First things first, not all creepy-crawlies and minibeasts are actually insects. There are a few different words that we use to describe different groups of animals and what makes them different from one another.
You might have already heard the term ‘vertebrate’ used to describe animals with backbones, such as mammals, birds or reptiles. The word ‘invertebrate’ refers to all animals without a backbone, such as insects, crustaceans or worms.
While all insects are invertebrates, not all invertebrates are insects!
The word ‘arthropod’ refers to all invertebrates with jointed legs. So now we know that all insects are also arthropods. Arthropods also have a hard exoskeleton, like you might see on a crab or on a beetle. They don’t have bones inside their bodies like humans. Instead, their ‘bones’ are on the outside, like a suit of armour, which is why it is called an exoskeleton.
The final way we decide whether an arthropod is an insect or not is by counting how many pairs of legs it has. Some arthropods, like crabs, have five pairs of legs (that’s 10 legs altogether!) and some have four pairs (like spiders).
Insects always have three pairs of legs, which makes six legs in total (if they haven’t lost any!).
Therefore, we can describe the animals we call insects as invertebrate arthropods with three pairs of legs.
Glossary
Arthropod – animals with no spine, a hard exoskeleton and jointed limbs.
Decapods – animals with ten legs (or five pairs of legs).
Exoskeleton – the hard outer casing seen on many insects and other arthropods, to protect their internal organs.
Invertebrate – any animal without a backbone.
Vertebrate - any animal with a backbone or spine.