Metamorphic remodelling of the larval midgut in Hermetia illucens

The black soldier fly (BSF), Hermetia illucens (Diptera: Stratiomyidae), is studied for its bioconversion ability of organic waste. Moreover, the high nutritional value of prepupae and pupae makes this insect useful for the production of feedstuffs. Despite the great interest for this species, data on BSF biology remain scarce and basic biological studies on BSF are limited. In particular, no information on the morphology, physiology and development of the midgut, which is implicated in food digestion and nutrient absorption, is available in the literature. In the present study we performed a structural and functional characterization of the midgut during the larva-pupa and pupa-adult transition. We also investigated how this organ changes during metamorphosis, a critical phase that, in holometabolous insects, leads to the rearrangement of different larval organs. To this aim, we analysed the morphology of the midgut epithelium, the behaviour of intestinal stem cells and the mobilization of long-term storage molecules, i.e. glycogen and lipids. Moreover, we performed experiments to investigate the functionality of this organ during the remodelling process. Our results demonstrate that, similarly to other holometabolous insects, the larval midgut of H. illucens is completely removed during metamorphosis and a new pupal-adult epithelium is progressively formed by the proliferation and differentiation of stem cells. During this process, glycogen deposits and lipid droplets are depleted. Moreover, functional experiments suggest that the newly forming epithelium is endowed with digestive capabilities. This work was supported by Fondazione Cariplo (Insect bioconversion: from vegetable waste to protein production for fish feed, ID 2014-0550). D.B. is partially supported by a grant from Consorzio Interuniversitario per le Biotecnologie (CIB).