Arthropod Classification: Insects, Arachnids, Crustaceans and Myriapods
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Arthropods have an exoskeleton and jointed legs. The four groups are insects (3 pairs of legs, 3 body parts), arachnids (4 pairs of legs, 2 body parts), crustaceans (more than 4 pairs of legs, 2 body parts) and myriapods (many legs, segmented body).
Arthropods are the largest animal phylum, and the 0610 syllabus requires you to distinguish four groups. The key features are the number of leg pairs, body segments and the presence of wings or antennae. Examiners test this through diagrams or descriptions of unfamiliar arthropods.
Comparing the Four Arthropod Groups
Feature
Insects
Arachnids
Crustaceans
Myriapods
Leg pairs
3 (6 legs)
4 (8 legs)
More than 4 pairs
Many pairs
Body sections
3 (head, thorax, abdomen)
2 (cephalothorax, abdomen)
2 (cephalothorax, abdomen)
2 (head, long segmented body)
Wings
Usually 1 or 2 pairs
None
None
None
Antennae
1 pair
None
2 pairs
1 pair
Examples
Butterfly, beetle, mosquito
Spider, scorpion, tick
Crab, lobster, woodlouse
Centipede, millipede
Shared Arthropod Features
All arthropods share:
Exoskeleton: a hard outer covering made of chitin that protects and supports the body. It must be shed (moulted) to allow growth.
Jointed legs: the defining feature of the phylum (arthropod means "jointed foot").
Segmented body: the body is divided into distinct sections.
Bilateral symmetry: the left and right sides are mirror images.
Exam Classification Strategy
When given an arthropod diagram, count the legs first:
6 legs → Insect. Confirm by checking for 3 body sections and wings.
8 legs → Arachnid. Confirm by checking for no antennae and 2 body sections.
10 or more legs, 2 body sections → Crustacean.
Many legs, long segmented body → Myriapod.
Tip: Spiders are arachnids, not insects. This is one of the most common errors in classification questions. Spiders have 8 legs and no antennae.
Try a focused question (2 marks)
An organism has an exoskeleton, 8 jointed legs, 2 body sections and no antennae. Identify the arthropod group and give one example. (2 marks)
Show the answer and marking guidance
Arachnid.
Example: spider / scorpion / tick.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Calling spiders insects: spiders are arachnids with 8 legs, not 6.
Forgetting that insects have 3 body parts (head, thorax, abdomen) while arachnids have 2.
Confusing crustaceans and myriapods: crustaceans typically have 2 body sections, myriapods have a head plus many identical segments.