So far we've learned a little bit about Arthropods and seen how all of the major groups of insects are related to each other.
Insects have three, and only three pairs of legs.
All other arthropods have more than three pairs of legs, and have either more or less than three body sections. Their skeletons are on the outside of their bodies, their heart runs in a thin line down their backs, and their nerve cords are found along their bellies. The insect's external anatomy is only slightly less bizarre than its internal anatomy. Breathing holes called spiracles are found in a line along the sides of the thorax and abdomen. These lead to a branching network of tiny tubes throughout the insect's body that supply oxygen to every cell.