School › Animal life cycles › Ages 10–11
Life cycles and metamorphosis
Compare the life cycles of insects, amphibians, birds and mammals. Learn the difference between complete and incomplete metamorphosis, and make a timeline to scale of a frog's life.
Read
Metamorphosis is the change of a young animal (a larva) into an adult. Butterflies and frogs both go through metamorphosis.[5][1]
Some insects go through complete metamorphosis: egg → larva → pupa → adult. The pupa is an inactive stage. Butterflies, moths, beetles, bees, ants and flies develop like this.[3][4]
Other insects go through incomplete metamorphosis: egg → nymph → adult. Nymphs look like small adults; they grow by moulting (shedding their skin). Grasshoppers, crickets, cockroaches and dragonflies develop like this.[3][4]
A caterpillar can grow to about 1,000 times its birth weight. Inside the chrysalis its body breaks down and reorganises, and many adult butterflies live for less than a month.[1]
Common frog tadpoles hatch about 3 weeks after the spawn is laid and breathe with gills. About 16 weeks after hatching the legs start to form, then the arms; the tail is absorbed and lungs form. Frogs are ready to breed at about 2 to 3 years old.[2]
Birds lay eggs: a chick's heart starts beating after about 2 days, and it hatches on day 21. Most mammals give birth to live young and feed them milk; the platypus and echidnas lay eggs, and marsupials carry their young in a pouch.[6][7]
Try it: A frog's life to scale
You need
- A long strip of paper (at least 2 m, or tape several sheets together)
- Metre stick or tape measure
- Coloured pens
- Calculator
Safety
Use a clear floor or corridor and keep the strip flat so nobody trips. This is a modelling and planning activity: do not take frogspawn or tadpoles from the wild.
What to do
- Use the scale 1 cm = 1 week. Mark 'spawn laid' at 0 cm at one end of the strip.
- Tadpoles hatch after about 3 weeks: mark them at 3 cm.
- Legs begin about 16 weeks after hatching: how many weeks is that from the spawn? Mark it.
- A frog is ready to breed at about 2 years (1 year ≈ 52 weeks). Work out how many cm that is and mark it.
- Colour the part of the strip when the frog lives in water and the part when it can live on land.
- Discuss: the NHM says cold water can delay metamorphosis by months. How would you plan a fair test of whether temperature changes how fast tadpoles develop? (Plan only: do not collect tadpoles.)
What happens, and why?
Hatching is at 3 cm and legs start at 3 + 16 = 19 cm, but breeding at 2 years is 2 × 52 = 104 cm along the strip. Most of a frog's life is spent as a growing froglet and adult, not as a tadpole. A fair test would change only the temperature and keep the food, water volume, light and number of tadpoles the same.[2]
Play: Complete or incomplete?
Does this insect go through complete metamorphosis (with a pupa) or incomplete metamorphosis (with nymphs)?
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Quiz
Other ages: Ages 6–7 · Ages 15–16