Thorax

Definition

The middle of the three primary body segments (tagmata) in insects and many other arthropods, situated between the head and abdomen. In insects, the thorax is the exclusive site of leg and wing attachment and contains the powerful locomotor musculature required for flight and walking. It is subdivided into three serial segments: the prothorax (anterior, bearing the first pair of legs), mesothorax (middle, bearing the second pair of legs and typically the forewings), and metathorax (posterior, bearing the third pair of legs and typically the hindwings). Each segment may bear dorsal plates (terga/nota), lateral plates (pleura), and ventral plates (sterna). The thorax is generally more compact and sclerotized than the abdomen, reflecting its mechanical role in locomotion. In arachnids, the thorax is often fused with the head to form the cephalothorax (prosoma).

Etymology

From Greek thōrax, meaning breastplate or cuirass, referring to the armored, protective nature of this body region.

Example

In a grasshopper (Orthoptera), the greatly enlarged hind femora attach to the metathorax, whose musculature powers the explosive jumping for which the group is known; the prothorax remains relatively small and uninvolved in flight.

Related Terms

• Abdomen
• Head
• tagma
• prothorax
• Mesothorax
• Metathorax
• pronotum
• notum
• pleuron
• sternum
• Coxa
• Haltere
• Cephalothorax
• prosoma
• pterothorax

Usage Notes

In entomology, 'thorax' is absolute and refers specifically to the locomotor tagma, never to the whole body. Contrast with 'cephalothorax' in arachnids and crustaceans, where the thorax is fused with the head. Some specialists restrict 'thorax' to the wing-bearing segments (meso- and metathorax), calling the prothorax 'neck' in certain contexts, but this usage is not universal. The adjective 'thoracic' applies to structures on or of the thorax. Do not confuse with the mammalian thorax (chest), which is anatomically non-homologous.