Life Cycles
What is a Life Cycle?
A life cycle describes all the stages of an organism's life from birth (or germination) through growth, reproduction, and death. Reproduction restarts the cycle for the next generation. Life cycles vary greatly between different types of organisms — some are simple and direct, others involve dramatic transformations.
Insect Life Cycles
Insects show two main types of metamorphosis — transformation during development.
Complete Metamorphosis (Holometabolous)
Four distinct stages: Egg → Larva → Pupa → Adult
Examples: butterflies, moths, beetles, flies, bees, ants
- Egg: laid by the adult; may be in clusters or individually
- Larva: the feeding stage (caterpillar, grub, maggot); grows rapidly through several instars (moults)
- Pupa (chrysalis or cocoon): inactive transformation stage; body is completely reorganised
- Adult (imago): sexually mature; wings present; primary role is reproduction
The larva and adult look completely different and often eat different foods, reducing competition between life stages.
Incomplete Metamorphosis (Hemimetabolous)
Three stages: Egg → Nymph → Adult
Examples: grasshoppers, locusts, dragonflies, cockroaches
- Nymph: resembles a small adult; lacks wings and sexual organs; moults several times
- Each moult the nymph grows larger and develops wing buds
- Adult: fully winged and sexually mature
There is no pupal stage. The nymph and adult occupy similar habitats and eat similar food.
Frog Life Cycle
Frogs show a dramatic aquatic-to-terrestrial transformation:
Frogspawn (eggs) → Tadpole → Froglet → Adult frog
- Frogspawn: eggs laid in water, surrounded by jelly for protection
- Tadpole: aquatic larva; breathes through gills; has a tail; feeds on algae
- Froglet: hind legs develop, then front legs; tail gradually absorbed; begins breathing with lungs; moves onto land
- Adult frog: fully terrestrial; breathes through lungs and moist skin; returns to water to reproduce
This metamorphosis is triggered by hormones (thyroxine).
Plant Life Cycle
Flowering plants (angiosperms) follow this cycle:
Seed → Germination → Seedling → Mature plant → Flowering → Pollination → Fertilisation → Fruit and seed production
- Germination: seed absorbs water and splits; requires water, warmth, and oxygen (not light initially)
- Seedling: first leaves (cotyledons) appear; begins photosynthesis
- Mature plant: grows to full size; develops flowers
- Pollination: pollen transferred from anther to stigma (by wind, insects, or other vectors)
- Fertilisation: pollen nucleus fuses with ovule; embryo forms
- Seed dispersal: seeds spread by wind, animals, water, or explosion
Human Life Cycle
| Stage | Approximate age | Key features |
|---|---|---|
| Embryo | 0–8 weeks | Rapid cell division; major organs forming |
| Foetus | 8 weeks – birth | Growth and development of organs |
| Infancy | 0–2 years | Rapid physical growth; learning to walk and talk |
| Childhood | 2–12 years | Continued growth; language and social development |
| Adolescence | 12–18 years | Puberty; sexual maturity; growth spurt |
| Adulthood | 18–65 years | Reproduction possible; peak physical capacity |
| Old age | 65+ years | Declining physical function; eventual death |
Puberty in adolescence involves hormonal changes that cause physical development (secondary sexual characteristics) and the onset of fertility.
Comparing Life Cycles
| Feature | Complete metamorphosis | Incomplete metamorphosis | Frog |
|---|---|---|---|
| Number of stages | 4 | 3 | 4 |
| Pupal stage | Yes | No | No (froglet instead) |
| Larva resembles adult? | No | Yes (nymph) | No |
| Environment changes | Sometimes | Rarely | Yes (water to land) |
Key Terms
- Life cycle — the sequence of stages an organism passes through from birth to reproduction and death
- Metamorphosis — transformation during development
- Larva — the juvenile feeding stage in complete metamorphosis
- Pupa — the transformation stage in complete metamorphosis
- Nymph — the juvenile stage in incomplete metamorphosis
- Germination — the sprouting of a seed when conditions are favourable
- Pollination — transfer of pollen from anther to stigma
Common Mistakes
- Confusing the pupa (transformation stage) with the larva (feeding stage)
- Thinking a nymph is the same as a larva — nymphs resemble adults; larvae do not
- Saying plants need light to germinate — seeds only need water, warmth, and oxygen; light is needed once photosynthesis begins
- Forgetting that the froglet stage exists between tadpole and adult frog
Tips and Tricks
- Remember ELPA for complete metamorphosis: Egg, Larva, Pupa, Adult
- For incomplete metamorphosis: Egg, Nymph, Adult (ENA — no pupa)
- Frog life cycle involves a habitat switch: water (tadpole) to land (adult) — a useful distinguishing feature
- Human stages in order: Embryo → Foetus → Infant → Child → Adolescent → Adult → Old age