Transpiration Through the Cuticle of Insects

Abstract

Transpiration through the cuticle of insects is restricted by a thin layer of orientated wax on the outer surface of the epicuticle. In some insects at least this wax layer is covered by a thin layer of cement. When heated to a certain temperature the wax layer shows an abrupt increase in permeability to water. This ‘critical temperature’ varies widely in different species and in different stages of the same species. It is highest in those insects which are most resistant to desiccation. In the newly formed puparium of Calliphora the impermeable film of wax is wholly superficial as in other insects. But after pupation the main impermeable layer is on the surface of the true pupa. The critical temperature of the pupa is much higher than that of the puparium. Abrasion of the wax layer results in a great increase in transpiration through the cuticle. Inert dusts cause the desiccation of insects by getting between the moving surfaces of the cuticle and abrading the wax layer. Such dusts in stationary contact with the cuticle will not remove the wax by adsorption; hence they are without action on dead or motionless insects. (The cockroach is an exception to this. Here the water proof layer is a soft grease, freely exposed on the surface; it is largely removed by adsorption on to the dusts.) The places where the wax has been abraded can be demonstrated by immersing the insect in ammoniacal silver solution. The phenol-containing epicuticle stains deep brown only where the protective layer of wax has been removed. Although the epicuticle shows no visible injury as the result of abrasion, the underlying cells react as though they had been wounded, and growth processes in the epidermis are affected. Insect larvae from the soil show great but variable evaporation of water. This is the result of abrasion of the cuticle by soil particles. If the wireworm Agriotes is allowed to moult out of contact with the soil it has an impermeable cuticle like other insects. After abrasion the living insect is able to secrete more wax through the substance of the cuticle and so to restore its impermeability. Adsorption of the wax by dusts while it is being secreted interferes with this process of recovery. Lipoid solvents remove the wax layer from the surface and so increase transpiration. A long series of wetting agents and detergents has been tested. They show widely different effects on permeability of the cuticle to water. Removal of the wax layer by means of abrasive dusts or suitable detergents increases the rate of entry of insecticides through the cuticle.