Using keys

Part 1 of 6Why keys are used

• Biologists often need to identify an organism they have found.
• Looking at pictures in a book or on the internet may find a name, but it does not always work.
• A key = a set of questions about the organism to be identified.
  • The answer to each question leads to another question.
  • The questions continue until the name of the organism is reached.
• Keys are used by scientists in fieldwork to identify organisms quickly, even by people who are not experts.

Part 2 of 6Dichotomous keys

• 01Dichotomous = branching into two.
• 02At each step there are exactly two choices, such as yes or no.
• 03Each answer leads either to another question or to the name of an organism.
• 04Questions must be about features that can be seen clearly and answered the same way by everyone.

Part 3 of 6How to use a key

1. Choose one organism to identify.
2. Start at the top of the key and answer the first question.
3. Follow the line (or number) to the next question.
4. Keep going until a name is reached, which is the answer.
5. Start again at the top for the next organism.

• Imagine the whole animal is being examined, not just a picture.

Part 4 of 6Style 1: the branching key

• The branching key is drawn as a diagram, and each question has two arrows labelled Yes and No.

Question	If yes	If no
Does it have legs?	Does it have more than six legs?	Is its body made up of rings?
Does it have more than six legs?	crab	Does it have four wings?
Does it have four wings?	dragonfly	housefly
Is its body made up of rings?	earthworm	slug

• Five organisms are separated by four questions.

Part 5 of 6Style 2: the numbered key

• The same key is written with pairs of statements instead of questions.
• A number tells which pair of statements to go to next, instead of an arrow.

Step	Statement	Go to / answer
1a	It has legs.	go to 2
1b	It does not have legs.	go to 3
2a	It has exactly six legs.	go to 4
2b	It has more than six legs.	crab
3a	Its body is made up of rings.	earthworm
3b	Its body is not made up of rings.	slug
4a	It has four wings.	dragonfly
4b	It has two wings.	housefly

Part 6 of 6Features that suit a key

• Good features can be counted or seen clearly and do not change.
  • Number of legs.
  • Wings present or absent.
  • Body made of segments or not.
  • Presence of a shell.
• Poor features are opinions, change with growth or season, or need comparison.
  • Size, such as "big" or "small".
  • Colour, which varies between individuals.
  • Whether a plant has flowers, which changes with season.