Although increasingly targeted in animal nutrition, black soldier fly larvae or prepupae (BSF, Hermetia illucens L. 1758) require the characterization and modulation of their fatty acid profile to become fully integrated within the feed sector. This improvement will only be possible by the understanding of underlaying biochemical pathways of fatty acid synthesis in BSF. In this study, we hypothesized a labelling of de novo synthesized fatty acids in BSF by the incorporation of deuterated water (D 2 O) in their feed. Three batches of fifty larvae were reared on two diets with different polyunsaturated fatty acid profiles moistened with 40% of H 2 O or D 2 O: chicken feed or 40% of chicken feed and 60% of flax cake. Although the occurrence of D 2 O in insect feed increased the larval development time and decreased prepupal weight, it was possible to track the biosynthesis of fatty acids through deuterium labelling. Some fatty acids (decanoic, lauric or myristic acid) were exclusively present in their deuterated form while others (palmitic, palmitoleic or oleic acid) were found in two forms (deuterated or not) indicating that BSF can partially produce these fatty acids via biosynthesis pathways and not only by bioaccumulation from the diet. These results suggest the importance of carbohydrates as a source of acetyl-CoA in the constitution of the BSF fatty acid profile but also the potential importance of specific enzymes (e.g. thioesterase II or Δ12 fat2 desaturase) in BSF fatty acid metabolism. Finally, nearly no deuterated polyunsaturated fatty acids were found in BSF fed with deuterium confirming that BSF is not able to produce these types of fatty acids. Despite the high levels of linolenic acid in flax-enriched diets, BSF will simply bioaccumulate around 13% of this fatty acid and will metabolize approximately two-thirds of it into saturated fatty acids as lauric or myristic acid. Insects are increasingly identified as an environmentally sustainable source of proteins and lipids for food and feed but also for pharmaceutical or biodiesel applications 1-3. While yellow mealworm larvae (Tenebrio molitor L. 1758) appears to be the preferred model in human nutrition for environmental, nutritional and acceptability considerations 4-6 , larvae or prepupae of black soldier fly (BSF, Hermetia illucens L. 1758) emerges by their ability to develop rapidly and extensively on a wide range of decomposing material for the feed sector 7-9. Although possessing unquestionable nutritional qualities 9,10 , the incorporation of BSF into animal feed is still limited by several factors including their high lipid content (till approximately 40%) and their unbalanced fatty acid profile 10,11. As BSF fatty acid profile is known to be related with the one of their diet, several experiments have tried to manipulate their fatty acid profile with enriched feed formulation 8,12-14. By analysing the fatty acid profiles in BSF fed with different diets, it appeared that BSF larvae contain a high level of saturated fatty acids (SFAs, lauric acid...

The black soldier fly (BSF), Hermetia illucens (L., 1758) (Diptera: Stratiomyidae), is an endemic fly species from the tropical, subtropical and warm temperate zones of America. This saprophagous species relies on its environment where it finds the decomposing matter for the larvae to grow. The polyphagous diet and the macronutrient quality (mainly lipids and proteins) of these larvae make them excellent candidates for various applications such as waste and organic material management, incorporation in animal feed or alternative energy source. Although rearing development in temperate regions requires artificial processes to continuously produce high quality eggs and larvae, few studies have been conducted on the mating and oviposition processes governing H . illucens reproduction. Research conducted in semi-artificial rearing conditions showed that the number of mating varied according to the season. It has been speculated that this behavior could be due to differences in the intensity of sunlight caused by the change of seasons. This study aims at evaluating the influence of sex-ratio, density and nycthemeral cycle on H . illucens reproduction. In order to tackle this issue, an artificial set up for oviposition to collect eggs has been developed. This egg collection system aims at centralize oviposition and simplify eggs collection. Two populations with opposite sex-ratio (male-dominant and female-dominant) were selected. Their respective eggs productions have been evaluated for five breeding densities. Eggs weights varied significantly among the densities for each opposite sex-ratio population and female dominant population produced most eggs weight from 6500 individuals /m 3 . Finally, four nycthemeral cycles (2, 6, 12 and 18h of daily light) were simulated to evaluate the impact of light duration on reproduction. Early oviposition pic associated with a decrease of the oviposition period are shown when H . illucens are exposed to increasing light duration. These experiments enable improvement of the understanding on artificial reproduction of H . illucens .

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