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The Penetration of Insect Egg-shells: II.—The Properties and Permeability of Sub-chorial Membranes during Development of Rhodnius prolixus, Stål
- J. W. L. Beament
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- Journal:
- Bulletin of Entomological Research / Volume 39 / Issue 4 / March 1949
- Published online by Cambridge University Press:
- 10 July 2009, pp. 467-488
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During development, membranes are added to the inner surface of the chorion in eggs of Rhodnius 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.
The Penetration of the Insect Egg-shells. I.—Penetration of the Chorion of Rhodnius prolixus, Stål
- J. W. L. Beament
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- Journal:
- Bulletin of Entomological Research / Volume 39 / Issue 3 / December 1948
- Published online by Cambridge University Press:
- 10 July 2009, pp. 359-383
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The unspecialised portion of the shell and the cap of Rhodnius eggs are impermeable to almost all hydrophilic and lipophilic liquids. If water and very small ions pass through the chorion they must traverse a wax layer on the inside of the shell. Certain corrosive materials, e.g., glacial formic acid, may pass through the shell slowly.
These conclusions, based on experiments with pieces of shell, have been confirmed in ovicidal experiments. A range of materials with widely differing properties enter the embryo only through the micropyles, of which there are approximately fifteen in the rim of each shell. At least one micropyle must be traversed to kill an egg but many eggs were killed when only one had been penetrated.
A cement, applied by the female at oviposition, may occlude the outer orifice of a micropyle. The properties of the cement are described; it appears to be a tanned protein. Cement deposits are much more copious on the eggs laid by younger females. Such eggs are more resistant to ovicides because penetration is delayed. This increased resistance is more pronounced when oleophilic liquids are used owing to the rapidity with which they kill eggs from older females. The random distribution of cement is one cause of the variability of replicates in ovicidal tests.
A detailed investigation has been made of factors governing liquids traversing the micropyles. Hydrophilic liquids invade the outer lipophilic part of the micropyle slowly; the displacement of air is the most important factor and small changes in the wetting power of the liquid make little difference to the rate of entry.
Aqueous liquids aie absorbed into the protein lining of the inner portion of the micropyle. They reach the wax layer on the inside of the shell by migrating into and through the inner protein layer. The area which is invaded increases linearly with time. Mortality, therefore, increases as the square of the time of immersion, but it is proportional to the increase in concentration of a solute if the period of immersion is constant.
Oleophilic liquids wet the micropyle actively. They may by-pass air and flow rapidly to the wax at the inner end of the tube. Wax solvents kill very quickly and are much more toxic than other lipophiles.
Water in the micropyle and shell may affect the entrance of either type of liquid. In general it increases the toxicity of aqueous solutions and retards the entry of oils.
Wax-emulsifying materials added to aqueous solutions do not produce great increases in toxicity. They are “filtered out” at the protein lining of the micropyle and do not reach the wax layer for a considerable period of time.
Penetration through the Egg-shell of Pieris brassicae (L.)
- J. W. L. Beament, R. Lal
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- Journal:
- Bulletin of Entomological Research / Volume 48 / Issue 1 / March 1957
- Published online by Cambridge University Press:
- 10 July 2009, pp. 109-125
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The structure of the shell of eggs of Pieris brassicae (L.), together with changes in it and associated membranes during embryonic development, have been investigated in relation to the penetration and toxicity of simple chemicals. The rigid outer shell consists of two proteinaceous layers, covered externally by a relatively hydrofuge cement, by which the egg is attached to the leaf surface. The egg has respiratory pores over its surface, and a single apical micropyle penetrating these layers. The inside of the rigid shell is lined with a layer of unsaturated oil—an unusual feature for an insect egg. When the egg is first laid, the vitelline membrane is directly applied to the inner surface of the solid shell over the region immediately around the micropyle, but within four hours this contact is broken, and the oil layer flows into this region also, and becomes complete. As development proceeds, the vitelline layer is replaced by membranes of embryonic origin, but before eclosion both these epembryonic layers, and also the oil, are resorbed.
The egg is remarkably resistant to water-soluble poisons which have no oil-solubility, except during the first four hours of development. This resistance is attributed almost entirely to the oil layer, and the early susceptibility to its absence over the micropylar region. These changes are not reflected in the effect of oil-soluble poisons or fumigants. The solid portions of the shell do not seem to be of great importance in restricting the entry of liquid poisons, even though the cement is comparatively hydrofuge; from experiments with wetting agents and with eggs immersed in poisons under vacuum, it does not appear that the respiratory air spaces in the shell are preferential channels of access; rather, the poisons penetrate through the solid portions of the shell. This penetration, even of oil-soluble materials, is slow, for they can be effectively washed out of the shell again, some considerable time after dipping. On the other hand, non-volatile oily materials can interfere with the respiration of the egg by blocking the air spaces in the shell.
The secretion of epembryonic layers does not appear to change the resistance of the egg to water-soluble materials; this is to be expected, for they do not contain lipoid. On the other hand they do add appreciably to the resistance to oil-soluble materials. There is no evidence that poisons are accumulated in these epembryonic membranes, and released during the pre-eclosion period. Experiments with covalent compounds, such as mercuric chloride, suggest that their oil-solubility accounts for their toxicity, whereas electrovalent compounds containing similar heavy metals are only effective while the direct micropylar path of entry is available to them.