The structure and function of the digestive system during postembryonic development in Lytta nuttalli Say (Coleoptera: Meloidae)

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Abstract
The digestive tract of Lytta nuttalli goes through three structural phases, correlated with the insect's trophic phases. The wholly or partly carnivorous larvae have an almost straight tract with a relatively short stomodaeum and proctodaeum, while the phytophagous adult has a convoluted system with a long fore- and hind-gut. In the two intervening non-feeding larval instars the tract is greatly reduced. Changes in the structure and histology of each part of the system, including changes in the apparent activity of the midgut epithelium, are described and related to the changes in diet and feeding habits. The foregut of the adult has a complex armature and probably functions as a triturating mechanism. In each stage there apparently is only one mature midgut epithelial cell type. A brush border composed of microvilli is present in the feeding stages. Regenerative cells are located in basal nests in the feeding larvae, protruding crypts in the adult, and isolated clusters in the non-feeding larvae. A peritrophic membrane of dual origin is present in the feeding stages; it is thin and continuous in the larvae, thick and fragmented in the adult. The colon, with its associated cryptonephridial system, appears to be the main site of water absorption; it is especially well developed in the adult.

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