While enjoying a very spring-like late-winter afternoon beside the Trout River, at my home in northern Franklin County, I found myself greatly appreciating the sunshine and fascinated by the large number of black bugs that seemed to also be enjoying the sun’s long-awaited warmth.

The adult stoneflies were emerging in fairly-large numbers; making their way across the ice and snow and onto the tree trunks along the bank; some finding the half-dozen sap buckets I have hanging there. Unfortunately, several made their way into the sap contained in those pails.

A Good Sign 

Stoneflies are aquatic insects and one of very few insect groups that emerge in winter and early spring. Stonefly nymphs (the larvae or sexually immature form of the insect) are also called naiads; a term which stems from Greek mythology (Naiads were female deities associated with fresh water springs, streams, etc.).

Stonefly naiads live on or under rocks and stones resting on the bottom of rivers and streams, which is how the insects came to be called stoneflies. Their ability to withstand and even thrive in cold temperatures allows them to take advantage of superior conditions in their habitat, at a time when most other insects, aquatic and terrestrial, are absent or dormant.

Adult stoneflies have two pairs of wings (4 wings total) held close to their bodies, on top of their elongated abdomens, giving them a somewhat flattened appearance. They’re weak fliers however, and they like to stay close to the (snow and ice on the) ground; often walking and never traveling far from the streams or rivers from which they emerge.

A thriving stonefly population is an indicator of clean, oxygen-rich water and a healthy river environment. Because of this, I very-much enjoyed seeing such a significant number of adult stoneflies ambling along the river shore. The aquatic nymphs require highly-oxygenated water to stay alive and they’re quite sensitive to pollution in the water, as well. They often struggle to survive, even in streams and rivers with seemingly little contamination. (The same may be said about mayflies and caddisflies.)

Adult Stone Fly. Photo Credit: Michigan State University Plant and Pest Diagnostics.

Not a True Fly  

Stoneflies aren’t true flies. All true flies belong to the order, Diptera, a large and diverse group that includes not just flies of all kinds, but mosquitoes and no-see-ums, among others. Stoneflies belong to the order Plecoptera (folded wings); a primitive group of insects that have been around for more than 300-million years. Plecoptera is derived from the Greek plektos, meaning twisted or braided, and pteron meaning wing; most likely a reference to the lattice-like veins in their wings

According to some references, stoneflies may be closely related to earwigs, grasshoppers, and/or cockroaches. In fact, among researchers, one recurrent observation is that stoneflies, both as naiads and even more so as adults, look a lot like aquatic cockroaches.

There are approximately 2000 species of stoneflies worldwide, with new species still being discovered. Estimates for the number of species in North America vary between 450 and nearly 700.

A Stonefly’s Life Cycle – Egg, Naiad, Adult

Most of a stonefly’s life (typically one to three years, although almost all of the species we’re likely to encounter here have a one-year life cycle) is spent as a nymph, or naiad, in aquatic environments. The naiads aren’t very strong swimmers, but they have six sprawling legs with two powerful claws on each tarsus (foot), which enable them to cling to objects (rocks, gravel, branches, tree roots), and endure, even against powerful currents and swiftly moving water.

Naiads of North American stonefly species have long antennae and two antenna-like tails (cerci). They breathe using gills on the underside of their bodies.

All species of stoneflies molt several times. But unlike other common aquatic insects, stoneflies undergo their final molt, to become winged adults, on land. A soon-to-be fully-mature naiad must crawl from the water in search of an exposed surface (bare rock or ground, a stick, a tree trunk) where it can shed its nymphal exoskeleton for the last time.

The exoskeleton splits right down the center of the insect’s back, and the adult stonefly emerges with new wings, new legs, and an entirely new body. Then it rests, until its new form has dried, its wings have fully-hardened, and it’s ready for flight. The cast-off exoskeletons, called exuviae, are sometimes spotted near the water’s edge.

Stoneflies live as adults for just a few days to a few weeks; just long enough to mate and lay eggs. To attract a mate, the males drum by beating their abdomens on whatever they’re standing on. The vibration is transmitted through the substrate, not through the air. When a female drums in reply, the male moves toward the female and drums again. And again, the female replies. This is repeated until the two meet and mate.

Females mate only once. Then they lay their eggs, in masses, in moving water. The eggs develop and hatch quickly, although the early instar naiads remain more or less dormant during the late spring and early summer months; after which they start feeding and growing rapidly.

Photo at top: Stonefly Nymph. Photo Credit: Michigan State University Plant and Pest Diagnostics.