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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Ever seen a stone fly? - Winter stoneflies, Family Taeniopterygidae


Winter stoneflies are active even on chilly days in January.


If you haven’t seen a stonefly, now is the time to do so. While many insects are hiding out or hibernating during January, winter stoneflies are in their glory. These relatives of grasshoppers and termites can be found on stones, vegetation, and bridges near small, fast-moving streams. Adult winter stoneflies are dark brown or black and are active day and night. In their youth, stoneflies live the life aquatic. Juvenile winter stoneflies, called nymphs, graze on submerged aquatic vegetation or decaying organic matter. Other species have abandoned the vegan feeding mode and eat aquatic insects, including other stoneflies. 

Stonefly nymphs search for food and hide among the small stones at the bottom of streams.

Stonefly nymphs obtain oxygen from the water through delicate gills lining the neck, thorax, or abdomen. Most immature insects shed their skin or molt just a few times as they develop. However, some species of stoneflies may molt more than 20 times before leaving the water to become adults. When the nymph has completed development, it moves to the edge of the stream and latches onto a stone or plant. The skin splits and the adult stonefly emerges from the cast skin like a wraith. As adults, stoneflies differ in their choice of food depending on their species. Some eat lichens, algae, or vegetation but others gain all the nutrients they need as nymphs and never feed as adults. Winter stoneflies are relatively weak fliers and seem to prefer walking and running to flying. However, some species are good fliers and are attracted to porch lights or bug zappers. Stonefly courtship is a curious matter. A hopeful guy stonefly strikes the surface of a resting place such as a small branch with its abdomen to create a specific drum beat. If a female of the same species is nearby and favorably impressed by his rhythm and sound, she will drum a reply with her abdomen. The percussive duet continues and if both like what they sense, the deal is sealed and they mate.

Clean, fast-moving streams and rivers are excellent places to hunt winter stoneflies.

After mating, the female stonefly will swoop to the surface of the water to deposit her eggs. This is a time of joy for fish living below. Trout, steelhead, and other freshwater fish find stonefly nymphs and adults delectable. Both adult and juvenile stoneflies are an important source of food for denizens of our streams. Fishermen have taken advantage of this passion and created a variety of lures that mimic stoneflies with colorful names like Montana Stone Yellow and Henry's Fork Yellow Sally. Stoneflies are also important indicators of water quality. Streams with heavy sediments, low oxygen content, or pollutants do not support a diversity or abundance of stoneflies. Stoneflies emerging from your local stream are a positive sign of a healthy environment. So, during the next few months take a walk on a sunny afternoon and visit a small stream or river to seek the stonefly. The best viewing occurs when stream banks are covered with snow and stoneflies clamber from the chilly waters below. The winter stoneflies featured in this bug of the week were captured on a balmy night on a small footbridge spanning the Little Patuxent River in Columbia.