Growth and Development in Plants and Animals

Lessons

Senior 4 · Biology

Growth and Development in Plants and Animals

5 lessons

Lesson 5 of 5

Metamorphosis and growth patterns in animals

25 min

Wrigglers in the water jar

Leave a bucket or an old tyre full of rainwater outside for a week and you may see tiny "wrigglers" jerking through the water. They are mosquito larvae. They have no wings and no biting mouthparts, and they breathe air at the surface. Within days each one becomes a comma-shaped pupa, and then a flying adult emerges from it. In the Anopheles mosquito, the female adult can carry the malaria parasite. The World Health Organization (fact sheet "Malaria", December 2025) estimates 282 million malaria cases and 610 000 deaths worldwide in 2024, 95 % of them in the WHO African Region. Because the larva and the adult live in completely different ways, the mosquito can be attacked at two points of its life. That change of body form is metamorphosis, and it is one of the growth patterns in this lesson.

Definition

Metamorphosis: a marked change in body form during the development of an animal after hatching, as in insects and amphibians.

Complete metamorphosis (holometaboly): four stages, egg → larva → pupa → adult, in which the larva is very different from the adult. Incomplete metamorphosis (hemimetaboly): three stages, egg → nymph → adult, in which the nymph looks like a small, wingless adult.

Moulting (ecdysis): shedding the old exoskeleton so that the body can enlarge before the new one hardens. Each stage between two moults is an instar.

Intermittent growth: growth in a series of steps, as in arthropods. Continuous growth: growth without breaks, as in mammals.

Allometric growth: parts of the body grow at different rates, so body proportions change. Isometric growth: all parts grow at the same rate, so proportions stay the same.

Complete and incomplete metamorphosis

Figure 1. Complete and incomplete metamorphosis in insects.

Feature	Complete metamorphosis	Incomplete metamorphosis
Stages	Egg, larva, pupa, adult	Egg, nymph, adult
Young stage	Larva (caterpillar, maggot, grub, wriggler): different body, food and often habitat from the adult	Nymph: similar body and food to the adult, but smaller, with wing buds instead of wings
Pupa	Present: a non-feeding stage in which larval tissues are broken down and the adult body is built	Absent
Examples	Butterflies and moths, flies, mosquitoes, beetles, bees, ants	Grasshoppers, locusts, cockroaches, termites, dragonflies
Advantage	Larva and adult do not compete for the same food or space	The young can feed in the same way as the adult from the start

Amphibians also undergo metamorphosis: a tadpole that lives in water, feeds on algae and breathes with gills grows legs, loses its tail and gills, and becomes a frog that breathes air and eats insects.

Hormones control moulting and metamorphosis

An insect's exoskeleton cannot stretch, so it must moult to grow. Two hormones decide what each moult produces:

Ecdysone, the moulting hormone, is released from glands in the thorax when the brain signals that the insect is large enough. It starts every moult.
Juvenile hormone (JH) decides the result. With a high level of JH, a larva moults into a bigger larva; with a low level, the final larva moults into a pupa; with no JH, the pupa becomes an adult.

In amphibians, the hormone thyroxine from the thyroid gland triggers metamorphosis. Thyroxine needs iodine, the mineral you met in Unit 6: tadpoles kept without iodine stay as tadpoles.

Worked example 1: what juvenile hormone does

Juvenile hormone is made by a pair of small glands behind the insect's brain. Predict and explain the result of (a) removing these glands from a young larva, (b) implanting active glands into a final-instar larva.

(a) No juvenile hormone, so at its next moult the young larva turns into a pupa early, and then into a miniature adult.
(b) Juvenile hormone stays high, so at the next moult the larva becomes an extra, larger larva instead of a pupa: a giant larva.
Conclusion: ecdysone causes moulting, but the level of juvenile hormone decides whether the insect stays young or moves to the next stage.

Growth patterns in animals

Figure 2. Intermittent growth of an arthropod and continuous growth of a human (schematic).

Intermittent growth (arthropods): the body length rises in steps. Just after each moult the new cuticle is soft, the animal takes in air or water to stretch it, and it then hardens; between moults length stays almost constant, although tissues keep growing inside. Adult insects do not moult or grow any more.
Continuous growth (mammals and birds): growth is smooth, following a sigmoid curve, with growth spurts in infancy and at puberty in humans. Their growth is limited (determinate): it stops at adult size. Many fish, reptiles and trees show unlimited (indeterminate) growth, slowly growing throughout life.
Allometric growth: in humans, the head is about a quarter of body length at birth but only about an eighth in an adult, because the brain and head grow early while the legs grow most later.

Worked example 2: a rule for each moult

A grasshopper's body length was measured just after each moult (illustrative): 8, 11, 15, 21 and 29 mm. (a) Calculate the ratio of each length to the one before. (b) Predict the length after the next moult.

