The abdomen, within vertebrates, refers to the portion of the body situated below the thorax, encompassing the digestive organs. In the context of insects and other arthropods, the abdomen constitutes the hind part of the body. Mammals, including humans, have a distinct separation between the abdomen and thorax facilitated by the diaphragm, a muscular sheet of tissue. Conversely, arthropods typically feature a narrower constriction to differentiate the abdomen from the rest of the body.
Within mammals, the female reproductive organs are located in the abdomen. In insects and spiders, the abdomen is characterized by the absence of limbs, contributing to their distinctive anatomical structure. This anatomical region plays a vital role in housing and protecting essential internal organs critical for digestion, reproduction, and other physiological functions.