Figure 5

Dorsal view of a male O. binodis as (A) pupa and (B) corresponding adult. Note presence of pronounced lateral concavities, or depressions, in the prothorax of the adult but not pupae (marked by arrows on left side of the adult). a indicates pupal and adult horn length measurements used in this study, which failed to reveal a male dimorphism in this species. b indicates adult horn length measurement used by previous studies, which relies on μ as the posterior landmark, marked by the posterior-most edge of the two lateral prothoracic depressions. (C) Use of μ as posterior landmark, and b as a measure of horn length, recovers a male horn dimorphism in O. binodis adults similar to what has been documented in earlier studies. (D) Scaling relationship between c (which measures the proportion of the prothorax that does not participate in concavity formation) and adult male size. c initially increases with male size and then declines rapidly in large males, suggesting that large males devote a disproportionately larger fraction of their lateral prothorax toward exposing the medial thoracic horn (solid line = log normal regression: r² = 0.42, p < 0.0001; in contrast a linear regression (not shown) fails to yield a significant fit; r² = 0.06; p = ns). Combined, these results are consistent with the hypothesis that dimorphic remodeling of the lateral prothorax during the pupal stage, rather than dimorphic growth of actual horn tissue during the prepupal stage, generates male dimorphism in O. binodis. To increase statistical power data shown in (C) and (D) include an additional 23 male O. binodis reared under identical conditions excpet that no measurements were taken during the pupal stage.