Morphological and functional characterization of the larval midgut of Hermetia illucens (diptera: Stratiomyidae), a promising insect for bioconversion and feed production

The Black Soldier Fly (BSF), Hermetia illucens, is among the most promising insect species for the reduction of organic waste. The high nutritional value of the larvae and pupae makes them an interesting and sustainable protein source for the production of animal feed. Despite the great interest toward this insect, the current literature mainly provides information on the rearing methods for BSF, and indications on its use for waste treatment and feed production, while little is known about H. illucens biology. In particular, 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 substrates. In the present study, we provide a detailed structural and functional characterization of the midgut of last instar larvae, 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. The midgut epithelium is formed by different cell types that accomplish nutrient digestion and absorption, acidification of the midgut lumen, endocrine regulation, and growth of the epithelium. A detailed characterization of the activity of enzymes involved in nutrient digestion and their mRNA expression levels reveals that protein and sugar digestion is associated to specific regions of this organ. According to the collected evidence, we propose a functional model of the larval midgut of H. illucens in which each region is characterized by peculiar features to accomplish specific functions. This work represents the first morphofunctional characterization of the midgut of H. illucens larvae and sets the stage for the best exploitation of the bioconversion ability of this insect. Work supported by Fondazione Cariplo (Insect bioconversion: from vegetable waste to protein production for fish feed, ID 2014-0550).