Oxygen levels in the foreguts of most caterpillars and grasshoppers rapidly drop to 0–3 mm Hg (compared to oxygen levels of 150 mm Hg in air).
Steady-state levels of oxygen in the midgut are consistently low, ranging from 0 to 6 mm Hg.
Especially in the high pH of caterpillar midguts,low oxygen levels are critical for minimizing the autoxidation of ingested tannins.