The structure and function of the respiratory horns of the eggs of some flies

Abstract

Eggs of flies of the families Sepsidae, Drosophilidae, Sphaeroceridae, Muscidae, and Cordiluridae may have one or more tubular or flattened projections from the anterior part of the shell. These projections are called respiratory horns. The eggs are laid in decomposing vegetable or animal matter. The respiratory horns project above the surface of the material in which the egg is embedded. The surface of most of each horn consists of an open hydrophobe meshwork of chorionin that provides a large water/air interface when the egg is submerged. The layer of air held in the meshwork is retained even when subjected to a considerable pressure. This layer of air is thus a plastron. The plastron found most resistant to wetting by excess pressures was that ofDrosophila funebris, which resists 1.3 atm. The plastron air is continuous with the layer of air held in the shell. When it rains, a layer of well-aerated water flows over the eggs, and the plastron of the respiratory horn serves to extract oxygen from that dissolved in the ambient water. The plastron of all species resists wetting by excess pressures considerably greater than those to which it is likely to be exposed in nature. To be an efficient respiratory structure the plastron must also resist wetting by surface-active substances, the concentrations of which are likely to be high in the environments in which the eggs are normally laid. Any change in the geometry or the nature of the surface of the plastron meshwork that increases its resistance to wetting by surface-active substances also increases its resistance to wetting by excess pressures, since wetting by excess pressures always occurs before there is a mechanical breakdown of the meshwork. Thus selection for greater resistance to wetting by surface-active substances results in an increase in resistance to wetting by excess pressures. These facts provide an explanation of the paradox that the plastron of some terrestrial eggs is more resistant to wetting by excess pressures than is the plastron of many wholly aquatic insects.