Comparison of the morphofunctional properties of the larval midgut of Hermetia illucens (Diptera: Stratiomyidae) reared on different diets

Hermetia illucens (Diptera: Stratiomyidae) is among the most promising organisms for the bioconversion of organic waste in proteins for feed production, because the larvae are able to grow on a wide variety of organic substrates and the dry-matter of the prepupae contains a very high percentage of protein with high nutritional value. One of the potential substrates for bioconversion by H. illucens is Fruit and Vegetable Waste (FVW), which could be provided in large amounts by large-scale retail trade and wholesale markets. However, in view of a possible application of this system, it is fundamental to evaluate the biological performance and the morphological, physiological, and molecular responses of H. illucens reared on this food substrate. In the present study we compared larvae reared on a standard diet for dipteran larvae and on FVW. In particular, after evaluating the growth performances, we focused our attention on the midgut, which is responsible for nutrient digestion and absorption. The morphological changes occurring in this organ after the ingestion of the two diets were investigated by optical microscopy. Moreover, an evaluation of midgut functionality was performed. An analysis based on enzymatic histochemistry was carried out to evaluate specific differences in lipids and glycogen accumulation in midgut cells. Finally, we assessed the activity and the mRNA expression levels of enzymes involved in digestion, focusing on proteolytic and amylolytic ones. Our results demonstrate that the extraordinary feeding plasticity of this insect corresponds to an extraordinary biological, morphological, physiological and molecular plasticity,that allows larvae to perform an effective bioconversion of FVW, opening up interesting application perspectives. This work was supported by Fondazione Cariplo (Insect bioconversion: from vegetable waste to protein production for fish feed, ID 2014-0550).