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The thorax in a human and an ant.
Diagram of a tsetse fly, showing the head, thorax and abdomen

The thorax is a division of an animal's body that lies between the head and the abdomen.

In mammals, the thorax is the region of the body formed by the sternum, the thoracic vertebrae and the ribs. It extends from the neck to the diaphragm, and does not include the upper limbs. The heart and the lungs reside in the thoracic cavity, as well as many blood vessels. The inner organs are protected by the rib cage and the sternum.

In insects and the extinct trilobites, the thorax is one of the three main divisions (or tagmata) of the creature's body, each of which is in turn composed of multiple segments. It is the area where the wings and legs attach in insects, or an area of multiple articulating plates in trilobites. In most insects, the thorax itself is composed of three segments; the prothorax, the mesothorax, and the metathorax. In extant insects, the prothorax never has wings, though legs are always present in adults; wings (when present) are restricted to at least the mesothorax, and typically also the metathorax, though the wings may be reduced or modified on either or both segments (as in the fly shown, where the metathoracic wings have been reduced to tiny balancing organs called halteres). In the Apocritan Hymenoptera, the first abdominal segment is fused to the metathorax, where it forms a structure known as the propodeum. Accordingly, in these insects, the functional thorax is composed of four segments, and is therefore typically called the mesosoma to distinguish it from the "thorax" of other insects.

Each thoracic segment in an insect is further subdivided into various parts, the most significant of which are the dorsal portion (the notum), the lateral portion (the pleuron; one on each side), and the ventral portion (the sternum). In some insects, each of these parts is composed of one to several independent exoskeletal plates with membrane between them (called sclerites), though in many cases the sclerites are fused to various degrees.

The trilobite body is divided into three major sections, a cephalon with eyes, mouthparts and sensory organs such as antennae, a thorax of multiple similar segments (that in some species allowed them to roll up into a ball), and a pygidium, or tail section.

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