A scientist is classifying two species of beetle and two species of spider. Explain how using a dichotomous key would allow the scientist to identify each organism, and state one feature that would place the spiders in a different class from the beetles.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A dichotomous key gives two contrasting statements at each step. The scientist looks at the beetle or spider, chooses the statement that matches, and follows the indicated line to the next pair of statements until a species name is reached. This step‑by‑step elimination allows each organism to be identified.
Spiders have four pairs of jointed legs, whereas beetles have three pairs. Thus spiders belong to class Arachnida and beetles to class Insecta.
Spiders have four pairs of jointed legs, whereas beetles have three pairs. Thus spiders belong to class Arachnida and beetles to class Insecta.
Examiner tips
- Use the phrase ‘dichotomous key’ and explain the two‑choice, stepwise process. Show the key feature (four vs. three pairs of legs) that separates the classes. Mention the body division (cephalothorax + abdomen vs. head, thorax, abdomen).
Common mistakes
- Confusing the number of leg pairs or stating spiders have three pairs. Failing to mention the key’s stepwise nature. Using vague terms like ‘different body parts’ without specifying the division.
Mark scheme (4 marks)
- A dichotomous key presents a series of paired choices / questions based on observable features
- At each step the scientist selects the option that matches the organism, leading to the next question or an identification
- Beetles have three pairs of jointed legs (insects), whereas spiders have four pairs of jointed legs (arachnids)
- Beetles belong to class Insecta and spiders belong to class Arachnida (accept: spiders' body is divided into cephalothorax and abdomen whereas beetles' body is divided into head, thorax and abdomen)
Key terms in this question
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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