Invertebrates are animals without a backbone. Of the planet’s estimated 15-30 million animal species, 90% or more are invertebrates. Invertebrates live just about anywhere. There is a good chance that you have seen an invertebrate recently. Do you recall batting away a fly, unearthing a worm, or admiring a spider as it waited to catch food in its web. Well guess what? All of these animals and many more are collectively known as invertebrates – animals that lack a backbone.
Invertebrates are everywhere. There are so many invertebrates on this planet that it is impossible to count them all. They come in many shapes and sizes, live practically anywhere and provide many services that are vital for our survival. Invertebrates are all around us and yet amazingly most go about their daily business unnoticed. Much of this has to do with the size of invertebrates. On land, invertebrates range from fractions of a millimetre to approximately 150 centimetres in length, though most are less than five centimetres.
Invertebrate groups. Terrestrial (land) invertebrates include the following groups, many of which also have members that live in freshwater or marine environments: Insects, Spiders, Centipedes, Millipedes, Worms, Velvet worms, Slaters, Landhoppers
Other invertebrates include land-dwelling members such as: Snails and slugs.
Source: https://australianmuseum.net.au/learn/teachers/learning/what-are-invertebrates/
Note: Insects have three body regions (head, thorax and abdomen), three pairs of legs and a pair of antennae. One gift most insects have that other arthropods lack is the ability to fly.