# 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.

> Cambridge International IGCSE Biology (0610) — 1.2 Concept and use of a classification system · Explain · 4 marks

## Mark scheme (4 marks)

1. A dichotomous key presents a series of paired choices / questions based on observable features
2. At each step the scientist selects the option that matches the organism, leading to the next question or an identification
3. Beetles have three pairs of jointed legs (insects), whereas spiders have four pairs of jointed legs (arachnids)
4. 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

- [dichotomous key](https://www.gradenine.co.uk/glossary/dichotomous-key)
- [class](https://www.gradenine.co.uk/glossary/class)

## Related

- [Revision notes for Cambridge International IGCSE Biology (0610)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-scientist-is-classifying-two-species-13dca35f) · Published by Druglandscape Ltd.