Alternate tactics in male bladder grasshoppers Bullacris membracioides (Orthoptera: Pneumoridae)

Bladder grasshoppers (Orthoptera, Pneumoridae) are a unique group of nocturnal, specialized herbivores, endemic to coastal regions of Southern Africa. Adult males are characterized by an inflated abdomen, which develops at the final molt, and a signaling system capable of producing high intensity calls detectable by conspecifics at distances of up to 2 km. Female response to male calls leads to reciprocal duetting and pair formation. We have found an alternative form to the dominant inflated male tactic in three pneumorid species. These alternate males are incapable of flight and lack the air-filled abdominal bladder used in long-range acoustic communication; but may be found in copula with mature females in the field. Here we address the issue of neoteny/paedogenesis in the alternate male by comparing the morphology of inflated and uninflated adult male Bullacris membracioides with that of ultimate and penultimate nymphal instars, to determine whether these forms follow a common developmental trajectory. We then compare the behavioral responsiveness of inflated and noninflated morphs to conspecific advertisement calls, to ascertain whether alternate males have the potential to actively exploit the mate location system of duetting pairs. Morphometric analyses indicate that alternate males are a distinctly divergent form from the primary male developmental tactic. We conclude that males following the alternate tactic eavesdrop on the duets of inflated males and females to exploit the acoustic mate location system. This satellite tactic has potential impact on several aspects of pneumorid biology contributing to biasing sexual selection, sensory drive, and ultimately speciation events.