Forestflies (Spring Stoneflies)

Ever since we decriminalized the sale of marijuana in Colorado, people here seem to think that “stonefly” is a term that describes an insect trapped in a bong. That would be a stoned fly. Anglers know the difference, and right now is the time for “hatches” of spring stoneflies in the family Nemouridae along the Front Range.

The species I find most commonly probably belong to the genus Zapada, known as “forestflies.” I haven’t bothered to catch any specimens and turn them over. The cervical (“neck”) gills of the larval stage (nymph or naiad) are retained by the adult and are diagnostic. Two sets of branched gills on each side of the body mean it is in the genus Zapada.

© Arlo Pelegrin via Bugguide.net

Stoneflies in general are excellent indicators of water quality in streams and rivers because they are more sensitive to pollution, dredging, and other alterations to their habitat than many other aquatic insects. Spring stoneflies, which are also known as “little brown stoneflies,” are tolerant of a moderate amount of pollution and other human disturbance.

© J. Bodenham in Ward & Kondratieff, 1992

Nymphs of Zapada cling to submerged rocks and other heavy objects in flowing waters. There is one generation per year for the common species Z. cinctipes, meaning it is “univoltine.” Some other species require 2-3 years to complete their life cycle. The nymphs feed on vegetable matter, mostly leaves that fall into the water. Aquatic insects that chew up leaves are known as “shredders.”

© Tim Loh via Bugguide.net

Adult forestflies can begin emerging as early as late February, but most have their peak emergence from mid-March to mid-April. Mature nymphs, and adults, measure only 5-8 millimeters in body length (not including the wings of the adult), so these are not conspicuous insects. Adults are most often seen on bridges or logs that stretch across streams and creeks. They are quite active, running rapidly and flying when startled.

Considering the rich history of mining in the Pikes Peak region, and various chemical spills that have contaminated the few creeks and rivers here in Colorado Springs, I am rather surprised that forestflies continue to prosper.

Sources: Adams, Jeff and Mace Vaughan. 2003. Macroinvertebrates of the Pacific Northwest: A Field Guide. Portland, Oregon: The Xerces Society. 16 pp.
Brown, Wendy S. 2005. “Plecoptera of Gunnison County, Colorado,” The Aquatic Insects of Gunnison County, Colorado.
Stewart, Kenneth W. and Bill P. Stark. 1993. Nymphs of North American Stonefly Genera (Plecoptera). Denton, Texas: University of North Texas Press. 460 pp.
Ward, J.V. and B.C. Kondratieff. 1992. An Illustrated Guide to the Mountain Stream Insects of Colorado. Niwot, Colorado: University Press of Colorado. 191 pp.

Cicada shells

Almost every summer I receive several questions about strange bugs sitting motionless on fence posts, tree trunks, and other upright objects. I thought I would do an entry here to explain the mystery.

The descriptions of the creature that people give to me vary from a “cross between a crayfish and a beetle,” to “gnome-like,” and all mention the large claws on the “front end.” No one has ever seen anything like it, and it is no wonder. Normally, the creature they are seeing lives fairly deep underground.

I have taken to calling these bugs “former insects,” since the objects being seen are not entire insects but the cast exoskeletons (“skins”) of cicadas, family Cicadidae. Scientists call these shed skins “exuviae,” all that remains behind when the insect molts from the nymph stage into an adult. Dragonflies, damselflies, stoneflies, and mayflies also leave behind these ghostly but tangible shadows of their former selves.

The average person rarely sees an animated version of a cicada nymph because the mature nymphs emerge from the soil at night, climbing the nearest vertical surface, and then splitting the exoskeleton down the middle of the back to allow the escape of the wet, soft adult under the cover of darkness. The fresh, pale adults are extremely vulnerable, but their principle predators are diurnal, so they avoid instant death by coming out at night. Cicadas are large insects, so they can be quite conspicuous under the best of cryptic circumstances.

An adult "annual" cicada, Neotibicen sp.

Most people in eastern North America see the shells of “annual” or “dog day” cicadas in the genus Tibicen. They then hear the loud “songs” of the adult male cicadas, not often seeing the insect that makes such a racket. Despite the name, annual cicadas still take a long time to grow up. They live a subterranean existence as nymphs, sucking the sap from tree roots for at least five to seven years. The generations are staggered, though, so some adults emerge every summer. This is in contrast to the synchronous broods (populations) of the periodical cicads or “17-year locusts” that emerge en masse in the late spring or early summer every 13 or 17 years, depending on the latitude of the population.

I like to get questions about things like cicada exuviae. It shows that people are observant and curious, two qualities I really admire in my own species