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Adult head and body varies from pale to straw yellow. The wings are light brown with dark brown mottling and straw coloured legs (LaFontaine, 1981). Male clasper is huge, directed postero-dorsally and distally widened to an approximately rounded object with a dentate perimeter.

Adult emergence throughout the season with a 5 to 6 month egg diapause. Both emergence and egg laying occur in the evening, the females will flop aimlessly on or near the water edge while extruding a mass of eggs (hangs off the tip of the abdomen), then later attaches the mass to a solid object. H. borealis adults look different in flight from other Caddisflies. They fly with both wings in unison because the hindwing is attched to the forewing with a row of tiny hooks; normal Caddisflies beat their wings as two spread-out sets (LaFontaine, 1981).