Discrete changes in silkmoth choriogenesis have occurred during evolution, as exemplified in the present report in Antheraea polyphemus and Hyalophora cecropia. At the level of morphology, the chorion of A. polyphemus has surface structures, called aeropyle crowns, that are absent from H. cecropia. Aeropyle crowns form during the very late period of choriogenesis and consist of two substructures--lamellae and filler. Filler is present in H. cecropia in greatly reduced amounts. At the level of protein synthesis, overall similarities in the two species are maintained until the very late period of choriogenesis, when synthesis of aeropyle crown components is maximal. In H. cecropia, very late period-specific proteins are reduced in number and abundance. Several of these minor proteins are candidates for E1 and E2, the components of filler. E1 and E2 RNAs are about 35 times more abundant in A. polyphemus, despite very similar gene copy numbers and times of expression in the two species. These results support the hypothesis that evolutionary changes in chorion morphology have resulted from regulatory changes in the expression of chorion genes, either at the level of transcription or mRNA decay. The hypothesis that evolutionary changes in chorion morphology are based on terminal addition onto a preexisting developmental program is discussed.