Themesothorax is the middle of the three segments of thethorax ofhexapods, and bears the second pair of legs. Its principalsclerites (exoskeletal plates) are themesonotum (dorsal), themesosternum (ventral), and themesopleuron (lateral) on each side. The mesothorax is the segment that bears the forewings in all winged insects, though sometimes these may be reduced or modified, as inbeetles (Coleoptera) orDermaptera, in which they are sclerotized to form theelytra ("wing covers"), and theStrepsiptera, in which they are reduced to formhalteres that attach to the mesonotum.[1] All adult insects possess legs on the mesothorax. In some groups of insects, the mesonotum is hypertrophied, such as inDiptera,Hymenoptera, andLepidoptera), in which the anterior portion of the mesonotum (called themesoscutum, or simply "scutum") forms most of the dorsal surface of the thorax. In these orders, there is also typically a small sclerite attached to the mesonotum that covers the wing base, called thetegula. In one group of insects, theHemiptera, the dorsal surface of the thorax is typically formed primarily of the prothorax, but also in part by the enlarged posterior portion of the mesonotum, called thescutellum; in theColeoptera, the scutellum may or may not be visible, usually as a small triangular plate between the elytral bases, thus similar in position to theHemipteran scutellum. InDiptera andHymenoptera the mesothoracic scutellum is also distinct, but much smaller than the mesoscutum.
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