Gene families implicated in arthropod immunity suggest unique features of the pea aphid immune system. Black indicates present (copy number is indicated, when known), white indicates absent, and gray indicates equivocal or unknown. Values for D. melanogaster, A. gambiae, T. castanateum, A. mellifera, and some D. pulex genes are based on published analyses [13, 14, 16, 17, 40]. For previously unannotated D. pulex genes, as well as for I. scapularis and P. humanus genes, we determined presence via cursory BLAST searches against available genome databases [127, 128] (wfleabase.org, vectorbase.org) using both D. melanogaster and A. pisum protein sequences as queries. Gene presence for Ixodes was confirmed based on previous studies . Future comprehensive annotation of the Pedicularis and Ixodes immune gene sets may reveal the presence of additional genes and lack of functionality of others. PPO, prophenoloxidase.