The process in which a relatively unspecialized cell derived from the embryonic head mesoderm acquires the specialized features of a mature hemocyte. Hemocytes are blood cells associated with a hemocoel (the cavity containing most of the major organs of the arthropod body) which are involved in defense and clotting of hemolymph, but not involved in transport of oxygen. [ http://www.ncbi.nlm.nih.gov/pubmed/14602069 GOC:bf GOC:mtg_sensu ]