In New England, some of the first emergers of the year show up in mid to late February. Two years ago, we observed stoneflies on the Swift River as early as February 14. This year, similar reports have come in from the Swift, and observations of little winter stoneflies (probably Capniidae family) were recorded for the Hoosic River (Williamstown, MA) on February 22 (with earlier anecdotal reports), and near the mouth of Hop Brook, on Quabbin Reservoir on March 2. The critters don't seem at all fazed by the foot or more of snow that covered the ground, right to the riverbanks. As the Hoosic River 2/22/14 images show (check the observations pages), they were quite active; mating and producing eggs.

Henry Thoreau's journal of March 24, 1857, in which he describes a paddle up the the Assabet River (which runs through Concord, MA), includes this entry:

"I see many of those narrow four-winged insects (perla?) of the ice now fluttering on the water like ephemerae. They have two pairs of wings indistinctly spotted dark and light."

Did he mean the genus Perla, one of several in the Perlidae (golden stonefly) family, or did he use "perla" as a generic term for stonelfies? Throughout his diaries, Thoreau uses a variety of common terms for various plant and animal species that may be confusing to modern readers. In the same diary entry, for instance, he mentions a fish hawk (osprey) and a striped squirrel (chipmunk).

It's possible that what he saw was one of the winter stoneflies - Capniidae or Taeniopterygidae - both of which fit the description of narrow (and 4 winged) insects, may be out this time of year, and are commonly found on snow or ice. The description of a spotted wing may be a bit misleading. Here are examples of adults of

genus Perla (from the UK)
http://www.discoverlife.org/mp/20p?see=I_MWS78704&res=640,

Capniidae (western MA)
http://www.inaturalist.org/observations/576170

and Taeniopterygidae (I think - from western MA)
http://www.inaturalist.org/observations/296485

Thoreau didn't offer any information about the size of the insects he saw. Nor did he mention whether any fish were rising.
Perhaps he didn't want the word to get out.