Plastron respiration in the eggs of blowflies

Abstract

In the egg-shells of the blowflies Calliphora and Lucilia a continuous film of air is held between vertical columns in the inner part of the chorion. This film of air is continuous with that held in the median area between the hatching lines. The chorion of the median area between the hatching lines consists of three distinct layers. The outer and inner layers consist of struts or columns that arise perpendicularly from the middle layer. A continuous film of air is held between the columns of each layer. Holes through the middle layer of the chorion effect the continuity of the air films of the outer and inner layers. The columns of the inner layer are connected to a thin and continuous inner sheet of chorion. The columns of the outer layer are branched at their apices in a plane normal to their long axes. These horizontal branches form a fine and open hydrofuge network that provides a large water-air interface when the egg is immersed. The film of air held in the outer layer of the area between the hatching lines functions as a plastron when it rains or the egg is otherwise immersed in water. The eggs of blowflies are normally laid on decomposing materials, and the surface tension of water in contact with these is much lowered by surface active substances. The resistance of the plastron to hydrostatic pressures varies directly as the surface tension of the liquid. These considerations would seem to provide an explanation for the fact that the plastrons of blowflies not only resist wetting in clean water by hydrostatic pressures far greater than any to which they are at all likely to be exposed in nature, but, paradoxically, are more resistant to pressure than are the plastrons of some wholly aquatic insects that live in clean water.