The Common Yellow Sally is a medium-sized stonefly and quite plentiful on fast-flowing stony rivers and streams; its slow, creeping nymph spends most of the time beneath stones where trout are unable to find it, but during spates when stones get disturbed some nymphs may end up drifting helplessly on the current.
When it is ready to transpose to a winged adult, the nymph moves into the shallows, usually leaving the water either by crawling up emergent vegetation or by climbing onto a partly submerged stone.
The Common Yellow Sally adult appears as a sulphur yellow blur in the summer sunshine, and it is one of the very few insects that 'hatch' during the heat of the day.
Another yellow stonefly species, Siphonoperla torrentium, is very similar and is also most likely to be seen during the daytime.
Pryce, D., Macadam, C., & Brooks, S. (2007) Guide to the British Stonefly (Plecoptera) Families: adults and larvae. Taunton: Field Studies Council.
Hynes, H.B.N. (1977) Adults and Nymphs of British Stoneflies (Plecoptera). Ambleside: Freshwater Biological Association.