What is a dichotomous key?
A dichotomous key is a tool used to identify an unknown organism. The word dichotomous means “dividing into two”. At each step the key offers two contrasting statements, and you choose the one that matches your specimen. Each choice either names the organism or directs you to the next numbered pair of statements, until an identity is reached.
Key idea
A key should use clear, observable, external features (for example number of legs, presence of wings, leaf shape) rather than behaviour or features that vary within a species. Good statements are precise and give a definite yes/no answer.
Using a key
Start at question 1, read both options, and follow the instruction beside the option that fits your organism. Repeat until you reach a name.
Worked example
To identify a small animal:
1. Has six legs → go to 2 | Has eight legs → Spider
2. Has wings → Bee | No wings → Ant
An animal with six legs and no wings is therefore identified as an ant.
Remember
- Each step has exactly two choices – that is what “dichotomous” means.
- Use visible structural features, not colour that may fade or behaviour.
- Follow one path at a time; do not skip steps.