"front part of crustaceans, spiders, etc., consisting of the head and thorax blended together," 1829, from cephalo- "head" + thorax. Perhaps from French or German.
Entries linking to cephalothorax
cephalo-
before vowels, cephal-, word-forming element meaning "head, skull, brain," Modern Latin combining form of Greek kephalē "head, uppermost or top part, source," from PIE *ghebh-el- (source also of Tocharian spal "head;" Old High German gebal "skull;" also, via the notion of "front," Gothic gibla, Old Norse gafl "side of a facade").
thorax n.
"chest of the body," late 14c., from Latin thorax "the breast, chest; breastplate," from Greek thōrax (genitive thōrakos) "breastplate, chest," of unknown origin.