What is a Dichotomous Key?

• A dichotomous key is used to identify an organism using clues about physical characteristics and traits.
• It is a useful tool for biological classification.

Understanding the Setup

• The word “dichotomous” has Greek roots and means to have “two parts.” Thus, in a dichotomous key, there are two options to pick from.
• For example, it may ask if a leaf is broad or needle-like:
  
• When the user chooses an option, it leads them to the next pair of clues to choose from and so on.
• Eventually, if followed correctly, the user will reach the end of the key and correctly identify the organism.
• Questions are usually laid out in a listed, easy-to-follow form.

How to use most effectively

• To avoid simple mistakes, double-check to make sure you have the right “next step” in mind before you accidentally choose the wrong one.
• It can be easy to forget that sometimes a dichotomous key requires you to skip a step. It isn’t always in chronological order.
• When you’ve arrived at your answer, start from the beginning once again and make sure you haven’t accidentally taken the wrong path at some point. Making the wrong call at one step can lead to getting an entirely different organism.
• Take your time!

Example

(Example from: University of British Columbia. (2017). Key to common insect orders. FRST 307 Introduction to Entomology.)

Solving Steps

• If the specimen/organism in question has hind legs adapted for jumping, it is an Orthoptera. There is no need to look further.
• If, however, the specimen/organism has hind legs not adapted for jumping, proceed to part 13
• If the body is flattened from top-to-bottom (13a) it is a Blattodea
• If the body is stick-like, not flattened, it is a Phasmatodea
• If there was a number 14 next to either of those options, one would proceed if the organism does not yet meet the exact description