Oxygen levels in the gut lumens of herbivorous insects.

Oxygen levels were measured in the foregut and midgut lumens of ten species of caterpillars and three species of grasshoppers. In most species, the foregut was nearly anoxic, with oxygen levels ranging from 0 to 2.5 mm Hg. However, two caterpillar species with large foreguts (Malacosoma disstria and Lymantria dispar) had elevated oxygen levels (27.9 and 32.1 mm Hg) in this region when they were fed artificial diet. In all of the species surveyed, the anterior and posterior midgut were nearly anoxic, with oxygen levels ranging from 0 to 7.3 mm Hg. Oxygen levels in the midgut lumen of Helicoverpa zea did not differ when caterpillars were fed artificial diet or tomato foliage, suggesting that the insect is capable of reducing the level of ingested oxygen in its gut. An examination of the radial microgradient of oxygen in the gut lumen demonstrated that the midgut epithelium is not a sink for ingested oxygen. However, the midgut contents of larvae fed artificial diet were capable of depleting oxygen. This capacity was reduced by boiling, suggesting that the nearly anoxic state of the midgut lumen in some insects is maintained by endogenous chemical processes. We conclude that low oxygen levels in the gut lumens of most herbivorous insects may greatly reduce the rates of oxidation of ingested plant compounds by oxygen-dependent reactions.