Science · Grades 3–6

Life Cycle of Animals

Reading comprehension lesson about animal life cycles — from egg to adult, and the dramatic transformations of metamorphosis.

Reading Passage

Life Cycle of Animals

All animals go through a life cycle — the series of stages from birth to maturity to reproduction and death. Life cycles vary enormously across the animal kingdom. Some animals look like miniature versions of their parents from birth; others undergo dramatic transformations called metamorphosis before reaching their adult form. Understanding life cycles helps scientists predict population changes, protect endangered species, and control pests.

Complete metamorphosis, or holometabolism, involves four distinct stages: egg, larva, pupa, and adult. Butterflies are a classic example. A butterfly lays eggs on a host plant. The eggs hatch into caterpillars (larvae) that eat constantly and grow rapidly. The caterpillar then forms a chrysalis (pupa), inside which its body is broken down and completely reorganized into the adult butterfly form. The adult butterfly emerges, mates, and lays eggs, completing the cycle. Beetles, flies, bees, and ants also undergo complete metamorphosis.

Incomplete metamorphosis, or hemimetabolism, involves three stages: egg, nymph, and adult. Grasshoppers are a familiar example. A grasshopper egg hatches into a nymph that looks like a small wingless version of the adult. The nymph goes through several molts — shedding its outer skeleton to grow — gradually developing wings and reproductive organs until it reaches its adult form. Dragonflies, cockroaches, and true bugs also undergo incomplete metamorphosis.

Mammals have a different type of life cycle. Most mammals give birth to live young and nurse them with milk. The young of different mammal species vary greatly in how developed they are at birth. Foals and calves can stand and run within hours; human babies are helpless for months. Marsupials like kangaroos give birth to tiny, undeveloped young that continue developing in a pouch. All mammals share the same general stages: birth, growth to sexual maturity, reproduction, and eventually death.

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