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The first three pairs of ganglia are fused into the brain, while the three following pairs are fused into a structure of three pairs of ganglia under the insect's esophagus, called the subesophageal ganglion. [1]: 57 The thoracic segments have one ganglion on each side, which are connected into a pair, one pair per segment.
In some insect pupae, like the mosquitoes', the head and thorax can be fused in a cephalothorax. Members of suborder Apocrita (wasps, ants and bees) in the order Hymenoptera have the first segment of the abdomen fused with the thorax, which is called the propodeum .
The thoracic segments have one ganglion on each side, which are connected into a pair, one pair per segment. This arrangement is also seen in the abdomen but only in the first eight segments. Many species of insects have reduced numbers of ganglia due to fusion or reduction. [8]
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.
Two thoracic segments are fused into the head; one thoracic segment is in the posterior tagma. Other kinds of copepod also have two tagmata but formed by different segments. The development of distinct tagmata is believed to be a feature of the evolution of segmented animals, especially arthropods. In the ancestral arthropod, the body was made ...
Prothorax of a beetle. The prothorax is the foremost of the three segments in the thorax of an insect, and bears the first pair of legs.Its principal sclerites (exoskeletal plates) are the pronotum (), the prosternum (), and the propleuron on each side.
While the tergum is positioned on the top (dorsal), and the sternum on the bottom (ventral), the pleuron is positioned to the side (lateral). The terms pro-, meso- and metapleuron are used respectively for the pleura of the first, second and third thoracic segments. A pleuron usually consists of a epimeron and an episternum. [2]
The anterior edge is called the 'base' and posterior edge is called the 'apex' or 'margin'. A given tergum may be divided into hardened plates or sclerites commonly referred to as tergites. [1] In a thoracic segment, for example, the tergum may be divided into an anterior notum and a posterior scutellum.