• Webster's Revised Unabridged Dictionary
    • n Mesonotum (Zoöl) The dorsal portion of the mesothorax of insects.
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Century Dictionary and Cyclopedia
    • n mesonotum The middle one of the three divisions of the notum of an insect, succeeding the pronotum and preceding the metanotum; the dorsal division of the mesothorax; the upper part of the middle thoracic segment. It consists typically of four sclerites, called præscutum, scutum, scutellum, and postsculellum, which may or may not be distinguishable by means of sutures between them. In Hymenoptera, Lepidoptera, and Dip-tera it is very large, forming the principal part of the upper surface of the thorax: in these insects its divisions are usually named without the prefix meso-. In insects having wing-covers the mesonotum is generally concealed by them, except a piece called the scutellum, which may be very small, as in most. Coleoptera, or large, as in many Hemiptera.
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Webster's Revised Unabridged Dictionary
NL., fr. Gr. me`sos middle + nw^ton the back


In literature:

Mesonotum: the primitively upper surface of the 2d or middle thoracic ring.
"Explanation of Terms Used in Entomology" by John. B. Smith