Concept:The diagram shows the body parts of an arthropod, and each label points to a specific external region.
Explanation:The part labelled I is the maxillae, which are mouthpart appendages.
The part labelled II is the head, the anterior body region.
The part labelled III is the thorax, the middle body region that carries the legs.
The part labelled IV is the abdomen, the posterior body region.
Therefore, the structure labelled III is the thorax.
Answer:D. thorax
