You may have heard of soldier ants - whose primary function is to guard their nest from intruders. Now, scientists have discovered a new soldier, in the usually much less confrontational world of bees. A University of Sussex team found that, in colonies of Jatai bees (Tetragonisca angustula), some insects are born soldiers. The study, reported in the journal PNAS, is the first known example of a soldier bee. While the caste system is common in ants and termites, with insects of different shapes and sizes assuming defined roles, the division of labour in bees is usually much more transient. \"Workers carry out different tasks at different ages,\" explained Prof Francis Ratnieks from the University of Sussex, who led the research team. \"They start out cleaning the nest, then feeding the larvae... then foraging and [eventually] guarding.\" But while most bee guards take on their role for about a day, Jatai bee guards stand guard at the wax entrance tube to their nest for about a week, which, in the insect realm, is a relatively long career. To find this out, the team observed the bees\' nests on a farm in Fazenda Aretuzina, Brazil. They used dots of paint to mark the bees that were hovering and perching close to the entrance, which revealed that these guards assumed that role for extended periods of time. \"We then took some of these [guard bees] back to the lab to examine them more closely,\" explained Prof Ratnieks. From this examination, he and his colleagues realised that the bees were not just behaviourally different, they were also a different size and shape to the worker bees. \"The Jatai soldiers are 30% larger than worker bees,\" said Prof Ratnieks. \"They also have larger legs that they probably use for grappling.\" Battling robber bees Prof Ratnieks and his colleagues think that the Jatai soldier bees may the product of an evolutionary arms race against the diminutive species\' worst enemy - the robber bee (Lestrimelitta limao). Large robber bee and Jatai soldier bee fighting Much smaller Jatai soldier bees clamp onto the wings of robber bees Robber bees are so-called because, rather than forage, they simply invade other bees\' nests and steal their food reserves. \"They\'re much bigger than Jatai bees and a full-blown attack can destroy a [Jatai] colony,\" explained Prof Ratnieks. Soldier bees appear to help prevent an attack by tackling individual robber \"scouts\" that set out to find a suitable victim colony to invade. The scientists actually tested the soldier bees\' ability to fend off a robber, \"staging fights\" between the two insects. They held a robber bee close to the entrance tube of a Jatai bee nest and watched the Jatai soldiers\' reaction. The much smaller Jatai soldier bees used their jaws to clamp onto the robber bees\' wings, immobilising their attacker. The outmatched Jatai\'s are ultimately killed during these fights. They seem to \"sacrifice themselves\" to protect the colony, Prof Ratnieks said. He added: \"These bees represent the pinnacle of social living.\" Dr Richard Gill, a bee specialist from Royal Holloway, University of London explained that insect societies could \"act more efficiently\" if individuals were the right size and shape for a particular job. \"Take nightclub bouncers, security guards, and rugby players,\" he said. \"It often helps if you are big when tackling a conflicting situation. \"The same seems to be true for these bees when deterring nest robbers.\" Prof Ratnieks and his team have dedicated their discovery to the researcher on whose farm the study took place. \"Dr Paulo Nogueira-Neto, is one of the world\'s leading experts on stingless bees,\" Prof Ratnieks told BBC Nature. \"We wish to dedicate this work to him on the occasion of his 90th birthday.\"