How we do define animals? Why cant animals be insects, or insects be animals?
Let’s start with how we define animals:
It is estimated that around 9 or 10 million species of animals inhabit the earth; the exact number is not known and all estimates are rough.
All animals are members of the Kingdom Animalia, also called Metazoa.
All members of this Kingdom are multicellular, and all are heterotrophs (they rely directly or indirectly on other organisms for their nourishment). Most ingest food and digest it in an internal cavity.
The kingdom of Animalia does not contain prokaryotes (organisms made up of cell Membersls that lack a cell nucleus or any membrane-encased organelles). Examples of prokaryotes are bacteria and blue-green algae. It also does not contain orprotists (Kingdom Protista, which includes unicellular eukaryotic organisms)
Animals range in size from no more than a few cells to massive organisms weighing many tons. They include vertebrates–animals such as ourselves, and other mammals we often associate with the animal kingdom
Which leads us to part 2 of your question:
Insects are part of a bigger animal group called arthropods. The word Arthropod literally means jointed limbs.
Our own group, the vertebrates, are relatively inconsequential from a population and diversity perspective. So not only are insects, in fact, animals–they are the most prevalent and diverse animals on planet Earth!