Growth patterns in animals
Animals grow until they reach adult size. Growth is isometric when all parts grow at the same rate, and allometric when some parts grow faster than others, as in the human head, which is large in a baby and relatively smaller in an adult. Humans show a sigmoid pattern with two fast periods: infancy and puberty.
Growth in arthropods
The hard exoskeleton of an insect cannot stretch, so the animal grows by moulting (ecdysis). The old skin is shed and the new one expands before it hardens, so growth happens in steps.
Metamorphosis
In complete metamorphosis the life cycle has four stages: egg, larva, pupa and adult. The larva (caterpillar, maggot, wriggler) is different from the adult and feeds, while the pupa is a resting stage in which the body is rebuilt. This is found in butterflies, flies and mosquitoes. In incomplete metamorphosis the egg hatches into a nymph that looks like the adult and grows through several moults, as in grasshoppers and cockroaches. There is no pupa.
| Feature | Complete | Incomplete |
|---|---|---|
| Stages | Egg, larva, pupa, adult | Egg, nymph, adult |
| Young | Larva, unlike the adult | Nymph, like a small adult |
| Examples | Butterfly, housefly, mosquito | Grasshopper, cockroach, termite |
Metamorphosis is controlled by hormones: ecdysone triggers moulting and juvenile hormone keeps the animal in the larval stage until the right time.
A larva grows fast, then a pupa stage follows. Choose the starting mass and the daily increase.
Key terms
- Allometric growth
- Different body parts growing at different rates.
- Moulting
- Shedding of the exoskeleton.
- Larva
- A feeding stage that differs from the adult.
- Nymph
- A young stage that resembles the adult.
Summary
- Animal growth is sigmoid, with different parts growing at different rates.
- Arthropods grow by moulting.
- Complete metamorphosis has a pupa stage; incomplete has nymphs.
- Hormones ecdysone and juvenile hormone control moulting.