In which arthropod group are the two major body regions the cephalothorax and abdomen?

• A. Arachnids
• B. Insects
• C. Crustaceans
• D. Myriapods

Answer: (A)

In arthropods, the arrangement of body segments into functional regions is a key feature for grouping species. The front part that is a fused head and thorax, called the cephalothorax, paired with a distinct posterior abdomen, is the classic layout seen in arachnids. The cephalothorax in these animals carries the eyes, mouthparts, and the legs, while the abdomen houses most of the internal organs and, in spiders, the silk-spinning structures. This two-part body plan—cephalothorax plus abdomen—is the hallmark that sets arachnids apart from other groups. Insects have three separate regions (head, thorax, abdomen), myriapods have a head plus a long trunk, and crustaceans vary but often show different arrangements rather than a strict two-region body plan.