Two major proteins (P1 and P2) are synthesized by the fifth instar larval fat body of Manduca sexta and then released into the hemolymph. 2. These proteins are later sequestered by the pre-pupal fat body. 20-Hydroxyecdysone does not appear to affect the synthesis of either protein. 3. When day 2 fifth instar larvae are neck-ligated there is an excessive synthesis (supersynthesis) of P2 (arylphorin). 4. Juvenile hormone I (JH I) applications to ligated animals had no effect, but brain homogenate injections resulted in the inhibition of P2 synthesis. 5. Neck ligations of larvae between days 5 and 6 revealed a head critical period between day 5 + 12 hr and day 5 + 18 hr, after which the head is unnecessary for the sequestration of either protein by the fat body. 6. JH I and JH III applications to ligated larvae before the head critical period do not restore the ability of the fat body to sequester the storage proteins. 7. P1 and P2 appear to be synthesized differentially and P2 is sequestered by the fat body to a much lesser extent than P1. 8. P2 is the hemolymph storage protein of both larval and pupal stages, whereas P1 appears to be the storage protein of the pupal fat body. 9. The data indicate that the synthesis of arylphorin and the resorption of both proteins are controlled by a putative head factor(s).