(a) $\frac{11}{8} = 1.38$ , $\frac{15}{11} = 1.36$ , $\frac{21}{15} = 1.40$ , $\frac{29}{21} = 1.38$ . The ratio is almost constant, about 1.4: each moult increases length by roughly 40 %.
(b) $29 \times 1.38 = 40\,\mathrm{mm}$ (2 s.f.).
A constant ratio means the steps get bigger as the insect gets bigger: growth at each moult is proportional to the size already reached.

Worked example 3: using the life cycle against malaria

The egg, larva and pupa of Anopheles all live in water and together last 5–14 days, depending on species and temperature. WHO reported 265 million of the world's 282 million malaria cases in 2024 in its African Region. (a) What percentage is that? (b) Explain why emptying containers of standing water every week reduces mosquitoes. (c) Why are treated bed nets still needed?

(a) $\frac{265}{282} \times 100 = 94\,\%$ (WHO rounds this to 95 %).
(b) The aquatic stages need at least about 5 days to become adults, so emptying water at least weekly kills larvae and pupae before any adults emerge.
(c) Mosquitoes also breed in ponds, swamps and rice fields that cannot be emptied, so adults will still appear; nets protect sleepers from the night-biting adult females. Attacking two stages of the life cycle works better than attacking one.

TED-Ed: "The life cycle of the butterfly" by Franziska Bauer. Watch what happens inside the pupa, where most of the caterpillar's body is broken down and the adult is built from groups of cells set aside in the larva.

Warning

Common mistakes

Saying the pupa is "resting". It does not feed or move much, but inside it the body is being completely rebuilt.
Calling a young grasshopper a larva. It is a nymph: grasshoppers have incomplete metamorphosis and no pupa.
Thinking adult butterflies keep growing. Adult insects do not moult, so they do not grow.
Saying arthropods only grow at moults. Tissues grow continuously inside; only the outer size increases in steps.

Tip

Memory hook. "Little People Arrive" for complete metamorphosis after the egg: Larva, Pupa, Adult. For the hormones: "Ecdysone = Exit the old skin; Juvenile hormone = Just stay young."

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Practice: metamorphosis and growth patterns

Compare complete and incomplete metamorphosis, giving two differences and an example of each.
State the roles of ecdysone and juvenile hormone in insect development.
Explain why the growth curve of a locust is a series of steps while that of a goat is smooth.
A locust measured 12 mm after one moult and 17 mm after the next. Calculate the percentage increase.
At birth a baby is 50 cm long with a head 12.5 cm long; as an adult the person is 172 cm tall with a head 21.5 cm long. Calculate how many times each grew, and say what kind of growth this shows.
Suggest one advantage of complete metamorphosis to an insect such as a butterfly.
Evaluate the claim: "Spraying adult mosquitoes is the only way to reduce malaria."

Show answersHide answers

Complete: four stages including a pupa; the larva is very different from the adult (butterfly, mosquito). Incomplete: three stages, no pupa; the nymph resembles a small, wingless adult (grasshopper, cockroach).
Ecdysone starts each moult; juvenile hormone decides its result: high JH gives another larval (or nymphal) stage, low JH a pupa, no JH the adult.
The locust has a rigid exoskeleton, so its size increases only just after each moult, before the new cuticle hardens; the goat has an internal skeleton that grows with the body, so growth is continuous.
$\frac{17 - 12}{12} \times 100 = 42\,\%$ (2 s.f.).
Head: $\frac{21.5}{12.5} = 1.72$ times; whole body: $\frac{172}{50} = 3.44$ times. Different parts grow at different rates, so this is allometric growth (the head goes from a quarter to an eighth of body length).
Larvae and adults use different foods and habitats (leaves against nectar), so they do not compete; the larva is a feeding and growing stage, the adult a dispersal and breeding stage.
Wrong. The larvae and pupae live in water, so removing standing water and treating breeding sites stops adults emerging; treated bed nets and repellents stop bites; and treating infected people with medicines cuts transmission. Combining methods works best.

Quick check

Put the stages of complete metamorphosis in order, starting with the egg.

Tap the cards in the right order.

Key points

Complete metamorphosis: egg → larva → pupa → adult; incomplete: egg → nymph → adult. Amphibians also metamorphose, under thyroxine.
Ecdysone starts each moult; juvenile hormone keeps the insect young (high JH: larva; low: pupa; none: adult).
Arthropods grow intermittently (steps at moults); mammals grow continuously along a sigmoid curve, with limited growth; many fish and reptiles grow throughout life. Allometric growth changes body proportions.
The aquatic larvae and pupae of mosquitoes are a target for malaria control, alongside bed nets and treatment.

1Wrigglers in the water jar

2Complete and incomplete metamorphosis

3Hormones control moulting and metamorphosis

4Growth patterns in animals

Wrigglers in the water jar

Complete and incomplete metamorphosis

Hormones control moulting and metamorphosis

Growth patterns in animals