Abstract: Ant colonies are organized similarly to those of wasps and bees: reproductive altruism, age polyethism, and complex communication. Yet ants exhibit more species, much higher total biomass, and their lifestyles and diet are more diverse. Hence, factors additional to sociality must be involved in this evolutionary diversification. We argue that loss of flight permitted extensive changes in body size of ant workers and queens. Wingless helpers revolutionized colonial economy because they are cheaper to manufacture. Flightlessness also removed constraints on the evolution of dwarf workers (head width 1 mm or less); these exist in 229 / 286 ant genera examined but not in social wasps and bees. Miniaturisation involves simplification of tissues and organs (compound eyes, sting apparatus, ovaries, exoskeleton), and dwarf workers are cheaper per capita. Comparison of ovariole numbers in 106 genera indicates reduction of ovaries in dwarf workers, and complete loss in six genera of Ponerinae and eight genera of Myrmicinae. Body size influences trophic ecology, but also the pattern in which a colony's finite energy budget is "packaged", allowing increases in colony size if adaptive. Dwarf workers together with big queens enabled the evolution of claustral independent colony foundation that is predominant in three large subfamilies (Dolichoderinae, Formicinae and Myrmicinae). Winglessness allows this divergence of costs between workers and queens, but also novel activity schedules and adaptations for defence. Highly dimorphic queens and workers promoted the evolution of mosaic phenotypes (soldiers and ergatoid queens), which added to colonial complexity (Molet & al. 2012 The American Naturalist 180: 328-341). We speculate that cheaper workers caused a shift away from a carnivorous diet to carbohydrates such as honeydew. Wasp and bee workers – infertile just as in ants – need to fly and this constrained extensive divergence from queens, which prevented bigger colonies. The winglessness of ant helpers maximized the benefits of having two morphological castes.

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