In warm-blooded vertebrates the gross pathologies of essential fatty acid deficiency include reduced growth, epidermal lesions, increased water loss through the skin resulting in increased drinking, and infertility in both males and females. Various subcellular lesions have been observed to precede or accompany the gross pathologies; these lesions mostly affect lipid membranes of cells and their organelles, causing changes in membrane permeability under certain conditions of stress, and altered kinetics of membrane-bound enzymes. Biochemically, deficiency is characterized by a reduction in the levels of the tetraene, arachidonic acid in tissue lipids, most marked in the phospholipids, which predominate in organs such as nerves and glands having a high component of lipid membranes. The earliest work on insect essential fatty acids, that of G. Fraenkel and M. Blewett with flour moths of the genus Ephestia, serves to illustrate the main features of deficiency.