Halictus rubicundus, the orange-legged furrow bee, is a species of sweat bee found throughout the Northern Hemisphere. H. rubicundus was introduced into North America from the Old World during one of two main invasions of Halictus subgenera. These invasions likely occurred via the Bering land bridge at times of low sea level during the Pleistocene epoch. The species exhibits different social behaviors depending on climate: it is a solitary species in high elevations or latitudes where the season is short, but eusocial in other areas. Often, solitary and eusocial colonies appear simultaneously in the same population. These sweat bees are extensively studied for their variability in social behavior, which has become a model for social plasticity. This variability has contributed to an understanding of evolution of social behavior. Taxonomy and phylogeny Halictus rubicundus is a species of Hymenoptera in the bee family Halictidae, more commonly known as sweat bees. This common name comes from their frequent attraction to perspiration. This species exhibits polymorphic social behavior that varies with environmental conditions, and other species of the family Halictidae are thought to have similar variability in sociality. Description and identification Many members of the family Halictidae are metallic in appearance but Halictus rubicundus are not metallic. Females are about 1 cm in body length and brown in color, with fine white bands across the apices of the abdominal segments, and rusty-orange legs. The males are more slender, with longer antennae and yellow markings on the face and legs; they can be distinguished from males of similar species by the absence of an apical hair band on the terminal abdominal segment. In social populations, females of the first brood, mostly workers, can be recognized because they are typically slightly smaller than the foundresses. Distribution and habitat H. rubicundus has one of the widest natural distributions of any bee species, occurring throughout the temperate regions of the Holarctic region. It is believed that differences in climate across this vast range actually contribute to variation in their social behavior. Those living in more northern geographic locations or higher elevations are often more solitary in behavior than those in southern areas or lower elevations. This difference is widely studied, as it provides insight into the evolutionary transition from solitary to social behavior. Nests are haplometrotic, meaning that they are founded by single females. Social populations typically are found in warmer regions, such as Kansas, while solitary