Whoever said size mattered? Male genital allometry in the black soldier fly, Hermetia illucens, is hypoallometric

The black soldier fly, Hermetia illucens, is mass reared to convert organic waste into insect biomass. However, little is known about how the morphology of heterogeneously-sized individuals interacts with mate choice within breeding colonies. This study investigated the nutritional static allometric relationship between male genitalia and body size. Larvae were reared at three densities to produce a gradient of body sizes. Adults were dissected and measured using microscope imaging software. Proxies for body size included head and thorax, whereas proxies for genital size included parameral sheath and gonostylus. Fitting a generalized linear log-log model to the data showed for every 10.0% increase in body size, genital size only increased by 1.8%. Hence, body size and genital size were demonstrated to be hypoallometric, irrespective of rearing density. Principal component analysis supported this finding, revealing body size measures to be inversely related to density, and both to have little-to-no relationship with genital size. Because male genitalia size is stabilized relative to body size, this study indicates female choice in black soldier flies may either favor males with intermediate genital size, be weak, or nested within a larger network of mechanisms. Therefore, future investigations should investigate the impacts of cryptic choice, as well as how concomitant increases in body size impact territoriality, choosiness, and mate success. In summary, this study reveals female preference for genitalia of a particular size is unlikely to be upstream cause of disruptions in fertile egg production because males generally have similar genital size irrespective of body size.