The
mesothorax is the middle of the three segments in the
thorax of an
insect, and bears the second pair of legs. Its principal
sclerites (exoskeletal plates) are the
mesonotum (
dorsal), the
mesosternum (
ventral), and the
mesopleuron (
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 in
beetles (
Coleoptera) or
Dermaptera, in which they are sclerotized to form the
elytra ("wing covers"), and the
Strepsiptera, in which they are reduced to form
halteres. All adult insects possess legs on the mesothorax. In some groups of insects, the mesonotum is hypertrophied, such as in
Diptera,
Hymenoptera, and
Lepidoptera), in which the anterior portion of the mesonotum (called the
mesoscutum, 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 the
tegula. In one group of insects, the
Hemiptera, 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 the
scutellum; in the
Coleoptera, the scutellum may or may not be visible, usually as a small triangular plate between the elytral bases, thus similar in position to the
Hemipteran scutellum. In
Diptera and
Hymenoptera the mesothoracic scutellum is also distinct, but much smaller than the mesoscutum.
