Relationships between venom gland mass and body size, measured as pronotum width, for S. noctilio and S. nigricornis. Sirex noctilio data were analyzed by presence or absence of parasitism by nematodes (Deladenus siricidicola) while numbers of S. nigricornis parasitized by nematodes were too low for analysis. A Sirex noctilio females without parasitism by nematodes B S. noctilio females parasitized by nematodes, and C S. nigricornis without nematode parasitism.