Queen size dimorphism in social insects

Abstract

Variation in queen size distribution in social insects is sometimes discontinuous. Consequently, such species are divided into two size morphs, macrogynes and microgynes. We review the occurrence of queen size dimorphism (QSD) in social insects, discuss the causes of the phenomenon and show which aspects are important to identify and distinguish the causes of QSD. Until now, QSD has been connected mainly with alternative reproductive strategies or social parasitism, but we show that ecological speciation and selfish larvae development can also cause QSD. We identify some precursors leading towards it and examine its prevalence across taxa. Different mechanisms have selected for the evolution of queen dimorphism in stingless bees and ants. We argue that especially in ants, the occurrence of microgynes is not a stable state, but rather a transition from an intraspecific polymorphism manifested as alternative reproductive strategies to the emergence of a new species, often combined with social parasitism. Thus, studying QSD can give valuable insights into the evolution of social parasitism and speciation processes in general.