A few weeks back I found some small animals on some fungi I had collected. They appear to be three different species of springtail. (Collembola).
[Note added 2012-12-25: They were actually on the Xylaria longipes shown here. I have since also found them on Xylaria hypoxylon. Maybe there is an association between Collembola and Xylaria spp or with the places that Xylaria spp grow?]
Springtails are arthropods but are not classed as insects.
Specimens collected in the Wilderness, Whiteknights Park, Reading, UK on 2012-12-09.