Size-stabilized, Hypoallometric, Genitalia Determined for Male Black Soldier Fly, Hermetia illucens (Diptera: Stratiomyidae)

The black soldier fly, Hermetia illucens, is mass-reared throughout the world to convert organic waste into ingredients for pet and livestock feed, as well as frass and other biological products. To promote the optimization of breeding regimes carried out by industrial black soldier fly production plants, it is critical to better understand adult reproductive physiology and its potential relationship with fertile egg production. However, in this species, little is known about how reproductive morphology changes with respect to increases in body size, nor the potential impacts selection can have on the form of reproductive structures. Hence, this study investigated the nutritional static allometric relationship between the external genitalia of adult male black soldier flies and their body size. Size differences were induced top-down by varying larval rearing densities, which has a downstream effect on adult body size. For each rearing density, a random sample of 30 individuals was selected, and measurements taken three times each for the head, thorax, parameral sheath, and gonostylus. Fitting a generalized linear log-log model to the data revealed that for every 10.0% increase in body size (thorax length), genital length (parameral shealth) only increased by 1.8%. The resulting allometric slope of genitalia to thorax size was 0.195, indicating a pattern of hypoallometry. The presence of hypoallometric genitalia in a domestic population indicates individuals can copulate irregardless of differences in body size, which is consistent with most other insects. Moreover, it is likely that the ancestral population of black soldier flies historically underwent selection to maintain a single genitalia-size independent of body size that continues to persist in captivity. To confirm, future work should investigate the direct impacts of hypoallometric genitalia on fitness, especially in flies which have been genetically edited or artificially selected to be increasingly large. LAY SUMMARYAdult male black soldier fly shown to have similar sized genitals despite differences in body size. SHORT SUMMARYThe black soldier fly, Hermetia illucens, is an economically important insect mass-reared throughout the globe; however, a large knowledge gap exists in terms of its reproductive physiology. This study examined the relationship of male genitalia to body size, finding a 10% increase in body size corresponded with a 1.8% increase in genitalia size, meaning the structures are hypoallometric. This finding is important because it indicates little adverse consequences for selecting larger flies, as differently-sized individuals should be able to copulate.