Every fact in the three animal life cycle lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the facts are simplified or less certain: the times for each stage are typical values and vary with species and conditions. The frog timings are for the common frog in the UK [2]: cold water or plenty of food can delay metamorphosis by months, and predators can speed it up. Butterfly times differ greatly between species [1]. The chick timings (heart beating at about 42–44 hours, hatching on day 21) are for chickens kept at the right incubation conditions [6]. The survival model in the 15–16 lesson uses made-up survival rates to show the idea; it is not real data for any species. The hormonal control of metamorphosis is described in a simplified way [5].
All sources
[1] The Butterfly Life Cycle Explained
American Museum of Natural History · Reliability: High
Type: Natural history museum
Four stages: egg, larva (caterpillar), pupa (chrysalis) and adult. Females lay eggs on leaves; the eggs are about the size of a pin and typically hatch in a week or two. Caterpillars eat plant material and can grow to about 1,000 times their birth weight; some species take just a few weeks, the Carpenter Worm up to three years. Inside the chrysalis the caterpillar's body breaks down and reorganises; the adult emerges after several weeks. Adults mostly search for mates, and many butterfly species live for less than a month.
Natural History Museum, London · Reliability: High
Type: Natural history museum
Common frogspawn appears in shallow still water from around February. Tadpoles hatch after about three weeks. Young tadpoles eat algae, then leaves, moss and sometimes small insects; they breathe through gills, external in younger tadpoles. About 16 weeks after hatching the legs begin to form, followed by the arms; the tail is gradually absorbed and lungs form. Cold or abundant food can delay this by months; predators such as fish can speed it up. Froglets leave the pond and eat invertebrates. Adults live around five to 10 years and are ready to breed at about two to three years. Frogs lay large numbers of eggs, which increases the chance that some survive to adulthood.
Holometabolous insects undergo complete change: egg, larva, an inactive pupa, adult (for example beetles, butterflies and moths, bees and wasps, flies). Hemimetabolous insects undergo partial or incomplete change (for example grasshoppers and crickets, praying mantids, cockroaches, dragonflies); their young are called nymphs and gradually increase in size and change form. As an insect grows it sheds its skin (moulting).
[4] Insect Metamorphosis: The Key to a Fresh New Start
Carnegie Museum of Natural History · Reliability: High
Type: Natural history museum
In incomplete metamorphosis the insect moults through increasingly large, adult-like stages (grasshoppers, stink bugs, dragonflies, termites, mantises). In complete metamorphosis the larva goes through an inactive pupal stage (beetles, ants, bees, wasps, flies, moths). In 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.
Metamorphosis is the transformation of the larva into an adult. In amphibians, thyrotropic hormone from the pituitary (hypophysis) activates the thyroid to make thyroxine, which drives metamorphosis; the tadpole loses its gills and larval mouthparts, the tail muscles degenerate and tail cells are destroyed and eaten by phagocytic cells. In insects, brain hormone triggers the prothoracic gland to release the moulting hormone ecdysone, which causes a new cuticle to form and the old one to be shed; juvenile hormone keeps the larval form, and its decline lets the larva become a pupa and then an adult.
Mississippi State University Extension · Reliability: High
Type: University extension service
The chick hatches on day 21, after about three weeks of incubation. The eye begins at about 24 hours; the heart starts beating at about 42–44 hours; wing and leg buds are visible at the end of day 3; digits appear by day 7; feathers become visible after day 10; the embryo is in hatching position by day 20.
Mammals have mammary glands which produce milk. Most mammals give birth to live young, except the monotremes (platypus and echidnas), which lay soft-shelled eggs. Marsupials give birth to poorly developed young which are then carried in a pouch.