But first: that strange head. The various species of Cephalotes have a range of head shapes. Some are almost perfectly circular, like a manhole cover. These ants typically establish their colonies in dead branches of living trees, where wood-boring beetles have conveniently left cavities. “The size of the soldier head is perfectly matched to the size of the beetles that came out of the tree,” said tropical ecologist Stephen Yanoviak of the University of Louisville. The Cephalotes move in, and at any given time a soldier’s head serves as a door to keep the ants’ many enemies at bay.