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.