Egg

Definition

The first developmental stage in the life cycle of most insects, arachnids, and other oviparous animals, consisting of a fertilized ovum enclosed within protective layers. In arthropods, the egg typically comprises a yolk-laden oocyte surrounded by the vitelline membrane and an external chorion that may be smooth, sculptured, or elaborately structured to resist desiccation, parasitoids, or mechanical damage. Eggs may be deposited singly, in clusters, or enclosed within protective cases (oothecae); oviposition site selection is often tightly linked to larval or nymphal food requirements.

Etymology

From Middle English egge, from Old Norse egg, cognate with similar Germanic terms for 'egg'.

Example

Female monarch butterflies (Danaus plexippus) deposit their ribbed, conical eggs singly on the undersides of milkweed leaves, ensuring that the emerging caterpillars have immediate access to their obligate host plant.

Synonyms

• Ovum

Related Terms

• oviparity
• ovipositor
• Chorion
• Ootheca
• Eclosion
• larva
• nymph
• Fecundity
• Vitelline membrane
• embryogenesis

Usage Notes

In entomological usage, 'egg' refers to the entire structure laid by the female, while 'ovum' strictly denotes the female gamete before fertilization and encapsulation. The term is absolute in developmental sequence (always precedes larva/nymph) but relative in duration and morphology across taxa—dipteran eggs may hatch in hours, while diapause eggs of temperate insects can overwinter for months. Desiccation susceptibility drives much of the diversity in egg chorion structure and oviposition microhabitat selection.