The notum is the dorsal portion of an insect's thoracic segment. It is typically a single large sclerite. It can be further divided into the alinotum which bears the wings and the postnotum which bears the phragma.
In most winged insects, the structure of each successive notum is quite variable; in the Neoptera, the main flight muscles insert on the notum, so the segment that bears the main pair of wings is typically the one with the most highly developed notum. For example, in Diptera, Hymenoptera, and Lepidoptera, the mesonotum is the most hypertrophied sclerite, and is commonly called the scutum. However, there are other groups of insects in which, for reasons other than flight, the pronotum is hypertrophied, such as in all beetles (Coleoptera), and most treehoppers (family Membracidae, order Hemiptera).
The notum on the first thoracic segment is called pronotum, the second mesonotum, and the third metanotum.
The back surface of nudibranchs is also known as the notum.
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