What makes an animal an arthropod?
"Arthro" means joint and "pod" means foot, so arthropod means "jointed foot". Every arthropod has:
- Jointed legs: each leg bends at the joints, like your knee.
- Hard outer skeleton (exoskeleton): a tough coat made of chitin that protects the body and stops water loss.
- Segmented body: the body is made of parts joined in a row.
- No backbone: they are invertebrates.
More than 8 out of 10 known animal species are arthropods, so it is the biggest animal group.
Moulting: how a hard-skinned animal grows
The exoskeleton cannot stretch. When the body inside gets too big, the animal moults: it splits the old shell, crawls out with a soft new shell, and waits for it to harden. During that time it is weak. Try it in the 3D: grow the body until the shell turns red, then press Moult.
Insects and their orders
Insects have 3 body parts (head, thorax, abdomen), 6 legs on the thorax, 2 antennae and usually 2 pairs of wings. Main orders:
- Beetles (ladybird, dung beetle): hard front wings that cover the soft ones.
- Butterflies and moths: wings with tiny scales, sucking tube mouth.
- Flies and mosquitoes: only one pair of wings.
- Bees, wasps and ants: often live in colonies.
- Grasshoppers and crickets: strong back legs for jumping.
- True bugs (bed bug, stink bug): piercing mouth.
Many insects change shape in life: egg, larva (caterpillar), pupa, adult. This is called metamorphosis.
Arachnids: spiders, scorpions and ticks
Arachnids have 2 body parts (head-chest joined, and abdomen), 8 legs, and no antennae and no wings. Spiders make silk and many trap insects in webs. Scorpions have claws and a sting tail. Ticks and mites are tiny arachnids with a rounded body; ticks suck blood from animals and can spread disease, so check pets and wear covered clothes in tall grass.
Crustaceans: crabs, prawns and woodlice
Crustaceans have 2 pairs of antennae (4 in all), usually 10 legs (5 pairs, the first pair is often claws), and gills for breathing. Most live in water. Examples: crab, prawn, lobster, shrimp and water flea. Woodlice (the grey "pill bugs") are land crustaceans and live in damp places.
Try it
Try it: Find three small animals at home or in a garden (ant, fly, spider). Count their legs and body parts. Fill a table: legs, body parts, antennae, wings. Which are insects and which are arachnids? Then use the 3D picker and check your answer.
Key formulas and definitions
- Arthropod = jointed legs + exoskeleton + segmented body
- Insect: 3 body parts, 6 legs, 2 antennae, usually wings
- Arachnid: 2 body parts, 8 legs, 0 antennae, no wings
- Crustacean: usually 10 legs, 4 antennae, gills
- Insect life cycle: egg → larva → pupa → adult
Worked examples
1. An animal has 8 legs, 2 body parts and no antennae. Which group is it?
8 legs, 2 body parts, no antennae: arachnid (for example a spider).
2. A prawn has 10 legs and 4 antennae. Is it an insect?
No. Insects have 6 legs and 2 antennae. A prawn with 10 legs and 4 antennae is a crustacean.
3. Why do arthropods have to moult?
Their hard shell cannot stretch. When the body grows too big, they shed the old shell and form a bigger one.
4. A garden has 5 ants and 3 spiders. How many legs are there in all?
Ants: 5 × 6 = 30. Spiders: 3 × 8 = 24. Total = 54 legs.
5. A bug has 6 legs, 3 body parts and two pairs of wings, one pair hard and covering the other. What kind of insect is it?
Hard front wings that cover the soft pair are the sign of a beetle.
Common mistakes
- Calling a spider an insect. It has 8 legs and 2 body parts, so it is an arachnid.
- Forgetting that ticks and scorpions are also arachnids, not insects.
- Thinking a caterpillar is a different animal from the butterfly. It is the young stage of the same insect.
- Saying arthropods have bones. They have an outside skeleton and no backbone.