Just a few years ago, it seemed like the scarce yellow sally stonefly had gone locally extinct.

In 1995, ecologists collected a single specimen of the aquatic insect in the River Dee near the Wales-England boundary, the species’ only known refuge. For the next two decades, every survey there failed to find another of the stonefly, which is only about a half an inch long.

“There had been so much work done to refind this beast,” said Craig Macadam, conservation director at the Invertebrate Conservation Trust, more commonly known as Buglife, a charity in Britain. “We were all beginning to give up hope.”

Small, isolated populations of stoneflies reside in pristine brooks, where they are vulnerable to pollution and habitat fragmentation. Scientists have described stoneflies as one of the most threatened insect groups, one that has experienced high extinction rates in recent decades.

Even among the numerous species of its family, the scarce yellow sally stonefly (“scarce” is part of its name) is noted for its rarity, said John Davy-Bowker, a freshwater biologist who has surveyed the insect’s population since the 1990s. Without any new evidence of its survival in the River Dee, the scarce yellow sally stonefly would be declared locally extinct, Mr. Macadam said; it already had vanished from an assortment of European countries.