Why most insects have very low proportions of C20 polyunsaturated fatty acids: The oxidative stress hypothesis.

Eicosanoids, a group of C20 oxygenated polyunsaturated fatty acids (PUFAs), mediate various physiological processes, such as immunity, reproduction, excretion, and metabolism in insects. Arachidonic acid (AA) is used for the main precursor for the production of various eicosanoids. However, most terrestrial insects possess relatively low AA levels. Insects are presumed to be evolved since the Paleozoic era, at which oxygen levels might be much higher than current conditions. Compared with other animals, they exhibit relatively high metabolic rates with the well-developed tracheal system, which directly supply enough oxygen to active tissues like flight muscles. This might allow insects to be susceptible to reactive oxygen species (ROS) generated from high oxidative catabolism. Long-chain PUFAs including AA is usually reacted with ROS and become peroxidized. Peroxidized PUFAs cause various cellular damage. Thus, we propose a hypothesis that terrestrial insects minimize AA levels to minimize oxidative stress.