Structural and functional characterization of the larval midgut of Hermetia illucens for the best exploitation of its bioconversion ability

The use of insects as a primary agent for organic waste reduction and bioconversion is a promising and sustainable strategy to produce protein that can be used for feed pro-duction. The non-pest black soldier fly (BSF), Hermetia illucens, is among the most promising insect species to this purpose because of the ability of the larvae to grow on a wide variety of organic substrates and their efficiency in the bioconversion process. Moreover, the high nutritional value of the larvae and pupae makes them an interesting alternative protein source for the production of fish feed. Although the literature provides information on the rearing methods for BSF and indica-tions on its use for waste treatment, little is known about H. illucens biology. In particu-lar, a deep understanding of the physiology of the midgut, which is implicated in food digestion and nutrient absorption, is essential to better comprehend the extraordinary dietary plasticity of the larva, which is able to grow on different food substrates. In the present study we performed a structural and functional characterization of the midgut of last instar larvae grown on two different food substrates, i.e., a standard diet for Diptera and a vegetable mix, by using morphological, biochemical, and molecular approaches. Our results demonstrate that the larval midgut is composed of three distinct anatomical regions with different luminal pH. They are characterized by different cell types that ac-complish digestion and absorption activities (columnar cells), acidification of the midgut lumen (cuprophilic cells), and growth of the epithelium (stem cells). The proteolytic ac-tivity is high in the posterior region, where the pH is around 8, and the major activities are due to trypsin- and chymotrypsin-like enzymes. Moreover, the analysis of the amylo-lytic activity showed that amylases are secreted in the first part of the midgut lumen. Larvae reared on the vegetable mix do not show alterations in the general morphology of the midgut, but a significant increment in digestive enzyme activities can be ob-served, especially for those involved in protein digestion. This work represents the first attempt to characterize from a morphofunctional point of view the larval midgut of H. illucens and evaluate the effects of different diets on the features of this organ. Moreover, it sets the stage for the best exploitation of the biocon-version ability of this insect. This work was supported by Fondazione Cariplo (Insect bioconversion: from vegetable waste to protein production for fish feed, ID 2014-0550)