Isoperla grammatica - Yellow Sally

The Yellow Sally is a medium-sized stonefly and very common on Welsh rivers and streams; its slow, creeping nymph spends most of the time beneath stones where trout are unable to find it. When ready to transpose to a winged adult, the nymph moves into the shallows. Usually it leaves the water either by crawling up emergent vegetation or by climbing onto a partly submerged stone.

The 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. A related species, the Large Yellow Sally (Chloroperla torrentium) is very similar apart from size and is also most likely to be seen during the daytime.

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