The mode of sclerotization of Aedes aegypti pupal and adult cuticle was examined by employing biochemical and radioactive techniques. During larval-pupal metamorphosis, tyrosine is converted to tanning precursors and is incorporated into aryl-amino adducts and beta-crosslinks. The major hydrolysis product of beta-crosslinks in pupal cases is identified to be arterenone. Examination of tanning modes in five different mosquito species shows that the ratio of quinone to beta-sclerotization not only differs within the life stages of the insects, but also differs between species.