The reproductive tract of the black soldier fly (Hermetia illucens) is highly differentiated and suggests adaptations to sexual selection

The reproductive biology of the black soldier fly (BSF), Hermetia illucens (L.) (Diptera: Stratiomyidae), was investigated in males and females with a focus on how sperm is handled by males before -and by females after -copulation. As this insect has great economic importance for protein production from agri-food wastes, its reproduction needs to be precisely known. A detailed description of male and female reproductive organs was made, and sperm were counted in male and female storage organs. Measures of testis and sperm counts were performed in males of various ages and their dynamics was compared to the ovaries of females. Both male and female tracts were long, and consisted of various successive parts. In males, testes fill the vas deferens with mature sperm, that are tangled as 'windrows', and sperm pass successively through a 'studded pipe' and three phallic 'wands' before being transferred to females. In virgin males, mean (+/- SEM) sperm number increased from approximately 9700 (+/- 1400, n = 5) to 39600 (+/- 3470, n = 13) in 20 days, and testes gradually decreased in size, indicating continuous spermatogenesis. Sperm cells are long, more than 3 mm. During copulation, sperm are deposited by phallic 'wands' at the base of each spermatheca in a smooth tube followed by a 'fishnet canal' at the top of which they accumulate before complete storage. They then pass through a ringed canal, an elbow, and a rigid rod strewn with glands, before reaching the spermathecal reservoir. Females store approximately 680 spermatozoa after one mating in each spermathecal reservoir. Such complex sperm storage organs of females may indicate strong female post-mating selection, and reflect competition for long and numerous sperm in this species, in which female multiple matings are suspected. Besides its economical interest, BSF is an appealing model for further investigations on sexual selection.