Among the Instars: Allocapnia

Hey bug folks! In Wisconsin, we have been blessed with an early spring this year. This means I was able to get out any collect some new bugs! This week, I have been looking at winter stoneflies since they have been emerging as adults for the past few weeks. With the warm weather, it was much easier to access open water and find some mature nymphs. Winter stoneflies are also known as snowflies due to their early appearance in the year, and they refer to insects in the families Capniidae and Taeniopterygidae (order: Plecoptera). I was able to find a few Capniidae specimens of the genus Allocapnia in a nearby spring-fed stream.

Allocapnia, a small winter stonefly in the family Capniidae

Allocapnia Life Cycle

The nymphs of winter stoneflies are small and slender, and they hatch a few weeks after their eggs were laid in a stream. For most species, this usually occurs around April to June. They grow through a few larval stages, known as instars, and they molt between each of these stages. Once warm summer months roll around, these nymphs go into a hibernation state called diapause where all activity and growth are suspended. This lasts for several months until autumn, when they resume eating and growing. Their growth continues throughout the winter months until they emerge. Studies of particular species of Allocapnia show different emergence times, but most occur in late winter months around February or March. At least one species, A. recta can reach maturity and emerge as early as December!

Stoneflies are hemimetabolous, which means that they do not have a pupal life stage. The nymphs grow to maturity and then emerge as terrestrial adults, ready to find food and mate. The adults of Allocapnia are dark colored and small, and they have long tails. Females may have wings that are reduced in size compared to their bodies, and males lack wings altogether! They don’t incorporate much flying into their lives. Females can climb up trees or houses and then glide on wind currents, but mostly they stay close to the streams they emerged from. These adults can also travel downstream easily by riding small blocks of ice and snow when streams first open in spring. Mating procedures are also fascinating with these stoneflies; males hit their abdomen repeatedly against the ice and snow surface in a vibration known as “drumming.” Females can feel the vibrations through the snow, so they know how to find their mate!

How cool can winter stoneflies get?

Now most of us probably don’t think to go looking for insects in the middle of winter. Most critters we are familiar with are flitting about in the spring, summer, and some into the fall. Why and how do these stoneflies emerge in the middle of winter? One of the benefits of this life cycle is the smaller numbers of active predators out and about while everything is still snow covered. Sure, there will always be cardinals around, but overall, there are much fewer chances to be eaten during frigid temperatures. It is also important to keep in mind that water temperatures and the border between ice and liquid water stay fairly close to 0° F. Aquatic critters need not deal with extra chilling temperatures until they leave the water and reach the top of the ice and snow.

On the topic of frigid temperatures, most everyone who has spent a winter in the Upper Midwest would agree that these late winter months are cold! So how does a tiny stonefly keep from freezing in these habitats? This is one of the coolest facts about winter stoneflies! Each Allocapnia nymph produces compounds, in the form of glycerol, proteins, or sugars, which essentially act like antifreeze in their bodies. The compounds prevent ice crystals from forming, ensuring that body fluids do not freeze, even at temperatures far below the normal freezing point. Adults can increase the amount of these compounds, so they can tolerate subzero air temperatures long enough to mate and lay eggs.

Ecology of Allocapnia

The ecological preferences of winter stoneflies are mostly observed by associating adults with their nearby water sources. Nymphs are not as well studied since mature specimens require a person to be out sampling in the middle of winter. Allocapnia is found widespread in North America east of the Rocky Mountains. There are many recognized species, so there some variety exists in the ideal habitat for each one. Most species prefer small to medium-sized streams with rocks or gravel as the primary substrate and a fair amount of leaf input from deciduous forests. Nymphs are primarily herbivores and detritivores, eating bacteria and fungi off of leaf litter in the stream. During diapause states, the nymphs move into the fast-flowing rocky or gravelly part of the stream and may burrow down into the substrate for this resting time.

