Dichotomous Keys

The format. Each question has two paired statements:

1a. Statement → go to question N (or NAME)
1b. Other statement → go to question M (or NAME)

Strategy.

1. Start at question 1.
2. Read both statements (1a and 1b).
3. Choose the one matching the unknown organism.
4. Follow the instruction (next question or final identification).
5. Repeat until you reach a name.

Worked example. Identify a creature with: 8 legs, 2 body parts, no wings, no antennae.

Key:

1a. Has 6 legs → go to 2
1b. Has 8 legs → ARACHNID
2a. Has wings → INSECT
2b. No wings → ...

Answer: ARACHNID (matched 1b on first question).

Cambridge tip. Always start at question 1 — don't skip ahead. Mark each branch you take in pencil to track.

Strategy for constructing a key.

1. List your organisms. Write all the organisms you need to identify.

2. Pick a feature that splits them in two. Use the most striking or universal observable feature first. (Leg count, has-wings, plant-vs-animal, body shape.)

3. Write Question 1 with two clear options.

4. Repeat for each subgroup. Within each branch, find another feature that splits it further.

5. End at a NAME when only one option remains.

Worked: construct a key for {ant, spider, crab, butterfly}.

Step 1. Group features.

• Ant: 6 legs, no wings (worker ants), small.
• Spider: 8 legs.
• Crab: 10 legs, in water.
• Butterfly: 6 legs, has wings, colourful.

Step 2. First split: leg count.

1a. Has 6 legs → go to 2
1b. Has 8 legs → SPIDER
1c. Has 10 legs → CRAB

WAIT — that's three branches, not dichotomous! Make it strictly two:

1a. Has 6 legs → go to 2
1b. More than 6 legs → go to 3

Step 3.

2a. Has wings → BUTTERFLY
2b. No wings → ANT

3a. Has 8 legs → SPIDER
3b. Has 10 legs → CRAB

Done — 4 organisms, 3 questions, all dichotomous.

Cambridge tip. Cambridge gives you a set of organisms (with images or descriptions). Your job: pick observable features and write a clear two-branch key. Mark schemes credit:

1. Use of observable features only.
2. Strict dichotomy (exactly two branches).
3. Each path leads to ONE name (no ambiguity).

Good observable features (Cambridge accepts):

• Number of legs.
• Number of body parts.
• Presence / absence of wings, antennae, fur, scales, feathers.
• Body length / size (with measurement).
• Colour / pattern.
• Habitat (where it's found — water vs land vs air).

Don't use:

• "Is it a mammal?" / "Is it an arthropod?" — that's classifying, not identifying. Circular.
• "Does it eat plants?" — diet isn't always observable.
• "Has it got DNA?" — invisible.
• "Is it nocturnal?" — behaviour, hard to observe.

Vague vs precise.

• ❌ "Is it big?" → big compared to what?
• ✓ "Is it longer than 5 cm?" — measurable.

Worked qualitative. A student writes Question 1 as "Is it living in water?". Is this a good question for a dichotomous key? Possibly — but only if the organisms have CLEAR habitat differences. If both options can be seen in water (e.g. a frog AND a fish), the question doesn't split. Pick a feature that creates a CLEAN split.