Peritrophic membrane structure and formation in the larva of a moth, Heliothis

The peritrophic membrane (PM) in tobacco budworm larvae (Heliothis virescens, Lepidoptera: Noctuidae), is a continuous sac which encloses the food bolus in the midgut and hindgut. The PM is a single-walled structure 3-5 mum thick which is comprised of two main layers or laminae. The laminae may be fused into a single structure or remain separated by a space which may contain additional thin strands of matrix. Staining with an anti-PM antibody and wheat germ agglutinin (WGA) illustrate the laminar nature of the PM and suggest that protein and chitin have co-incident spatial distributions within the matrix. By transmission electron microscopy, the PM is composed of a loose network of fibrils and small granules, the only structural difference among laminae being a compaction of the matrix along the edges of the two limiting laminae facing the endoperitrophic and ectoperitrophic spaces. By scanning electron microscopy, the PM surface has a wrinkled, felt-like texture without pores or slits. Contrary to the classical view that lepidopterans are Type I insects with respect to PM formation in which the PM forms along the full length of the midgut, the PM in the tobacco budworm forms primarily from secretions of specialized midgut epithelial cells at the junction of the foregut and midgut. The secretory cells, their secretions and the nascent PM stain intensely with the anti-PM antibody but not with WGA suggesting that chitin is added more posteriorly. The PM may be supplemented by the addition of minor amounts of matrix material along the length of the midgut. PM synthesis begins during embryogenesis prior to the initiation of feeding. The PM in neonates is only about 0.1 mum thick but otherwise is structurally similar to that in older larvae.