A mass of flies have made their home on a branch dangling over the River Torridge.
Louise Davis was walking alongside the riverbank in Torrington when she spotted the insects.
Louise works as the water resources advisory officer for Devon Wildlife Trust's Northern Devon Nature Improvement Area project.
She was conducting a survey along the stretch of the river when she saw the strange ball.
Louise said: "It was about the size of my fist, dangling from a branch.
"Then I saw several other similar shaped balls all hanging from the same tree.
"At first I thought they were just debris that had got stuck. On taking a closer look I discovered that each was a tightly-packed dome consisting of hundreds of flies. I had never seen anything like it before."
Louise took pictures of the balls to show her colleagues. However, they were not able to identify why the flies were behaving this way.
The photographs were given to Martin Harvey, a leading fly expert.
He said the species of fly were Atherix ibis, part of ibis fly family. Martin explained that the flies are known to adopt this strange behaviour, but that it was not seen often.
He said the female flies bunch together in preparation to lay their eggs.
These balls can consist of hundreds of flies and are formed where flood refuse has been caught in a branch, giving a good base on which the flies can cling.
The females laid their eggs in a glutinous goo and then die leaving a suspended mass of dead flies and eggs.
When the eggs hatch the fly larvae drop into the river's water to begin the next stage of their life cycle.
Louise said she had never seen anything like it before. She said: "Nature really is full of surprises."