The Structure of the Insect Egg-shell in Relation to the Respiration of the Embryo

Many insect eggs are able to avoid mechanical injury and desiccation because they are protected by an egg-shell or chorion which is tough, rigid and relatively waterproof. These properties tend to make it impermeable to gases, and since the egg contains a living embryo, there must be provision for the diffusion of an adequate volume of oxygen into the egg. The way in which this is achieved has never been satisfactorily explained. Specialized respiratory structures have been described in many species of insect egg, especially in those of parasites (Maple, 1937), and a respiratory function has sometimes been ascribed to the system of micropylar canals that penetrate the shell in hard-shell eggs, but until now there has been no direct evidence for the respiratory function of these structures. Indeed it seems unlikely, at first sight, that sufficient oxygen could diffuse through such small areas of the shell to satisfy the needs of the embryo, although in some species there can be little doubt that oxygen uptake is restricted to a small part of the shell; in certain species of capsid, for instance, the eggs are embedded in the tissues of a plant with only the caps exposed to the air. What follows is an account of an investigation of this problem in the eggs of the reduviid bug, Rhodnius prolixus (Stahl). Beament (1947) has shown that the eggshell in this species is composed of several layers, and its waterproofing properties are due to a thin continuous layer of wax on its inside surface. He has also shown that at the cap end of the egg the shell is penetrated by a series of micropyles and pseudomicropyles. I have been able to show that most of the oxygen consumed by the egg enters through the micropyles and pseudomicropyles, and that the relative impermeability of the rest of the shell is due, not to the wax layer, but to the components that give the shell its mechanical strength. From the theoretical point of view it can be shown that sufficient oxygen can only enter the egg through these pores if there is a continuous gas phase under the shell. A gas space can be demonstrated in the Rhodnius egg, and its minimum dimensions can be calculated.