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Insects: brain hormone triggers the prothoracic gland to release ecdysone, the moulting hormone, which causes a new cuticle to form and the old one to be shed. Juvenile hormone keeps the larval form while it circulates; when its level falls, the larva moults into a pupa and then an adult.

Inside the pupa the larval body breaks down, and imaginal discs (clusters of cells formed in the embryo) grow into adult legs, wings, antennae and mouthparts. The new adult pumps air and fluid into its wings to unfurl them before they harden.

Amphibians: thyrotropic hormone from the pituitary activates the thyroid gland to make thyroxine, which drives metamorphosis. The tadpole loses its gills and larval mouthparts; the tail muscles degenerate and the tail cells are destroyed and eaten by phagocytic cells.

Experiment: Modelling survival through a life cycle

1. Start with 100 counters: these are eggs. The survival rates in this model are made up to show the idea; they are not real data.
2. Egg → tadpole: for each counter roll the dice; a 1, 2 or 3 means it does not survive (50% survive). Count the survivors.
3. Tadpole → froglet: a 1 to 4 means it does not survive (about 33% survive). Count again.
4. Froglet → breeding adult: a 1 to 5 means it does not survive (about 17% survive). Count again.
5. Plot survivors (y) against stage (x). Calculate the overall survival as a percentage of the eggs and compare with the expected value 100 × 1/2 × 1/3 × 1/6.
6. Pool the class results and compare the spread between groups with the class mean. Why do small samples vary so much?

Stage	Survivors (my group)	Expected	Class mean
Eggs
Tadpoles
Froglets
Breeding adults

Questions

1. Compare how hormones control metamorphosis in insects and in amphibians.

2. Calculate the expected number of breeding adults from 500 eggs using the model survival rates (1/2, 1/3, 1/6). Explain why your group's result may differ.

3. A chick hatches on day 21 and feathers are visible after day 10. What percentage of incubation has passed when feathers appear?

4. Explain why producing huge numbers of eggs can be a successful strategy for frogs, even though most young die.

Sources for every fact on this sheet: echoesof.science/school/animal-life-cycles/sources/

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