A learner makes a note: in a spider, the head and thoracic regions are fused into one anterior body region. The missing scientific idea is:

The cephalothorax is a fused head-thorax region found in many arachnids and crustaceans. It is distinct from the abdomen. This MCQ belongs to the Arthropoda topic in Animal Diversity-I and is written to test concept understanding rather than memorized wording.