AbstractArlyphorin (Ap) is the principal protein of the last larval instar hemolymph ofTrichoplusia ni. It was shown to be homologous with the Aps ofManduca sextaandLymantria disparby Western blot and quantitative immunoelectrophoresis. Another hemolymph storage protein inT.niof lesser titer was shown to be homologous with larval hemolymph protein (LSP) ofM.sexta. Ap titer increased dramatically in the last larval instar ofT.ni, as in other holometabolous insects studied. Parasitization byChelonus sp. caused the Ap titer to rise prematurely in the penultimate larval instar ofT.ni. This rise in Ap in the fourth instar is one of the earliest diagnostic signs of parasitization. Among the suite of behaviors of theChelonuslarva on exiting the host is depletion of the host cadaver of most remaining protein. TheT.niAp titer in the alimentary tract ofChelonuspeaks at that time and declines to zero in the first 24 h after parasitoid emergence, prior to its pupation. Aps are a source of phenolic storage compounds. Hence, premature induction ofT.niis advantageous for the parasitoid's own pupation and adult development.