1. A naturally occurring peptide, termed sarcophagine, has been isolated from larvae of the fleshfly Sarcophaga bullata. 2. The physicochemical properties of the natural peptide are identical with those of synthetic β-alanyl-l-tyrosine. 3. During larval growth the concentration of the dipeptide increases continuously; in the fully grown larva it is the predominant non-protein ninhydrin-positive material. 4. Sarcophagine is rapidly and extensively utilized at the time of hardening and darkening of the insect puparium. 5. By virtue of its much higher solubility than that of free tyrosine, the peptide may serve as a reservoir to provide large amounts of readily available tyrosine for subsequent puparium formation.

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