Fundamentals of Entomology

Eggs

• The first stage of development in all insects is egg. Majority of insects are oviparous. Egg stage is inconspicuous, inexpensive and inactive. Yolk contained in the egg supports the embryonic development. Eggs are laid under conditions where the food is available for feeding of the future Youngones. Eggs are laid either individually or in groups.
• The outer protective shell of the egg is called chorion. Near the anterior end of the egg, there is a small opening called micropyle which allows the sperm entry for fertilization. Chorion may have a variety of textures. Size and shape of the insect eggs vary widely.

TYPES OF EGGS
a) SINGLY LAID :
1) Sculptured egg : Chorion with reticulate markings and ridges e.g. Castor butterfly.
2) Elongate egg : Eggs are cigar shaped. e.g. Sorghum shoot fly.
3) Rounded egg: Eggs are either spherical or globular. e.g. Citrus butterfly
4) Nit : Egg of head louse is called nit. It is cemented to the base of the hair. There is an egg stigma at the posterior end, which assists in attachment. At the anterior end, there is an oval lid which is lifted at time of  hatching.
5) Egg with float : Egg is boat shaped with a conspicuous float on either side. The lateral sides are expanded. The expansions serve as floats. e.g. Anopheles mosquito.
b) EGGS LAID IN GROUPS :
1) Pedicellate eggs : Eggs are laid in silken stalks of about 1.25mm length in one groups on plants. e.g. Green lacewing fly.
2) Barrel shaped eggs : Eggs are barrel shaped. They look like miniature batteries. They are deposited in compactly arranged masses. e.g. Stink bug.
3) Ootheca (Pl. Oothecae) : Eggs are deposited by cockroach in a brown bean like chitinous capsule. Each ootheca consists of a double layered wrapper protecting two parallel rows of eggs.Each ootheca has 16 eggs arranged in two rows. Oothecae are carried for several days protruding from the abdomen of female prior to oviposition in a secluded spot. Along the top,there is a crest which has small pores which permit gaseous exchange without undue water loss. Chitinous egg case is produced out of the secretions of colleterial glands.
4) Egg pod : Grasshoppers secrete a frothy material that encases an egg mass which is deposited in the ground. The egg mass lacks a definite covering. On the top of the egg,the frothy substance hardens to form a plug which prevents the drying of eggs.
5) Egg cass : Mantids deposit their eggs on twigs in a foamy secretion called spumaline which eventually hardens to produce an egg case or ootheca. Inside the egg case, eggs are aligned in rows inside the egg chambers.
6) Egg mass: Moths lay eggs in groups in a mass of its body hairs. Anal tuft of hairs found at the end of the abdomen is mainly used for this purpose. e.g. Rice stem borer. Female silk worm moth under captivity lays eggs on egg card. Each egg mass is called a dfl (diseases free laying).
 7) Eff raft : In Culex mosquitoes, the eggs are laid in a compact mass consisting of 200-300 eggs called egg raft in water.