Classification of Kingdom Animalia
Senior 2 · Biology
Classification of Kingdom Animalia
Lesson 4 of 5
22 min
Let's begin
Think of the termite mounds of Bugesera, the bees kept by honey cooperatives, a millipede curling into a ring after rain, the spider in the corner of your classroom, and the tiny crustaceans in Lake Kivu that isambaza feed on. All of them are arthropods.
About 85% of all known animal species are arthropods. They pollinate crops, recycle dead material, feed fish and birds, and also spread diseases such as malaria.
The name means "jointed foot". Every arthropod has:
An exoskeleton cannot grow. To get bigger, an arthropod must moult: it sheds the old exoskeleton, swells, and waits for a new, larger one to harden. Until then it is soft and easily attacked.
Many insects change form as they grow. In complete metamorphosis the stages are egg → larva → pupa → adult (butterflies, mosquitoes, houseflies, bees). In incomplete metamorphosis they are egg → nymph → adult, and the nymph looks like a small adult without wings (grasshoppers, cockroaches, termites).
Both have a head and a long trunk of many similar segments, one pair of antennae, and tracheae. They live on land in damp places.
| Class Diplopoda (millipedes) | Class Chilopoda (centipedes) | |
|---|---|---|
| Body shape | round (cylindrical) | flattened |
| Legs per segment | two pairs | one pair |
| Speed | slow | fast |
| Food | dead leaves and rotting plants | small animals (carnivores) |
| Defence | rolls into a coil; some release bad-smelling chemicals | poison claws behind the head; the bite is painful |
This short key sorts arthropods into their classes.
| 1a | Three pairs of legs | Insecta |
| 1b | More than three pairs of legs | go to 2 |
| 2a | Four pairs of legs, no antennae | Arachnida |
| 2b | More than four pairs of legs, antennae present | go to 3 |
| 3a | Two pairs of antennae, body covered by a carapace | Crustacea |
| 3b | One pair of antennae, long body of many similar segments | go to 4 |
| 4a | Two pairs of legs on each segment | Diplopoda |
| 4b | One pair of legs on each segment | Chilopoda |
Question: A farmer finds a small, flat-bodied animal with eight legs and no antennae on a cow's ear. Use the key to name its class.
Solution: It has eight legs, which is four pairs and more than three, so step 1 sends you to 2. Four pairs of legs and no antennae matches 2a, so it is in Arachnida. (It is a tick.)
Tip
Memory hook: insects are 3-3-2-1: 3 body parts, 3 pairs of legs, usually 2 pairs of wings and 1 pair of antennae. Spiders are 2-4-0: 2 body parts, 4 pairs of legs and 0 antennae.
Diplopods have two pairs of legs per segment, as in di- for two. Centipedes catch prey.
Explain why an arthropod must moult in order to grow.
Solution: Its exoskeleton is hard and cannot stretch or grow. To get bigger, the animal sheds it, expands its soft body, and then a new, larger exoskeleton hardens.
Is a spider an insect? Give three reasons.
Solution: No. A spider has two body parts (insects have three), four pairs of legs (insects have three) and no antennae (insects have one pair). It also has no wings.
A mosquito has only one pair of wings. Is it still an insect?
Solution: Yes. It has three body parts, three pairs of legs and one pair of antennae. The number of wings varies among insects, so it does not decide the class.
Give two differences between a millipede and a centipede.
Solution: Any two: a millipede has two pairs of legs per segment and a centipede one; a millipede is round and a centipede flat; a millipede eats dead plant material and a centipede eats animals; a millipede is slow and a centipede fast; a centipede has poison claws.
Name the class of each: crab, tick, termite, centipede.
Solution: crab: Crustacea; tick: Arachnida; termite: Insecta; centipede: Chilopoda.
Summary
Sources and credits
The Rwandan examples, worked examples and practice questions are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
