[VFB] Winter flies ??

[iain short]

Yesterday I drove home across the bridge over the river wansbeck in the 
north east of england.  It is real cold with a six foot wide skin of ice 
from both banks.  What i couldnt understand was why every trout/greyling in 
the river was rising apparently to take flies.  whats hatching at 5 below 
freezing ???

iain