We describe the immature larvae of Coelioxys (Liothyrapis) decipiens Spinola and compare them with those of Coelioxys (Boreocoelioxys) octodentata Say and C. (B.) sayi Robertson. All three species have five larval instars, and, in each case, the third instar is primarily responsible for assassinating the host immature. However, the anatomy of the third instar of C. decipiens appears to be plesiomorphic compared with highly modified features of the third instars of the other two species. This suggests that a study of the third instars of other taxa in this diverse genus might play a significant role in sorting out the phylogenetic relationships within the genus, which contains more than 320 species worldwide.