably restricted to the holometabolous orders of insects.
