Egg-stage is the first life stage of an insect. Oviposition (egg deposition) takes place in diverse ways. Eggs may be laid singly or in groups, enclosed in gelatinous masses/scales or may be laid in protective case (egg-pod/ootheca). Mostly, the eggs are laid in a situation where they are afforded some protection or where the young ones on hatching, will have suitable conditions for their development. The number of eggs laid by a female varies (50 to few hundreds) with the insect species.
Majority of insects are oviparous that is young ones hatch from the eggs after they have been laid. However, in few insects (e.g. Aphids) the eggs develop within the uterus of the female and directly living young ones are deposited.
Different Forms of Eggs: Egg of different insects varies greatly in appearance. Most eggs arte spherical, over or elongate, but some are barrel shaped, some are disk-shaped and others are of other shapes. The egg is covered with a shell what varies in thickness, sculpturing and colour. Many eggs are provided with characteristic ridges, spines or other processes and some are brightly coloured.
Structure of typical insect egg:
i. Chorion / Egg shell: It is secreted by the cells of the follicular epithelium of the ovary and is composed of lipo-proteins (arranged in a number of layers) and is devoid of chitin. It is formed by two layers viz. Exochorion (outer) and Endochorion (inner). In some insects, there is a Wax layer coated on the exochorion as outer most layers.
ii. Micropyle: One (or more) small openings through the Chorion, usually at one end of the egg. Spermatozoa enter through the micropyle to fertilize the ovum. Besides, this opening serves as respiratory channel.
iii. Viteline membrane (cell wall): It is a delicate membrane, completely lining the egg shell from inner side which encloses the cytoplasm. Yolk and nucleus. Periplasm is another more delicate lining prevails from inner side of the vetelline membrane.
iv. Cytoplasm: It is the living substance of an egg.
v. Yolk/Deutoplasm: It is the non-living (lifeless) substance of an egg which consists of carbohydrates, proteins and lipids scattered as globules throughout the reticulum of the cytoplasm.
vi. Nucleus: It is highly organized dynamic part of the egg containing chromatin which forms the chromosomes. These are composed of large number of giant molecules of deoxyribonucleic acid (DNA), which are the genes, the bearer of heredity characters.