The developmental basis of worker polymorphism in fire ants

The fire ant, Solenopsis invicta, has a relatively simple worker caste. Workers show a wide size range but maintain near proportionality over that range. Data on normal and hormonally manipulated growth suggest a developmental mechanism underlying the production of two populations of workers that together form a single, highly skewed size-frequency distribution. The mechanism involves regulation of two sets of critical sizes for metamorphosis. Larvae that reached a large size during the 3rd instar relative to their cohort became major workers. Major workers were much larger than could be predicted by the relationship between larval and pupal sizes of minor workers. Methoprene treatment increased the mean size of workers. The mechanism of these increases differs with timing of application. Treatment during early instars increased size at the moult to the last instar, and this increase was translated into a large pupal size. Treatment during the 4th instar caused a metamorphic delay during which larvae continued to grow. Although worker size can be manipulated with methoprene, it is not clear if juvenile hormone plays any role in resetting critical size. The type of control of size-frequency distributions found in S. invicta may apply to other ant species.