The Penetration of Insect Egg-shells

Abstract

During development, membranes are added to the inner surface of the chorion in eggs ofRhodnius prolixus. The chemistry and permeability of the membranes have been investigated.A true fertilisation membrane is produced immediately before oviposition when the egg is fertilised. It is attached to the inner surface of the primary wax layer, recessed into the micropylar region and covers the whole inner surface of the shell.The membrane is very thin when first formed. It is colourless, comparatively resistant to solvents and apparently composed of “vulcanised” protein. It is semi-permeable to salt solutions but made very permeable to small molecules when immersed in absolute alcohols.In the following five days of incubation, further material is added to the fertilisation membrane. Over the main part of the shell and cap, this does not cause a great increase in the thickness of the, membrane. The added material is probably proteinaceous, with tanning- and vulcanising substances and it is mostly a product of the serosa.Opposite the inner openings of the micropyles, material is accumulated at a much greater rate and by six days incubation the membrane may be fifteen microns thick. The inner surface of the egg-shell thus becomes a uniformly ellipsoidal body. The thickened material has been called the epembryonic ring.At about the, sixth day of incubation, shortly before blastokinesis, the membrane is partially impregnated with a high-melting-point wax. This raises the “transition point” in the egg's water-loss/temperature curve from 42·5°C. to 68°C. Evidence is against this material being arranged as a second wax layer on the inner surface of the membrane.No further changes were detected until the thirteenth day when the innermost part of the membranes are broken down by the embryo. On the day before eclosion the embryo is surrounded by a liquid containing emulsified wax and proteinaceous material. The properties of the membranes return towards those of the one-day-old egg but do not attain them ; the egg hatches on the 16th day.The changes in the membranes produce considerable changes in the apparent toxicity of ovicidal liquids; ovicidal experiments are recorded and explained. In general, the egg becomes more resistant to lipophiles over the first six days and less resistant afterwards, due to the wax impregnation. The resistance to hydrophiles increases during development due to the epembryonic layer and secondary wax, but decreases when the membranes are broken down just before eclosion.