الأسبوع 19 · المحور الثالث «حماية كوكبنا»، الوحدة الثالثة «الموارد الموجودة على سطح الأرض»، المفهوم 3.2 «الماء كأهم الموارد الطبيعية على سطح الأرض» (ب): الماء العذب ومستجمع المياه · 3 دروس
3
Insects that tell if water is clean
Discovery Education Science P5: Term 2, Unit 3, pp. 48–53
GoalWrite an explanation of why water is a valuable resource, and use water insects as clues to how clean a stream is.
New words
aquatic biologist · a scientist who studies living things in waterlarva · a young insect that looks different from the adultnymph · a young insect that looks like a small adult without wings
Explanation
Water is a valuable resource because every living thing needs it, farms and factories use it, and only a small part of Earth’s water is fresh and easy to reach. A good scientific explanation has a claim, evidence and reasoning. Aquatic biologists study living things in water. They know that some insects can live only in clean water. Mayfly nymphs and caddisfly larvae are two such insects. Dragonfly and damselfly nymphs live in water with a little pollution. Midge larvae can live in poorer water. When a scientist finds many kinds of clean-water insects in a stream, the stream is probably healthy. When there are only the kinds that live in poor water, the stream is probably polluted. The insects are living evidence of the water’s quality. People near streams look from the bank with a grown-up and never collect from canals or drains.
Insect
Water it lives in
mayfly nymph, caddisfly larva
very clean water
dragonfly nymph, damselfly nymph
a little pollution
midge larva
poorer water
Worked examples
Part
What it says
Claim
Water is a valuable resource.
Evidence
Only about 1 part in 100 of fresh water is easy to reach, and all living things need water.
Reasoning
A resource that everyone needs and that is hard to reach must be used wisely.
An explanation about water as a resource.
Insect
Creek A
Creek B
mayfly nymphs
3
1
caddisfly larvae
2
0
midge larvae
1
5
A scientist finds these insects in two creeks. Creek A has many clean-water insects and Creek B has mostly a poor-water insect, so Creek A is cleaner.
count the insects→see which kinds live there→judge how clean the water is
Insects are clues, not proof. A scientist uses them together with other tests of the water before deciding.
Class activity
Creek detective cards (a grown-up helps; use only paper cards, no collecting from water). Write the name of each insect on cards, with several cards of each kind. Deal six cards for “Creek 1” and six for “Creek 2”. Sort the cards into clean-water, a-little-pollution and poor-water insects. Decide which creek is cleaner and write a claim, evidence and reasoning.
Practice
1
What does an aquatic biologist study?
Living things in water, such as insects, fish and plants.
2
Which insect lives only in very clean water: a mayfly nymph or a midge larva?
The mayfly nymph.
3
Which creek is cleaner? Give evidence from the table.
Animal
Creek 1
Creek 2
mayfly nymphs
4
0
caddisfly larvae
3
0
midge larvae
0
6
flatworms
0
2
Creek 1, because it has 4 mayfly nymphs and 3 caddisfly larvae, which need clean water, while Creek 2 has only midge larvae and flatworms.
4
Write a claim that answers: why is water a valuable natural resource?
For example: Water is valuable because all living things need it and only a small part of Earth’s water is fresh and easy to reach.
