Primitive insects have two pairs of membranous flight
wings, but during the evolution of the beetle lineage the
forewings lost their flight function and became modified
as hard, rigid covers called elytra for protection of soft
body parts of the abdomen and also the delicate flexible
hindwings, which retained their flight function. This
transformation is manifested by a greatly thickened and
rigid (sclerotized) exoskeletal cuticle secreted by the
forewing epidermis. We demonstrate that this evolutionary
modification is accompanied by the incorporation of two
highly abundant structural proteins into the elytral cuticle,
namely TcCPR18 and TcCPR27. Depletion of these proteins
by RNA interference results in malformation and weakening
of the elytra, culminating in insect death. These
proteins are also abundant in hard cuticle from other
regions such as the pronotum and ventral abdomen, but
are absent in soft cuticles, and therefore may function as
key determinants of rigid cuticle. Expression of such
proteins at high levels in the modified forewing appears
to have been a fundamental evolutionary step in the
transformation of the membranous wing into a thickened
and rigid elytron in the Coleoptera.