Ever since Darwin identified it as the force responsible for the evolution of exaggerated male characters, sexual selection has been the focus of research aimed at understanding the most bizarre and intriguing morphologies and behaviors in Nature. Two congeneric species in the firefly genus Photinus, P. pyralis and P. macdermotti, afford a unique opportunity to examine the interaction between sexual and countervailing natural selection that act to shape the evolution of mating behavior and body size in closely related species with very different courtship strategies. Photinus pyralis males emit very bright flashes during their extended patrolling flights and form ‘love knots’ of competing suitors, while P. macdermotti males, whose courtships seldom exceed two competing males, produce weaker flashes during a shorter patrolling period.
Possibly as a consequence of their scramble competition and long flights, not only is there an extremely wide range of body sizes in P. pyralis males, but they also exhibit wing allometric slopes greater than one, and lantern allometries less than one. In contrast, P. macdermotti males do not have allometric slopes significantly different from one. Small males, when tested in an artificial scramble situation move faster than large males, an advantage in the intense competition that ensues once a female is located. Females answer several males in alternation and fail to respond to all male flashes, with the evident consequence that love knots frequently develop around such fickle females. Allometric relationships in two non-luminescent beetle species with non-visual courtship protocols are compared. Visual, sexually selected characters showed positive allometric slopes, while non-visual characters showed isometry or negative slopes. Data presented here support the existence of distinct patterns in modality-specific sexual selection.