The amount of chitin and of protein in the blowfly larval cuticle remains essentially unchanged during the process of hardening and darkening of the puparium. In the formation of the puparium there is a gain of weight of about 6% over the larval cuticle weight, and this can be accounted for by the incorporation of phenolic substances. These are derived from the free tyrosine in the blood which decreases by an amount sufficient to account for the weight increase of the cuticle. An insoluble and highly resistant fraction has been isolated from the puparium, and this consists of about equal quantities of protein and pigment and possibly represents the natural association of these two substances in parts of the puparium. All the evidence suggests that the tyrosine is deaminated before giving the derivatives which combine with the cuticle substance, hardening and darkening it. Further properties of arthropodin, the water-soluble protein of soft arthropodal cuticles, are described. X-ray and related studies give a picture of the polysaccharide/protein complex in the cuticle which implies a model with alternating monolayers of protein and chitin. This appears to be based on a ratio of 3 amino acid residues to 1 chitobiose residue, or for equal lengths of protein and chitin chains a weight ratio of 45:55. This basic weight-ratio of protein and chitin appears to occur in the soft cuticles of blowfly larvae and other arthropods. A review is given of the composition and properties of various cuticles, and it is evident that hardening may be achieved without darkening of the cuticle. The chemical basis of this type of cuticle stabilization merits further study.