All the animals in Group 2 belong to the group called arthropods. All arthropods have:

• a segmented body with a thick outer skeleton
• a head with sense organs and paired mouth parts
• jointed legs

Differences:

1. Arthropods can be further divided into subgroups. 
2. Crustaceans have many legs and breathe through gills, e.g., crab. 
3. Arachnids have two body parts, two pairs of feelers or pincers, and four pairs of legs, e.g., spiders.
4. Insects in their adult form have three body parts, three pairs of legs, and, often but not always, one or two pairs of wings.
5. Millipedes and Centipedes.

Students at Level 3 are likely not to have a clear understanding of the scientific classification system. They often, when they start learning about classification, don't recognise that the model is hierarchical, with each group being subdivided into smaller groups. While they initially may put spiders and insects in the same group, once they learn the differences, they may then not recognise that they also belong to a common group.

Common way of classifying by children

How scientists classify animals

At Level 3 it is sufficient that children understand that insects, spiders, and crustaceans have differences, but are related. Using the analogy of families may be useful.