Development and metamorphosis

Abstract

A typical insect egg (Fig. 49a) is covered by an outer shell, or chorion, which is variously sculptured or ornamented. The chorion is secreted by the follicular epithelium and may consist of several layers; these include a very thin wax layer, which reduces water loss, and others containing protein but no chitin. Beneath the chorion is the delicate vitelline membrane, which is a product of the egg itself; further inner layers may be deposited after fertilization and during embryonic development. Insect eggs usually contain a large amount of yolk. This is a mixture of protein, lipid, carbohydrate and other materials lying within the meshes of a cytoplasmic reticulum. As in other Arthropods, the egg of insects is centrolecithal, a thin peripheral layer of cytoplasm — the periplasm — surrounding the yolk. One or more specialized pores or canals in the chorion are known as micropyles; through these the spermatozoa enter the egg at fertilization. In some insects the micropyles are also the channels through which oxygen diffuses into the egg; in other cases special respiratory channels, called aeropyles, occur separately and in some insects part or all of the chorion forms a plastron (p. 76), allowing respiration under water. Maturation of the egg usually follows the entry of spermatozoa. The egg nucleus moves towards the periphery and undergoes meiotic division, the polar bodies are segregated and the female pronucleus fuses with one of the spermatozoa, thus restoring the diploid chromosome number.