Interspecific mating in eusocial Hymenoptera can be favored under certain conditions even if all hybrid offspring are completely infertile. This exploits two key features of the eusocial Hymenoptera: a haplodiploid genetic system and reproductive division of labor in females. Interspecifically mated queens can still produce viable sons that will mate intraspecifically. Apparent reduced fitness resulting from producing infertile daughter gynes can be also offset by advantages conferred by hybrid workers. An important advantage is likely to be superior ability at using marginal habitats. Interspecifically mated queens can nest in sites where intraspecific competition will be low. By mating interspecifically, a queen trades expected reproductive success through female offspring for a higher probability of achieving some reproductive success. Females that mate interspecifically can be considered "sperm parasites" on the males of the other species. I provide evidence that sperm parasitism is responsible for widespread hybridization in North America among two species of the ant subgenus Acanthomyops (genus Lasius), and review evidence for sperm parasitism in other hybridization phenomena in ants. Sperm parasitism in ants represents a novel form of social parasitism in ants and a dispersal polymorphism. It may also act as a precursor to the evolution of some other recently discovered phenomena, such as genetic caste determination.

The fulva–rudis–texana complex of the ant genus Aphaenogaster includes A. fulva, A. rudis, A. texana, and morphologically similar species. Morphometric and other morphological investigations were conducted on colony representatives of 10 forms (4 likely representing undescribed species) that were previously identified using cytogenetic and electrophoretic markers. In workers, most qualitative characters exhibit such a high degree of size-associated intraspecific variation relative to interspecific differences that they are not reliable for identification. Linear discriminant analysis and canonical variate analysis on 12 morphometric measurements were used to optimize classification and define a morphometric habitus for each form. Two-variable scatterplots clarify the nature of morphometric variation in the complex and provide simple characters that will reliably separate numerous pairs of forms. Indices appear to be of little taxonomic value in this complex. A preliminary key to the workers of the complex summarizes the most important taxonomic characters. This key substantially improves the ability to recognize morphologically most members of the complex, but sometimes only genetic evidence is definitive.

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Although the Hymenoptera represent a remarkably diverse and socioeconomically important group that is of considerable interest in genome biology, they remain understudied in terms of genome size. This study reports new genome size estimates for 89 species of ants, bees and wasps, representing 17 families and four superfamilies. These are used in a test of the hypothesis that genome sizes are constrained by traits associated with parasitism or eusociality. Not all parasitoid wasps exhibit small genomes, though a relationship based on specific types of parasitism may still occur; by contrast, there was no convincing evidence of a constraint relating to eusociality. The data provided here can be used to guide future research aimed at understanding the evolution of large-scale genomic properties in this order.

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