Some insects have the word fly in their names , but they are not flies at all . The common names of true flies are written as two words , e.g. , crane fly , robber fly , bee fly , moth fly , fruit fly . The common names of non-dipteran insects that have `` fly '' in their name are written as one word , e.g. , butterfly , stonefly , dragonfly , scorpionfly , sawfly , caddisfly , whitefly .