Animal Life Cycles✓ Updated 2026
Master animal life cycles for PSLE Science. Complete vs incomplete metamorphosis, Singapore examples, stage-by-stage explanations, and exam tips for P3/P4 students.
What Is a Life Cycle?
Every animal goes through a series of stages from birth to death to the birth of the next generation. This repeating sequence is called a life cycle. For many insects and amphibians, this involves dramatic physical transformation — a process called metamorphosis (from the Greek word for "change of form").
There are two types of metamorphosis in the PSLE syllabus: complete metamorphosis (4 stages, where the young looks completely different from the adult) and incomplete metamorphosis (3 stages, where the young looks like a miniature version of the adult). The key difference is the presence or absence of a pupa stage.
Life Cycles You Can Observe in Singapore
The Aedes mosquito — the one that spreads dengue fever in Singapore — is a perfect example of complete metamorphosis. The female lays eggs on the walls of containers with still water. When water touches the eggs, they hatch into larvae (wrigglers) that live in the water and feed on micro-organisms. They then become pupae (tumblers) that do not feed, and finally emerge as adult mosquitoes. This is why NEA's dengue prevention campaigns focus on removing stagnant water — you are destroying the larval stage and preventing the cycle from completing.
The common grasshopper found in parks like Bishan-Ang Mo Kio Park undergoes incomplete metamorphosis. The egg hatches into a nymph that already looks like a small adult grasshopper — same body plan, just smaller and without fully developed wings. It moults (sheds its exoskeleton) several times as it grows, and eventually becomes a fully winged adult.
Butterflies at the Butterfly Garden on Sentosa and in the Botanic Gardens undergo complete metamorphosis. The caterpillar (larva) you see munching leaves will eventually form a chrysalis (pupa) before emerging as an entirely different-looking adult butterfly that drinks nectar instead.
Egg → Larva → Pupa → Adult
In complete metamorphosis, the four stages are so different from each other that the larva and adult look like completely separate organisms. Here is what happens at each stage:
- Egg: The fertilised egg is laid by the female, usually near a food source suitable for the larva. Butterfly eggs are often laid on specific plant leaves that the caterpillar will eat.
- Larva: The main feeding and growing stage. The larva (caterpillar, maggot, or grub depending on the species) eats constantly to build up energy reserves. It does not look like the adult at all. This is the stage that causes the most crop damage in agriculture.
- Pupa (chrysalis or cocoon): The resting and transformation stage. The pupa does not eat or move significantly. Inside, the body is completely reorganised — tissues are broken down and rebuilt into the adult body plan. This is one of nature's most remarkable processes.
- Adult: Emerges fully formed and capable of reproduction. The diet is often completely different from the larva (caterpillar eats leaves; butterfly sips nectar). The adult's main biological purpose is to reproduce and start the cycle again.
Examples: butterfly, moth, housefly, bee, wasp, ant, beetle, mosquito. The frog also undergoes complete metamorphosis: tadpole (aquatic larva) → froglet → frog.
Egg → Nymph → Adult
In incomplete metamorphosis, there are only three stages and no pupa stage. The young, called a nymph, already resembles the adult in body plan — it has the same number of legs, the same diet, and lives in the same habitat. The main differences are size and the absence of fully developed wings.
- Egg: Fertilised egg laid by the female adult.
- Nymph: Hatches from the egg and already looks like a small adult. The nymph moults (sheds its hard exoskeleton) multiple times as it grows. Each stage between moults is called an instar. Wing buds gradually develop with each moult.
- Adult: Fully winged and capable of reproduction. No dramatic transformation — just a gradual increase in size and wing development.
Examples: grasshopper, cockroach, cricket, praying mantis, dragonfly, termite.
Why Did Two Types of Metamorphosis Evolve?
Complete metamorphosis evolved because it solves a critical problem: competition between young and adults of the same species. A caterpillar eats leaves; the adult butterfly drinks nectar. They live in different microhabitats and eat completely different food. This means the young and the adult are not competing with each other for the same resources — doubling the species' chances of finding enough food to survive.
The pupa stage, while it looks like a vulnerable and wasteful period, is actually brilliant. It allows the organism's body plan to be completely rebuilt to suit a totally different lifestyle. No other method allows such a dramatic change in body structure.
Incomplete metamorphosis, on the other hand, is simpler and more energy-efficient. The nymph can start feeding and growing almost immediately after hatching without going through a helpless pupal stage. For organisms like grasshoppers, where the juvenile and adult eat the same food and live in the same place, there is no benefit to complete metamorphosis.
Complete vs Incomplete — Side by Side
| Feature | Complete metamorphosis | Incomplete metamorphosis |
|---|---|---|
| Number of stages | 4 | 3 |
| Stages | Egg → Larva → Pupa → Adult | Egg → Nymph → Adult |
| Pupa stage? | Yes | No |
| Young called | Larva (caterpillar/maggot/grub) | Nymph |
| Young resembles adult? | No — completely different | Yes — similar body plan |
| Same diet as adult? | Usually different | Same diet |
| Examples | Butterfly, mosquito, fly, bee, beetle | Grasshopper, cockroach, dragonfly |
Common Mistakes
Key Points at a Glance
- Complete metamorphosis: 4 stages — Egg, Larva, Pupa, Adult. Young looks nothing like adult
- Incomplete metamorphosis: 3 stages — Egg, Nymph, Adult. No pupa stage
- The KEY difference: presence of a PUPA stage = complete metamorphosis
- Nymph = young of incomplete metamorphosis. Looks like small adult, moults as it grows
- Frog = complete metamorphosis (tadpole is the larva)
- Larva and adult in complete metamorphosis often eat different food and live in different places
🥑 Digestive System — Path of Food
Know every organ and its specific function. Vague answers lose marks in PSLE Section B.
| Organ | Function | Food type broken down |
|---|---|---|
| 🗣 Mouth | Chews food; saliva starts digestion | Carbohydrates (starch only) |
| Oesophagus | Muscular tube; pushes food down to stomach | None (transport only) |
| 😀 Stomach | Churns food; acid kills bacteria; enzymes digest | Proteins only |
| 🪐 Small Intestine | Digests all food; absorbs nutrients into blood via villi | ALL 3 types (with help of pancreas & liver) |
| Large Intestine | Absorbs water from undigested food | None (water absorption only) |
| Liver | Produces bile; emulsifies (breaks up) fats | Fats (emulsification only) |
| Pancreas | Produces enzymes that digest all food types | ALL 3 types ← KEY FACT |
🫡 Respiratory System — Gas Exchange
Gas exchange happens in the alveoli. O2 enters blood, CO2 leaves blood.
📄 Life Cycles: Complete vs Incomplete Metamorphosis
Examples: butterfly, moth, housefly, mosquito, beetle
Larva looks different from adult. Has pupa stage.
Examples: grasshopper, cockroach, dragonfly, praying mantis
NO pupa stage. Nymph resembles small adult.
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