Some Notes on the Integument of Insects in Relation to the Entry of Contact Insecticides

Abstract

Insects immersed in oils exude minute droplets of water over the surface of the cuticle. These appear more rapidly in light oils than in heavy oils and are more numerous in young insects than in old. Immersed in mixtures of oils and alcohols which separate in contact with water they show a vigorous effervescence at those regions where the mixture penetrates most rapidly to the water in the cuticle. There is a delay of varying duration at different parts of the cuticle when heavy oils are used. This delay disappears if the lipoid layer is first extracted with petroleum ether.Factors controlling the rate of entry of pyrethrum through the cuticle have been studied by the application of pyrethrum in oil to a restricted area of the abdomen of nymphs and adults of Rhodnius. Entry is more rapid in light than in heavy petroleum oils ; it is much accelerated if the cuticle is first treated with petroleum ether. Entry is very slow in vegetable oils. There are great individual variations, due chiefly to the thickness of the endocuticle, which in turn is determined by the size of meal taken before moulting, the age and the amount of food taken after moulting, and the degree of stretching of the cuticle. There is evidence that the pore canals are important in the passage of pyrethrum through the endocuticle.Histological examination of the integument shows that in the nymph of Rhodnius oils are taken up first by the epidermal cells in the zone around the bristles ; later by the general epidermis. The uptake of oil is greatly increased if 5 per cent. of oleic acid is added to refined petroleum. In the adult Rhodnius soon after moulting, oils are taken up by the general epidermis. In old insects they are taken up solely by the dermal glands. There is no visible entry through the sockets of the bristles.The entry of pyrethrum in oils is accelerated by the addition of oleic or other fatty acids.