Wax Secretion in the Cockroach

Abstract

An investigation has been made into the properties of the grease layer which waterproofs the cuticle of the cockroach Periplaneta americana. This grease changes slowly into a hard wax if stored in air. The melting-point rises during this process by 20° C. It is shown that the grease consists of a hard wax (m.p. 56° C.) dissolved in a ‘solvent’ which is liquid at room temperatures; there may be equal amounts of wax and of solvent. It is suggested that the series of paraffins and alcohols which probably compose the wax extend into the short-chain lengths, C₈-C₁₂ to provide the solvent. A micro-freezing-point method for determining the molecular weight of wax samples is described (based on Ramsay's method for aqueous solutions); the molecular weight of the solvent lies between 120 and 170, and of the hard wax--300-350. A method is also outlined for collecting large quantities of cockroach wax. Of a large range of lipophilic liquids, which dissolve waxes, only octane, decane, octyl and decyl alcohols are miscible in all proportions with insect waxes, and form synthetic greases with beeswax and with the hard wax of the cockroach. The surface-active properties of the natural and synthetic greases have been compared on a surface balance of novel design which is described in the Appendix. The two materials are remarkably similar; the surface balance has been used to confirm the size of the solvent molecules. An apparatus is described which enables the continuous measurement of water transport through an insect membrane in a water vapour/vapour gradient, and in the presence of other vapours. As opposed to the effect of the most organic wax solvents, the vapour of an octane-octyl alcohol mixture does not alter the permeability of a natural insect cuticle to water; exposure of an artificially waxed membrane to this vapour and water increases its impermeability. Evidence is provided for believing that octane-octyl alcohol mixtures will improve the waterproofness of most ‘imperfectly’ waxed membranes. The natural grease of the cockroach is a strong reducing agent, and evidence is produced for believing that the lipoid part is entirely saturated, but that it contains a small portion of substances resembling shellac. It is suggested that this represents the ‘cement’ layer of the cockroach cuticle. The importance of spreading agents with wax solubility in the formation of insect waterproof layers is discussed; it is suggested that the presence of a water-saturated substrate is an essential part of this process.