A biologist collected four small animals from the leaves and soil of a garden and photographed them side by side for comparison. All four animals are arthro...

Labelled answer diagram:

This question tests the shared features of arthropods and how the subgroups are told apart by countable external features rather than by variable ones like colour or size.

(a) Pairs of legs on an insect [1] - three pairs (six legs).

(b) Group with four pairs of legs [1] - arachnids (for example spiders), which have four pairs (eight legs).

(c) Two features, other than legs, separating an arachnid from an insect [2] - one mark each for any two of:

• An arachnid has two body regions (cephalothorax and abdomen) whereas an insect has three (head, thorax, abdomen).
• An arachnid has no antennae, whereas an insect has one pair.
• An arachnid has no wings, whereas many insects have wings.

(d) Three features shared by all arthropods [3] - one mark each:

1. Jointed legs (jointed appendages) (1).
2. A hard exoskeleton, made of chitin (1).
3. A segmented body (1). (Bilateral symmetry is also accepted.)

(e) Why an arthropod must shed its exoskeleton to grow [3]

1. The exoskeleton is hard and cannot stretch or grow with the animal (1).
2. So it is shed (moulting, or ecdysis) (1).
3. The animal then increases in size before the new, larger exoskeleton hardens (1).

Exam tip: classify arthropods by counting body regions and pairs of legs, because these features are constant within a group, unlike colour or overall size.