The Stonefly

Stoneflies are captivating insects that belong to the order Plecoptera, given the Latin name which means “folded wing.”  They are one of the oldest insect species of more than 3,500 different variations found around the world except Antarctica.  Stoneflies thrive in freshwater habitats, particularly in fast-flowing rivers, streams, and lakes.  Their unique life cycle enables them to flourish in healthy aquatic environments, undergoing an incomplete metamorphosis of three distinct life stages: egg, nymph, and adult.

Stonefly.  Image by Tyler Edkahl, Togens Pro Staff

Stoneflies have a unique appearance that makes them so different from other insects.  They typically have a soft, elongated body ending with two long tails, and with two pairs of membranous wings laid over their backs. Their wings are transparent or smoky in coloration.  They have three pairs of legs with two claws are essential for gripping onto rocks.  They have large, compound eyes with exceptional visual perception, along with lengthy, thread-like antennae that aid in sensing their surroundings.

Stonefly.  Image by Jon Baiocchi.

The lifecycle of the stonefly from egg to nymph and straight to adult is about one year to three years depending on the species.  The egg stage is the life cycle of a stonefly begins when the female lays eggs on the surface of the water.  Stonefly females have a unique way of laying their eggs involving dipping her egg-filled abdomen into the water and dropping them into the water while hovering just above the surface.  They can produce as many as a thousand eggs.  These eggs are then transported downstream by the current. The eggs are coated with a sticky residue which allows them to adhere themselves to rocks, gravel, or other submerged river materials.  Most stonefly eggs can hatch within few days or weeks.

Female stonefly about to lay her eggs on the surface of the water.

After the eggs hatch, the stonefly enters the nymph stage.  The nymph stage is the longest phase of life cycle lasting from a few months to several years, depending on the species and environmental conditions.  As nymphs, they breathe through their gill system or through their exoskeleton.  Stoneflies thrive in fast flowing highly oxygenated waters and clean habitats.    During this stage, they primarily reside in crevices between stones and pebbles.  Most stonefly nymphs are herbivores and primarily feed on algae, aquatic plants, and other small organisms.  Larger golden stoneflies  exhibit a carnivorous diet, preying on smaller stonefly nymphs, mayfly nymphs, and caddis nymphs.

Pat's Rubber Leg.

The nymph stage is also when the stonefly undergoes several molts, shedding its exoskeleton and growing a new one.  Stoneflies can molt up to 20 times during the nymph stage in growing and development into adults.  As they near maturity, stonefly nymphs migrate to swifter water currents near the shore and begin the metamorphosis process.  They congregate on the shoreline, climbing out onto nearby rocks and shore vegetation to start their pupa stage.

The pupa stage is when the stonefly is full grown adult.  Typically, stoneflies emerge during darkness or dusk.  They will adhere to rocks and trees where their husk separates open and the adult stonefly emerges from their pupa casing. After a few minutes, the wings of the new adult stonefly dry and harden, preparing them for their first flight.

Skwala Stonefly.  Image by Jon Biaocchi.

As adult stonefly, their primary purpose in this stage is to mate and reproduce.  The newly adults seek refuge in nearby trees, willows, and grass.  They live for maybe a few weeks to a few months where they will mate, lay eggs, and eventually die.

Stonefly.  Image by Scott Bowerman

The adult stoneflies discover another mate through a series of acoustical drumming.  Stoneflies drum by tapping, rubbing, or scraping their abdomens against substrates like rocks and twigs.  The male’s drumming is reciprocated by a female.  The drumming sounds are exclusive to the same species specifically.  Through continued drumming, the male and female locate one another and proceed to mate.

Stoneflies are remarkable insects.  They require optimal water conditions to survive.  From the egg stage to the adult, each stage of the of the stonefly’s life cycle with an incomplete metamorphosis.  The adult stoneflies period can be some of the most exhilarating fishing for the year.  See you on the river!