Scientists have discovered the first known "soldier" bee. The bee, which is larger and heavier than other worker bees in its nest and defends the entrance, has been identified in colonies of a Brazilian stingless bee known as a Jatai.

A team of researchers from Sussex University and the University of São Paulo studied the Jatai bee (Tetragonisca angustula) which lives in colonies of 10,000 individuals in cavity walls, tree trunks or in the ground.

The study, published in the journal Proceedings of the National Academy of Sciences, found that Jatai soldiers are 30% heavier than their forager nestmates, have larger legs and smaller heads. They stand on the tube leading to the entrance of their nest to provide early warning of attack by robber bees, which can kill entire colonies when raiding nests for food.

The new research shows that Jatai soldier bees, unlike guard bees in honeybee colonies, are physically specialised to perform the task of protecting their nest mates.

Jatai soldier bees guard the entrance to their nest. Photograph: PNAS

Francis Ratnieks, professor of apiculture at Sussex, one of the scientists involved in the study, said: "The discovery is significant in term of the evolution of advanced insect societies. Large-bodied soldier workers have long been known in ants and termites, but this is the first evidence of a soldier bee – a worker physically designed for active defence of their nest."