Which one is NOT one of the three insect body regions?

Insect bodies are primarily divided into three distinct regions: the head, thorax, and abdomen. Each of these regions has specific functions and characteristics. The head contains sensory organs and mouthparts, the thorax is responsible for locomotion due to its attachment points for legs and wings, and the abdomen houses reproductive and digestive systems.

The term "cephalothorax" refers to a composite body structure found in some arthropods, particularly arachnids like spiders and crustaceans, where the head and thorax are fused together. This structure does not apply to insects, making it the correct choice for what is not one of the three insect body regions. Insects are classified distinctly with the three clear regions, thereby making "cephalothorax" an incorrect term when discussing insect anatomy.