Stoneflies

Stoneflies are close relatives of the cockroaches. Generally drab-colored, small to medium sized, rather flattened insects usually found near streams or rocky lake shorelines. They are known as inhabitants of cool, clean, highly oxygenated waters, and, as such, are excellent biotic indicators of water quality. They are important as a food source for a variety of game fishes and are found year round in suitable habitats.

Stonefly nymphs are not active swimmers, but are accomplished crawlers among stones, gravel and detritus. They go through several molts or "instars" on their way to adulthood. They are able to maintain themselves in rapid flows and many species are drifters. They often look like mayfly nymphs, but a very general difference is stonefly nymphs have 2 tails, mayflies have 3.

Emergence of adults usually occurs at the stream or lake edge, rather than in mid-water. Many hatch at night, with males often preceding females. Both temperature and photoperiod appear to be important in emergence.

A Few Key Stoneflies . . .

(To reveal the adult forms of these immature larva/nymphs, place mouse over image)

"Slender winter" stoneflies (family: Capnia vernalis)			This widespread species occurs in habitats ranging from tiny springs to moderate-sized streams, often among detritus or mixed substrates. Generally, this family has been described as "little black" or "early black" stoneflies.
	larva = 3-6 mm	adult = 5-7 mm
"Giant" stoneflies (family: Pteronarcys californica)			A small group (10 species) of conspicuously large flies widespread but absent in Texas. Some require 3 year life cycle. The popular & important "Salmonfly" on western rivers is a legendary member of this family.
	larva = 15-50 mm	adult = 38-44 mm
"Common" stoneflies (family: Acroneuria evoluta)			Among some of the more common genera, with 3 families. Larva are generally brown with yellow markings, found in many lotic habitats under stones or in sandy substrates. Life cycles can require more than a year. Spring emergers.
	larva = 8-35 mm	adult = 28 mm
"Perlodid" stoneflies (family: Isoperla bilineata)			Larva of these small stoneflies are often highly patterned. Well represented in the drifting fauna of streams. Colors vary: pale yellow, olive, light green, light brown, reddish brown to black. Hatching occurs at waterline of certain rocks, smooth ledges, and deadfalls polished by current.
	larva = 8-16 mm	adult = 13-15 mm

Stoneflies (order Plecoptera)	Giant Stonefly adult	World species: 1150
		Species in N.A. north of Mexico: 500
		Source of name: "pleco" = folded/plaited & "ptera" = wings
		Nickname: "downwings"