Fig. 4. CO₂ emission (at 37°C) by batches of Manduca sexta eggs either 1.5 d (A and B) or 3 day old (C and D). Each batch contained 210–450 eggs. Eggs were exposed, in series, to either 21 or 9.5% O₂ with either He or N₂ making up the balance. Only variation in O₂ availability affected metabolic rates; swapping carrier gases (which altered the diffusion coefficient of O₂ in the air filled parts of the chorion and trabecular layer) had no effect.