To be useful, classification should be as natural as possible. Group should be derived from a common ancestor. Allows the most predictive framework. Can predict characteristics of new taxa based on existing framework. Honey bee, house fly.
Rules exist for common names.
If belong to that taxon, then two words. If not one word. The Biological Species Concept Species - groups of interbreeding natural populations which are reproductively isolated for other such groups Not an easy concept to apply. Insects can fly very well.