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Parasitic bee tibial spur. Macrophotograph of a tibial spur of the parasitic bee Acanthopus excellens. This is a little-known species of bee endemic to South America. Tibial spurs occur on the legs, and their function is unclear. Acanthopus excellens is a nest parasite of bees of the genus Centris. It is kleptoparasitic, that is, it steals food or other resources from another species. Photographed by the USGS Bee Inventory and Monitoring Lab: an organisation dedicated to identifying, cataloguing and monitoring bees and other insect species. (KEYSTONE/SCIENCE PHOTO LIBRARY/US GEOLOGICAL SURVEY/SCIENCE PHOTO LIBRARY)