The cephalothorax (calle prosoma in some groups) is an anatomical term used in arachnids and malacostracan crustaceans for the first (anterior) major body section. The remainder of the body is the abdomen (opisthosoma), which may also bear lateral appendages as well as the tail, if present. The term "prosoma" can also be applied to the head of insects, but as the two are always exactly synonymous in insects (not true for mesosoma vs. thorax or metasoma vs. abdomen), the simpler term - "head" - is used instead.