Meaning of dichotomous

A dichotomous key presents two alternatives at each step. Each choice directs the user to another pair of statements or to an identification. The alternatives must be mutually exclusive, so a specimen cannot satisfy both, and together they should cover every specimen being classified.

Selecting features

Begin with obvious features that divide the specimens into fairly large groups. Presence or absence of wings, number of legs and leaf-vein pattern are suitable. Avoid relative descriptions such as ‘large’ and ‘small’ unless a numerical boundary is given, because intermediate specimens may be difficult to place.

Writing paired statements

Both statements in a pair should refer to the same characteristic. For example: 1a wings present; go to 2. 1b wings absent; go to 3. A poor pair would compare wings in one statement and colour in the other. Use positive statements when possible and avoid vague language.

Testing the key

Run every specimen through the completed key. Check that each route ends in one identification and that no specimen becomes trapped between choices. If two specimens follow the same route, add another observable feature that distinguishes them.

Using a supplied key

Read both alternatives before choosing. Return to the specimen or image at every step rather than relying on memory. In an answer, show the sequence of choices if asked to justify an identification. The final name alone may not receive all available marks.

Review questions and suggested answers