The Cuticular Lipoids of Insects

The ability of insects to withstand desiccation was shown by Kühnelt (1928) to be due to the epicuticular layer of the cuticle. This outermost part was found to be highly resistant to strong acids and was shown, by chemical tests, to contain fatty acids and cholesterol type molecules. Wigglesworth (1933) described a complex fatty or waxy substance in the upper layers of the cuticle which he called ‘cuticulin’ ; Pryor (1940) found that this lipoid is entirely confined to the epicuticular layer, suggesting that it is used to impregnate a tanned protein. Lipoid material has also been extracted from the exuviae of the silkworm Bombyx mori (Bergmann, 1938). Bergmann found that hot alkali merely fused the epicuticle into oily droplets suggesting that saponification was taking place. The extract so obtained was a brown waxy substance, forming about 4% of the total weight of the cuticle, and it consisted of paraffins, saturated aliphatic acids and esters. A more detailed study of the waxes of certain insects specialising in wax formation has been given by Chibnail, Piper, Pollard, Williams & Sahai (1934b) ; Chibnall, Latner, Williams & Ayre (1934a), and Blount, Chibnall & el Mangouri (1937). All these substances were complicated mixtures of alcohols, acids, paraffins and esters with chain length of the order of C30. Hurst (1941) has suggested that the asymmetry of the insect cuticle is regulated by the ‘outer free lipoid’. He postulates a cuticle surface in the form of a lipoprotein mosaic. It is the object of this study to investigate the distribution and properties of these lipoid materials, to obtain evidence of the structure of the epicuticle by comparison of in vitro phenomena with those of living insect cuticle, and to indicate the function of lipoids in resisting the action of certain insecticides.