Stoneflies in general are considered indicators of good water quality since they usually prefer flowing waters with high oxygen content and minimal pollution. This mostly follows for Allocapnia as well since most species prefer those types of habitats. There are a few species, such as A. vivipara, that have been noted as tolerant of some organic and farmyard pollutants. They still likely wouldn’t be found in a heavily impacted stream or one with higher levels of urban runoff and pollution. A. granulata and A. vivipara have been found in larger rivers with slower currents and some turbidity. A. minima is found in large rivers located in areas of North America that have been glaciated. Most other species of Allocapnia fall into the category of small, clear streams with fast currents. In the Upper Midwest, this genus has a tolerance value of 3 out of 10, so they are considered intolerant of pollution and would only be found in streams with higher quality.

The blue box surrounds the hind windpads and the green box surrounds the front wingpads on this Allocapnia specimen.

Allocapnia Taxonomy

Order Level

The identification of these stonefly nymphs can be challenging based on their size, life cycle, and the overall knowledge known about them. Stoneflies can be identified to order level with a combination of two “tails,” a pair of claws on each leg, and long slender antennae. Small stoneflies are most often confused with mayfly larvae, especially when tails and gills break off during collection. A few mayfly species can have two long tails, but they will have gills present along both sides of the abdomen. Most mayflies have three tails, and all stoneflies will only have two. The claw count is a more reliable way to differentiate these two orders. Mayflies nearly always have a single claw on each leg, and stoneflies always have a pair of claws per leg.

Family Level

Once into the stonefly key, the lack of any external gills on this specimen will eliminate several family options. The mouthparts as pictured below are the most difficult to see and usually require a microscope to tell for sure. Highlighted in the image below are the paraglossae and glossae, all of which reach to about the same level. This characteristic, in combination with the parallel wingpads as shown in the image above, will get us to Capniidae or Leuctridae. These two families can be challenging to tell apart, especially if the specimen is immature. A few characteristics tell these families apart, most of which are difficult to see, much less photograph. The most notable feature seen here are the hind wingpads of this specimen. They are truncate and quite a bit shorter in length than the front pair of wingpads. In Leuctridae, the hind wingpads are very similar in appearance to the front pair, being long and slender, and usually they occur much closer together than the front pair.

Genus Level

For generic identification of Allocapnia, there are several keys that can be used including: Stewart and Stark, 1993; Hilsenhoff, 1995 (for Wisconsin or nearby specimens); and Merritt, Cummins, and Berg, 2019. This specimen has hairs on the edge of each segment on the tails, without any occurring in between. The hind wingpads also point to Allocapnia since they are truncate and notched near the tip. These are all characteristics that are best seen using a microscope; I would not recommend trying to field identify any small stoneflies.

The blue arrows point out some of the identifying mouthparts for the family Capniidae.

Special thanks to the Aquatic Biomonitoring Laboratory at UW-Stevens Point for allowing me the use of their microscope camera! Check out these photos in the gallery and view my other stonefly posts.

Sources

Harper, P.P., M. Lauzon, and F. Harper, 1991. Life cycles of 12 species of winter stoneflies from Quebec (Plecoptera; Capniidae and Taeniopterygidae). Canadian Journal of Zoology 69:787-796. https://doi.org/10.1139/z91-114

Hilsenhoff, W.L., 1995. Aquatic Insects of Wisconsin. Keys to Wisconsin genera and notes on biology, habitat, distribution, and species. Publication of the Natural History Council, University of Wisconsin-Madison 3:11-17.

Hitchcock, S.W., 1974. Guide to the insects of Connecticut. VII. The Plecoptera or stoneflies of Connecticut. State Geological and Natural History Survey of Connecticut Bulletin 107.

Merritt, R.W., K.W. Cummins, and M.B. Berg, 2019. An introduction to the aquatic insects of North America, 5th edn. Kendall/Hunt Publishing Company, Dubuque, IA.

Stark P.B. & Lacey J.W., 2005. Larvae of the winter stonefly genus Allocapnia (Plecoptera: Capniidae) in Mississippi, USA. Illiesia, 1(3):10-20.

Stewart, K.W. and B.P. Stark, 1993. Nymphs of North American stonefly genera (Plecoptera). University of North Texas Press, Denton, Texas.

Genus Plecoptera Capniidae Allocapnia (macroinvertebrates.org)

Winter Stoneflies: Insects with Antifreeze (constantcontact.com)

Winter stoneflies sure are supercool News and Research - Scientific American Blog Network