The Cuticular Lipoids of Insects

Abstract

The passage of water through isolated insect cuticles and exuviae, before and after extraction with lipoid solvents, has been measured. The impermeability of the cuticle is due to a thin continuous layer of lipoid over the epicuticular surface. Transmission of water through unextracted cuticle is more rapid in the direction epicuticle to endocuticle, than when reversed. This asymmetry is most marked in the exuviae. The lipoid layer is approximately 0.25 µ. thick on most of the insects investigated and is independent of cuticular thickness. The thickness of wax on a given species is very constant. The pure extracted lipoids are solid waxes, except in the blattids, which have a mobile grease. The waxes show a gradation of physical properties corresponding to their melting-points; the hardest are most hydrofuge and are found on the most impermeable insects. The impermeability of insect wax films, deposited on a membrane, depends on the degree of chemical orientation induced in the lowest layer of molecules by the membrane surface. The monolayer at the epicuticular interface of the insect is completely orientated and tightly packed, and is the main barrier to the passage of water. Waxed membranes show a sudden increase in permeability at a critical temperature corresponding to that of the insect from which the wax was obtained. The waxes undergo a crystalline transition at this temperature; their molecules become mobile, and the orientated layer is disorganized. In the presence of water, wax surfaces become more hydrophilic at the transition point, and the molecules are permanently orientated in this state. Boiling solvents must be used to extract waxes from the exuviae; the extracts are all soluble in cold chloroform. The vapour of wax solvents will destroy the orientation of the wax on the cuticle. Substances having both hydrophilic and hydrophobic properties are present in high meltingpoint waxes; they may be residues of natural emulsifiers. Inert dusts increase the transpiration of waxed membranes by adsorbing the lipoid, only if the membrane exerts little orientational force. They cannot overcome the orientational bonds of a wax layer deposited on a membrane of insect epicuticle, unless adsorption is augmented by abrasion. Dusts will adsorb all but the lowest orientated layer of the grease on the coackroach. Some analysis is given of the mode of action of emulsifiers and detergents, which increase the permeability of waxed membranes, and of the insect cuticle. Their efficiency depends on their own permeability to water, and possibly on their ability to penetrate the cemen't layer on the insect, as well as on their capacity for emulsifying wax.