The structure and function of the egg-shell in the Nepidae (Hemiptera)

Abstract

A comparative account is given of the respiratory system of the egg-shells of sixteen species of Nepidae. These include half of the genera at present known in the family. The egg-shell has two (Ranatrinae) or four to twenty-six (Nepinae) respiratory horns arising from the anterior pole. Each horn consists of a central gas-containing meshwork that is connected either directly or through aero-pyles with a peripheral plastron meshwork that may extend over most of the horn or may be confined to its apical part. The plastron meshwork consists of structs normal to the long axis of the horn. The struts 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 gas-containing meshworks of each horn are joined basally to the gas-containing meshworks of the inner shell wall, and the film of air in the plastron meshwork is thus continuous with the film of air in the inner shell wall. The shell fills with air only after the egg has left the common oviduct. The plastron of Ranatra linearis is more resistant than that of Nepa cinerea to wetting by excess pressures, but the resistance of both species is sufficient to provide what appears to be a very wide safety margin against natural contingencies. The differences between the respiratory systems of the Nepidae and those of the terrestrial Heteroptera are noted. In the Nepidae the aeropyles that extend from the surface inwards to the gas-containing meshworks on the shell wall are present over most of the shell: in Nepa cinerea there are about 2700 aeropyle-producing follicular cells which produce altogether about 17,000 aeropyles. In the terrestrial Heteroptera the aeropyles are confined to a restricted area of the shell wall. The possible functions of the aeropyles are discussed. It is shown that, contrary to previous accounts, the respiratory system of the egg-shell of Rhodnius prolixus is like that of other Reduviidae and the terrestrial Heteroptera generally. During incubation the egg absorbs water from the environment and in some species more than doubles its volume. The water is chiefly absorbed through a complex chorionic hydropyle. A serosal hydropyle is formed against the inner face of the chorionic hydropyle. The viscous, hygroscopic material on the outside of the shell of Nepa cinerea appears to be an acid mucopolysaccharide. It functions as a cement and may also serve to damp down humidity fluctuations of the ambient air. It is produced in the individual ovarioles and not in the accessory gland.