Like the wings of most insects, the wings of fruit flies are convered with many fine hairs. The wings had a kind of iridescent quality that can't really be seen in a stained mounted specimen.

Here we see the "tail end" of a fruit fly. The exact details of such features can be used to distinguish between closely related species. This is done using what is known as a dichotomous key, meaning that the answer (1 of 2 choices, usually yes or no) leads either to an identification or to another question and each question narrows the number of possiblities. This can be done to determine the order, the family and finally the genus and species. With some diverse families the genus and species keying can take up whole volumes the size of an unabridged dictionary. This is typical of the kind of material that you won't find at your neighborhood library but you may evenutally be able to access over the internet.