Follow a dichotomous identification key
Trace one of two alternatives at each step for a defined set of specimens.
Section 1.2 · Concept and uses of classification systems
Core and Extended; Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Read the stated observations before choosing a group.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What makes a key dichotomous?
What you will learn
- Use observable features to choose each branch.
- Identify a specimen while recording the route.
Two alternatives at a time
A dichotomous key offers two alternatives at each decision. Read both, inspect the relevant feature, and follow the indicated route until a name or label is reached. Each choice should separate the specimens under consideration without overlap.
The key shown here is for four stated adult arthropod specimens: an insect with three leg pairs, a spider with four, a lobster with two antenna pairs and more than four leg pairs, and a centipede with one antenna pair and more than four leg pairs. It is not a universal key for all arthropods.
Keep observations and routes consistent
At the first decision, three leg pairs identifies the insect. Other specimens go to a four-leg-pair decision. The spider ends there; specimens with more than four pairs continue to antenna-pair observations. Two antenna pairs identifies the lobster; one identifies the centipede in this supplied set.
If the needed feature is obscured or the specimen falls outside the stated set, report that the key cannot securely identify it. Do not invent a feature or force an endpoint. Keys identify supplied specimens; their branch order is not a measure of evolutionary distance.
Pause and explain
In this key, what follows a not-three-leg-pairs choice?
Worked example
A supplied specimen has more than four leg pairs and two antenna pairs. Follow the key.
Show the worked solution
- It does not have three leg pairs: go to decision 2.
- It does not have four leg pairs: go to decision 3.
- It has two antenna pairs: choose lobster.
Answer Lobster in the supplied specimen set.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A specimen has six individual legs and one antenna pair. Follow the supplied key.
Give me a hint
Six legs form three pairs.
Compare my reasoning
- Convert six individual legs to three pairs.
- At decision 1, choose three leg pairs.
- The endpoint is insect.
Insect.
Look for these in your work
- Correct leg-pair count.
- Record the first decision.
A specimen has many leg pairs and one antenna pair. Give its route.
Give me a hint
Use the three decisions in order.
Compare my reasoning
- Not three leg pairs.
- Not four leg pairs.
- One antenna pair.
Decision 1 no → decision 2 no → decision 3 one pair → centipede.
Look for these in your work
- Trace all required branches.
- Keep the conclusion within the supplied set.
Only four legs are visible because the underside is hidden. Why should you not select the spider endpoint immediately?
Give me a hint
Visible count may be incomplete.
Compare my reasoning
- Four visible legs are not four pairs.
- Hidden legs may change the total count.
- Observe the whole specimen before following that branch.
The leg-pair observation is incomplete; a clearer view is required.
Look for these in your work
- Distinguish legs from pairs.
- State the missing observation.
Common mistakes
- Count pairs rather than individual legs.
- Use the key only within its stated specimen set.
What can you explain now?
A supplied specimen has four leg pairs and no antennae. What is its endpoint?
Compare with the explanation
Spider.
It follows the not-three branch, then the four-leg-pair branch.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Trace all four provided specimens and explain why the key has a defined scope.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.