Mesothorax

Definition

The middle of the three thoracic segments in hexapods, bearing the second pair of legs and, in winged insects, the forewings. Its principal sclerites are the mesonotum (dorsal), mesosternum (ventral), and mesopleuron (lateral). In many insect orders—particularly Diptera, Hymenoptera, and Lepidoptera—the mesonotum is enlarged and dominates the dorsal thoracic surface, often with a tegula covering the wing base. The forewings may be modified: hardened into elytra in Coleoptera, reduced to halteres in Strepsiptera, or otherwise altered.

Etymology

Greek meso- (middle) + thorax (chest, breastplate)

Example

In a honey bee (Apis mellifera), the mesothorax is greatly enlarged; its mesoscutum and scutellum form the bulk of the visible thorax and anchor the powerful flight muscles that drive the forewings.

Related Terms

• prothorax
• Metathorax
• Mesonotum
• mesopleuron
• mesosternum
• tegula
• scutellum
• pterothorax
• elytron
• Haltere

Usage Notes

Distinguished from prothorax (anterior, bearing first legs) and metathorax (posterior, bearing hindwings when present). The term applies specifically to hexapods; other arthropods with thoracic regions lack this tripartite division. In Coleoptera, the mesothoracic scutellum is often visible as a small triangular plate between the elytral bases, though it may be concealed. In Diptera, the mesothorax is disproportionately developed while the metathorax is reduced, with the hindwings modified into halteres.