The bioconversion capability of Hermetia illucens larvae: a morphofunctional study of the larval midgut

The black soldier fly (BSF), Hermetia illucens, is among the most promising insect species for the reduction of organic waste. Moreover, the high nutritional value of larvae and pupae makes them an interesting and sustainable protein source for the production of animal feed. Despite the great interest toward this insect species, the current literature mainly provides information on the rearing methods of BSF and indications on its use for waste treatment, 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 food substrates. The main aim of the present study is to 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. The activity of the enzymes that are involved in protein and sugar digestion (i.e. trypsins, chymotrypsins, and amylases) is associated to specific districts of this organ. According to the evidence collected, 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. This work was supported by Fondazione Cariplo (Insect bioconversion: from vegetable waste to protein production for fish feed, ID 2014-0550).