@flo People say say “____ is not an insect because it doesn’t have six legs” because having six legs is a requirement for being an insect. In other words, all insects have six legs (anything that doesn’t have six legs isn’t an insect), but not all things that have six legs are insects (just because something has six legs doesn’t mean it automatically is an insect.
Here is an example that might help: a square has four sides, but not everything with four sides is a square. That’s because having four sides is just one requirement for being a square. To be a square, the four sides also have to be equal and they need to meet at right angles.
So back to insects. In scientific classification, “Insecta” is a class. But classes are a lower level (which, in this case, means more specific) level of classification. To belong to the class Insecta, a creature also has to fit the definition of all of the classification levels above it. Insects are invertebrate animals that have a chitinous exoskeleton, a body that is segmented into three parts, three pairs of jointed legs, compound eyes, and one pair of antennae.
Something counts as an insect only if a creature meets all of these specifications.