Great balls of fire, yes, but great balls of flies? Yet, this was the scene that
confronted Louise Davis on a recent walk alongside the picturesque River Torridge in
north Devon.
Louise, who works as the Water Resources Advisory Officer on Devon Wildlife Trust's
Northern Devon Nature Improvement Area project, was busy conducting a survey of a
stretch of the river near Torrington when she spotted something hanging over the
water from a tree. Louise takes up the story: '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, but
on taking a closer look I discovered that each was a tightly-packed dome consisting
of hundreds of flies. I'd never seen anything like it before.'
Louise took photos of the balls and showed them to colleagues, but they got no
closer to identifying the strange phenomenon. It was only when she passed the images
on to Martin Harvey, a leading fly expert, that the answer was revealed. The fly
concerned turned out to be Atherix ibis, part of ibis fly family. Martin explained
that the flies are known to adopt this strange behaviour, but that it was 'not often
seen'.
Martin explained that the behaviour takes place when the flies are preparing to lay
their eggs. Females come together to form dense balls on branches overhanging
rivers. Often these balls can consist of several hundred flies and are frequently
formed where flood refuse has been caught in a branch, giving a good base on which
the flies can cling. The females then lay 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.
The explanation solved a mystery for Louise: 'My role for the Northern Devon Nature
Improvement Area is about working with local landowners to improve the water quality
of the River Torridge for the benefit of local people and wildlife. This means I'm
out doors a lot amongst the beautiful countryside of north Devon. I also get to see
a lot of wonderful wildlife at close quarters, but I'd never seen anything like
this. Nature really is full of surprises!'
Photos copyright Devon Wildlife Trust
