High pH in the ectoperitrophic space of the larval lepidopteran midgut

The midgut lumen of lepidopteran larvae is highly alkaline (Waterhouse, 1949; Berenbaum, 1980), a condition maintained in the presence of a large and opposing transapical H + gradient from the midgut epithelial cells. Lumen pH values of approximately 11 may be common (Dow, 1984), whereas the cell cytoplasm itself is maintained at approximately neutral pH (Dow and O’Donnell, 1990; Chao et al. 1991), giving rise to cell/lumen H + concentration ratios of 10 4 or more. Dow (1984) reported a mean pH of 12.0 in the midgut of a sphingid larva, Acherontia atropos, and Schultz and Lechowicz (1986) recorded a maximum reading of pH12.4 in the European gypsy moth, Lymantria dispar. These values are among the highest recorded in any biological system.