(zoology) The middle part of the body, or tagma, of arthropods whose body is composed of three parts (the other two being the prosoma and the metasoma), bearing the legs and (in winged insects) the wings.
In higher specialized forms these branchial processes become first of all limited to five segments of the mesosoma, then sunk beneath the surface as pulmonary organs, and finally atrophied, their place being taken by a well-developed tracheal system.
- The primitive distinction between the mesosoma and the metasoma wholly or almost wholly obliterated, the two regions uniting to form an opisthosoma, which never consists of more than twelve somites and never bears appendages or breathing-organs behind the 4th somite.
- Appendages of 1st pair bisegmented, without poison gland; of 2nd pair prehensile, their basal segments underlying the proboscis, and furnished with sterno 1 to i 1, Somites of the opisthosoma (mesosoma plus metasoma).
- The possibility of another interpretation of the anterior somites of the mesosoma and the prae-genital somite must be borne in mind.
In each somite of the mesosoma is a small, free entosternite having a similar position, but below or ventral to the nerve cords, and having a smaller number of muscles attached to it.