Gone but not forgotten: dynamics of sperm storage and potential ejaculate digestion in the black soldier fly Hermetia illucens

When it comes to describe reproduction in internally fertilized species, understanding the dynamics of sperm storage is crucial to unravel the complexity of post-copulatory sexual selection processes. This physiological process goes from sperm transfer to its use for fertilization. In this framework, the spatiotemporal dynamics of sperm storage were described in the black soldier fly (BSF) with fluorescence and transmission electron microscopy (TEM). Besides being of great economic interest, BSF females have highly compartmentalized spermathecae. Spermatozoa were counted both during and after mating in two successive spermathecae compartments: the fishnet canals and the reservoirs. In addition to seminal fluids, male transfers a sperm plug in the fishnet canals, then only some spermatozoa reach the reservoirs over 2 days. TEM observations of the fishnet canals revealed potential digestive functions, explaining the decline in the number and the viability of spermatozoa that have not been stored in the reservoirs. After one mating, females laid up to three fertile clutches, evidencing no constraints on sperm quantity or quality. Spermatic and ultrastructural investigations strongly suggest that BSF ejaculate acts both as a sperm plug and a as nuptial gift, reinforcing the interest in studying this insect as a model for sexual selection.