SIZE-DIMORPHISM IN THE QUEENS OF THE NORTH-AMERICAN ANT LEPTOTHORAX-RUGATULUS (EMERY)

With recent findings of an unexpected variability in the reproductive behaviour of ant sexuals, their morphology has become an area of major evolutionary interest (Heinze and Tsuji, 1995). Here we report on the occurrence of two queen morphs in Leptothorax rugatulus (Hym., Formic idae): Microgynes (small queens), exceeding worker size only marginall y, and macrogynes, which are, typically for the subgenus Myrafant, abo ut twice as big as their workers. The frequency distribution of queen- size is clearly bimodal, in contrast to worker- and male-size. The ave rage size of queens is highly correlated with the size of daughters in field-collected colonies, whereas within colonies no correlation betw een the average queen-size and the size of workers or males exists. Th is gives additional support that size-dimorphism is due to a specific, transmissible size reduction of the microgynes which could be based o n genetics, the environment or both. This reduction is quasi-isometric , with a slightly smaller thorax-to-head ratio in microgynes, and scan ning electron microscopy does not reveal any significant degeneration of the pterothorax, ocelli or number of ommatidia. The frequency of mi crogynes at different sample sites is highly variable, correlating wel l with the prevailing social structure in the respective subpopulation s. Indeed, the majority of macrogynes is found in monogynous colonies, while microgynes abound in polygynous ones, which is strong evidence for an alternative dispersal tactic. However, the expected correlation to altitude or latitude was not found and further investigations are needed to reveal proximate and ultimate causes of this prevalent polym orphism between two types of female ant reproductives.