Metamorphosis is the process by which insects change their shape and form as they grow from eggs to adults. There are two types of metamorphosis:

Complete Metamorphosis

This type has four stages:

1. Egg — The insect starts as an egg.
2. Larva — The egg hatches into a larva (a worm-like stage that feeds and grows).
3. Pupa — The larva forms a protective case and changes inside.
4. Adult — The fully grown insect comes out.

Houseflies and mosquitoes go through complete metamorphosis.

Incomplete Metamorphosis

This type has three stages:

1. Egg — The insect starts as an egg.
2. Nymph — The egg hatches into a nymph, which looks like a small adult but without wings.
3. Adult — The nymph grows wings and becomes an adult.

Cockroaches go through incomplete metamorphosis.

The housefly undergoes complete metamorphosis with four stages:

1. Egg: The female housefly lays eggs in dirty places such as feces, food waste, and decaying matter. A single female can lay hundreds of eggs.

2. Larva: Within 24 hours, the eggs hatch into larvae (maggots). Larvae feed on the decaying material for about seven days, growing rapidly.

3. Pupa: The larva then changes into a pupa, which is enclosed in a hard, dark case called a puparium. During this stage, the insect does not feed. It takes about six days for the pupa to develop into an adult.

4. Adult: The adult housefly emerges from the pupa. It can fly and feed on various substances, including human food.

Worked Example: Observing Housefly Development

A student placed a piece of raw meat on a plate and left it in an open area where houseflies could land.

• Day 1: The student observed tiny white eggs on the meat.
• Day 2: The eggs hatched into small white larvae (maggots) that were moving on the meat.
• Day 7: The larvae stopped moving and turned into brown pupae.
• Day 13: Adult houseflies emerged from the pupae and started flying around.

This activity shows how houseflies develop from egg to adult through complete metamorphosis.

Mosquitoes also undergo complete metamorphosis:

1. Egg: Female mosquitoes lay eggs on or near stagnant water. The eggs are very small and can float on the water surface.

2. Larva: The eggs hatch into larvae (sometimes called wigglers). Larvae live in water and come to the surface to breathe. They feed on tiny organisms in the water.

3. Pupa: The larva changes into a pupa (called a tumbler). The pupa does not feed but moves in the water. This stage lasts about two days.

4. Adult: The adult mosquito emerges from the pupa and rests on the water surface until its wings dry and harden. Only female mosquitoes bite humans to get blood for laying eggs.

Worked Example: Collecting Mosquito Larvae

A student collected water from a stagnant puddle near the school compound. Using a hand lens, the student observed:

• Tiny black dots floating on the water surface (eggs)
• Small wiggling creatures moving in the water (larvae)
• C-shaped creatures that tumbled in the water (pupa)

After a few days, the student observed adult mosquitoes flying out of the container. This shows that mosquitoes need stagnant water to breed.

Cockroaches undergo incomplete metamorphosis with three stages:

1. Egg: Female cockroaches lay eggs in a protective case called an ootheca. They hide the eggs in dark, hidden places such as cracks, behind furniture, or in cupboards.

2. Nymph: The eggs hatch into nymphs. Nymphs are smaller versions of adult cockroaches but they do not have wings. Their outer covering is soft. Nymphs molt (shed their skin) several times as they grow.

3. Adult: After about three months, the nymph develops wings and becomes an adult cockroach. Adult cockroaches can fly (some species) and reproduce quickly.

These insects are called vectors because they carry disease-causing germs:

• Houseflies land on dirty places like feces and garbage, then land on food. They carry bacteria that cause cholera and diarrhea.

• Mosquitoes spread malaria (by female Anopheles mosquito) and other diseases when they bite humans.

• Cockroaches crawl in dirty places and can carry bacteria on their bodies. They contaminate food and utensils.

You can investigate these life cycles through simple activities:

For Houseflies

1. Place a piece of raw meat or fruit on a plate.
2. Leave the plate in an open area where flies can land.
3. Use a hand lens to observe changes daily.
4. Record what you see at each stage (egg, larva, pupa, adult).

For Mosquitoes

1. Collect water from a stagnant puddle or unused container.
2. Look for tiny wiggling creatures (larvae) in the water.
3. Observe them over several days to see them change into pupae and then adults.

For Cockroaches

1. Observe dark, hidden places like behind cupboards or in corners.
2. Look for cockroach eggs (ootheca) or nymphs.
3. Record your observations and compare with adult cockroaches.

• Keep your home clean — Remove garbage and food waste properly.
• Destroy breeding places — Empty containers that hold water, clear bushes and long grass.
• Use mosquito nets — Sleep under treated mosquito nets to prevent malaria.
• Cover food — Keep food covered to prevent flies from landing on it.
• Use insecticides — Apply insecticides to kill cockroaches in hiding places.

Understanding the life cycles of insect vectors helps us protect our families from diseases. In Tanzania, during the rainy season, stagnant water collects in puddles and containers around homes, providing breeding sites for mosquitoes. By emptying these containers and covering water storage tanks, we can reduce the number of mosquitoes and prevent malaria. This knowledge is used in community health programs where villagers are taught to keep their surroundings clean to stop insects that spread cholera and other diseases.