Eco-evolutionary implications for a possible contribution of cuticle hardening system in insect evolution and terrestrialisation

Previously we proposed one aspect of how insects could adapt to terrestrial environments during their evolution. The hypothesis is based on a theory that insects have evolved an insect-specific system for cuticle formation mediated by an insect-specific enzyme. This enzyme, multicopper oxidase-2 (MCO2), catalyses the oxidation of catecholamines using molecular oxygen as the acceptor of electrons from the substrates. A potential advantage of this MCO2-mediated system is the utilization of molecular oxygen abundant in the atmosphere, which is different from the case in crustaceans (close relatives of insects) that utilize calcium ions. Accumulation of calcium ions leads to an increase in weight, but the lightweight cuticle without calcification might have been a critical factor enabling insects to evolve flight first in the history of Metazoa. Our theory also provides a simple explanation to a long-standing question of why insects are so rare in marine environments. In previous reviews, we have mainly focused on the details of the biochemical processes occurring in arthropod cuticles, but here only the essence of our theory is extracted and briefly summarized with newly added information.