nEndothorax(Zoöl) An internal process of the sternal plates in the thorax of insects.
Century Dictionary and Cyclopedia
nendothoraxIn arthropods, as crustaceans and insects, the apodemal system of the thorax or the cephalothorax, formed by various processes and continuations of the dermal skeleton, and so constituting an interior framework of this part of the body, supporting and giving attachment to soft parts, as nerves and muscles.
Webster's Revised Unabridged Dictionary
Endo-, + thorax,
Endothorax: the internal framework or processes of the thorax.
"Explanation of Terms Used in Entomology" by John. B. Smith