For hundreds of years insects have been classified according to readily observable characteristics. 

This rather clumsy system has been adapted over the years to accommodate the millions of species known today. There are more species of insects, after all, than of all the other phyla together, from protozoa to primates. A systematic classification, then, was clearly necessary. 

During Carl Von Linneas’ lifetime, the preferred criteria were the simplest: morphological (observable physical and developmental traits). Later, Georges Cuvier (1769-1832) and Antoine-Laurent de Jussieu (1748-1836) incorporated physiological and anatomical considerations. Nowadays, classification is determined by molecular biology and, increasingly, by genetics. This new approach has revolutionized taxonomy, forcing us to reconsider groupings taught for 250 years. 

The basic question naturalists face - now as then - is, what is it that defines a species? A species is a number of individuals alike in their characteristics, in a particular environment, capable of reproducing young that are fertile and that reproduce in turn. This last is very important, as many entomologists have successfully obtained offspring by crossing different species. When the young survive, however, they are sterile. By itself this is proof that the parents belonged to different species. 

Generally, the naming protocol is said to be binomial, made up of two words Genus Species. Some animals are trinomial (Ornithoptera priamus poseidon). The third name indicates the subspecies and may be preceded by ssp. (Ornithoptera priamus ssp. poseidon). This naming convention makes it possible to identify groups belonging to a single species though they inhabit different geographical areas and would never meet in nature. 

Follow the links to learn more about Coleoptera and Lepidoptera.