Answer : Organisms are classified as insects on the basis of their external structures of body.
Insects are the most diversified group of organisms that belong to the phylum Arthropoda.
Their body is primarily divided into three distinct regions that are head, thorax, and abdomen.
They have three pairs of legs (that is why also called hexapods) and two pairs of wings, present in the thorax region.
They possess exoskeleton (external protective covering) that is made up of chitin.