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

Understanding the dynamics of sperm storage is essential to unravel the complexity of post-copulatory sexual selection processes in internally fertilized species. This physiological process goes from sperm transfer during copulation to its use for fertilization. In this context, the spatiotemporal dynamics of sperm storage were described in the black soldier fly (BSF) with fluorescence and transmission electron microscopy (TEM). BSF females have compartmentalized spermathecae with a transfer compartment, the fishnet canals, and a storage compartment, the reservoirs. Spermatozoa were counted both during and after mating in the two compartments. In addition to seminal fluids, the male transfers a mass of sperm in the fishnet canals, then only 49% of the transferred spermatozoa reach the reservoirs over two days. TEM observations of the fishnet canals revealed potential digestive functions, explaining the decline in the number and viability of spermatozoa in this compartment but not in the reservoirs. After one mating, females laid up to three fertile clutches, showing no constraints on sperm quantity or quality. Spermatic and ultrastructural investigations strongly suggest that BSF ejaculate acts both as a sperm plug and as a nuptial gift, reinforcing the interest in studying this farming insect as a new model for sexual selection.