Pacific needlefly (Megaleuctra complicata)

Physical description

Adult stoneflies have two sets of clear, fine-veined wings that lay close to the body. Adults Pacific needleflies are about ½ inch long. These insects have long, thin antennae. The larvae (nymphs) look like wingless adults, but often have external gills, which may be present on almost any part of the body. Depending on the species, stonefly nymphs range in size from ½ to 1 ½ inches in length not counting appendages.

Ecology and life history

The Pacific needlefly is restricted to springs, seeps and rheocrenes (springs that flow from a defined opening into a confined channel). They inhabit exclusively spring habitats, ranging from small seeps to large flowing springs. Even when it occurs in large springs, it is usually found along the edges instead of out in the area of flow.

Water quality must be consistently good and the temperature cold. The nymphs are often found in small, consistently wet seepage areas some distance from nearest the creek, river or lake habitat. The essential habitat for the nymphs is springs or seeps that might not even be visibly flowing.

Stoneflies usually live in areas with running water, and are important predators and shredders in aquatic ecosystems. Adults are terrestrial and can be found near aquatic habitats with running water, resting on rocks, debris, and vegetation. As nymphs, stoneflies live in aquatic habitats, mainly along the bottom of cool, clean, flowing waters with relatively high oxygen concentrations, mainly on rocky, stony, or gravel substrates.

The females lay hundreds or even thousands of eggs in a ball which they initially carry on their abdomens, and later deposit into the water. The eggs typically hatch in two to three weeks, but some species undergo diapause as eggs during the dry season. The nymphs physically resemble wingless adults, but often have external gills, which may be present on almost any part of the body. The nymphs (technically, "naiads") are aquatic and live in the benthic zone of well-oxygenated creeks and lakes.

In early stages (called instars), stoneflies tend to be herbivores or detritivores, feeding on plant material such as algae, leaves, and other fresh or decaying vegetation; in later instars, the nymphs of many species shift to being omnivores or carnivores, and some species become predators on other aquatic invertebrates.

The insects remain in the nymphal form for one to four years, depending on species, and undergo from 12 to 33 molts before emerging and becoming terrestrial as adults.

Stonefly adults are generally weak fliers and stay close to stream, river, or lake margins where the nymphs are likely to be found. The adults emerge only during specific times of the year and only survive one to four weeks.

As adults, very few stonefly species feed but those that do, feed on algae and lichens, nectar, or pollen.

Geographic range

Megaleuctra species are “always rare.” The Pacific needlefly is found in the Cascades in Washington, Oregon, and northern California. Washington records include King, Pierce, Lewis, Skamania, and Cowlitz Counties.