Among the Instars: Helicopsyche borealis
Hey bug folks! I just wanted to thank those who have patiently awaited updates after my long diapause period. The past year and a half have seen some drastic changes in my life, and I wasn’t able to prioritize this blog as much as I’d hoped. That being said, I’d like to splash back in with a very special critter that was personally requested: the widespread caddisfly Helicopsyche borealis (family: Helicopsychidae). In the image below, you may be able to guess why this taxon was given the common name “snail-case caddisfly.” I like to refer to them as swirly or spiral caddisflies, since they build their case in a circular form, similar to a snail’s shell. These critters also build a “door” to close the opening of their case, especially when entering a resting stage during winter or for pupation. This covering resembles a snail operculum so much that Helicopsyche was originally described as a snail named Valvata arenifera back in 1834! It wasn’t until 1856 that Dr. Carl Theodor Ernst von Siebold described the genus Helicopsyche in Germany. Then in 1861, Dr. Hermann August Hagen described and named the species H. borealis that is found across most of North America.
Helicopsyche borealis is a species of caddisfly in the family Helicopsychidae.
Helicopsyche borealis Life Cycle
Due to the widespread nature of Helicopsyche borealis, the life cycle is variable depending on location. In general, this species is known to be one of the first to emerge in the spring. In Ontario, they were found to overwinter during the final instars of their larval stage or as pupae. This allows them to be ready to emerge in May or June before most other insects are fully developed. However, adult Helicopsyche continue to emerge throughout the summer and into autumn. Most reports indicate that this caddisfly is univoltine, meaning they go through one life cycle per year. However, some studies conclude that this species can be multivoltine, completing multiple generations in a year. One such study took place in the Arbuckle Mountains of Oklahoma, where streams usually stay at a warmer temperature instead of fluctuating with the seasons.
The egg timeframe of Helicopsyche is also variable. In some parts of North America, an egg diapause period was observed for up to six months. However, in a California study, there was no egg diapause stage at all. Larvae go through an estimated five to seven instars before entering the pupal stage. Pupae remain in their case and seal the opening while they develop. Just as adults emerge throughout the summer, pupal specimens can be found at different times of year. As a caddisfly with such a wide range, they certainly have evolved to make the most of their local surroundings. Their adaptability allows them to thrive in a whole range of climates.
So what’s up with that spiral case?
Of all the case-making caddisflies, Helicopsyche is fairly unique in design with the spiral shape. Very few other caddisflies use this structure; I only read about one other caddisfly with a similar case – a taxon in the family Leptoceridae found in South Africa. In their first instar, Helicopsyche larvae begin constructing their case with very fine sand, progressively adding larger sand grains as they grow. They leave a small opening in the spire center to allow water to flow through.
There are a few theories about why Helicopsyche construct such unusual cases. The first is to provide camouflage protection from predators. The sand grains blend into substrates very well, but many other casemaker caddisflies also use this method with tube-shaped cases. Anyone who has tried to find caddisflies in the field knows that sometimes it’s easier to look for moving leaves, twigs, and other bunches of plant matter indicating a cased caddisfly, rather than looking for the critter itself. Another benefit of the spiral case is how water flows around it in fast flowing water. No matter which way the larva is facing, there is a convex edge facing upstream, pushing water around the case instead of sweeping it away.
One prominent advantage of the spiral case is how sturdy it is, providing optimal protection for the larva inside. The cases are very difficult to crush, and this allows Helicopsyche borealis to live in interstitial habitats, the area below a stream bed in the spaces between the substrates. Studies have found larvae living up to 30 cm below a stream bed, though a study in California found no Helicopsyche living below the substrate level. Supporting this theory is the difficulty in removing larvae from their cases. Many caddisflies will easily evacuate from their case at the first sign of trouble, but Helicopsyche will stay inside whenever possible, suggesting that they rely on their cases for protection.
The last thought I’ll include here has to do with the fact that Helicopsyche cases are in a dextral spiral, meaning that the opening is on the right side. It’s possible that the spiral case is intended to mimic the shape of aquatic snails. A majority of snails have dextral shells, and they are less desirable to predators because of their protective shell. It’s possible that Helicopsyche have evolved to replicate this shape to deter predatory species. Whatever the reason, it seems to be working well for them!
The dorsal view of Helicopsyche borealis shows the spiral case structure.
Ecology of Helicopsyche borealis
Habitat
Helicopsyche borealis is found across North America, especially east of the Rocky Mountains. Most often, they are found in flowing water, usually in small to medium sized streams, even inhabiting some larger rivers in the northern parts of their range. This caddisfly can be found in low-flow habitats, but it prefers to be in the current, where populations can thrive and become quite numerous. The study in California found communities often consisting of at least 10,000 individuals per square meter. This might seem crowded, but these caddisflies are not aggressive towards each other, but rather they coexist without problems. They also occur in lakes, particularly along windswept shorelines where they can attach to the bottom side of rocks.
This caddisfly is one of the most resilient and has a high tolerance for temperature extremes. They outlast most other caddisflies in warm thermal streams. A Wyoming stream at 34 degrees Celsius contained a community of Helicopsyche borealis, and a population in California could survive up to 36 degrees Celsius!
Food Preferences
The food preference of Helicopsyche borealis could best be described as opportunistic omnivore. Studies have found that they typically eat algae and diatoms, but they will also eat detritus and animal materials as needed. This caddisfly has specialized mouthparts to scrape periphyton from the surface of rocks. Other organisms that also have developed scraper mouthparts include the caddisfly family Glossosomatidae and the beetle family Psephenidae. Each of these specimens has mandibles with a flat edge to dislodge the periphyton from the rock surface and small brushes to move the food into its mouth.
Tolerance Values
Caddisflies usually indicate good water quality, and Helicopsyche borealis definitely contributes to this generalization. In the Upper Midwest where most of my specimens are found, their tolerance value is 3.0 on a scale from 0 to 10. This suggests that they prefer fairly clean streams without much pollution from organic inputs. The score is the same in the Northwest region, and even lower in the Midwest with a value of 1.8. In the Southeast region, Helicopsyche is attributed with the lowest possible score of 0.0. Part of the difference in scores can relate to how variable this species is depending on the location.
Helicopsyche borealis Taxonomy
Taxonomy for Helicopsyche borealis is fairly easy, since it’s one of the easiest to recognize in the field due to the distinct case shape. Caddisflies are distinguished at the order level from other insects by the combination of these features:
No developing wingpads
Last abdominal segment with a pair of anal claws
Antennae is one segment, usually inconspicuous
No prolegs
May have a case or free-living
Within the caddisfly order, many keys will separate the Helicopsychidae from the rest within the first few couplets. For example, Merritt, Cummins and Berg (2019) lists them right in the first couplet with the primary characteristic being the portable case in the shape of a snail shell. In the event you find a specimen that is not in a case, there are a few other characteristics you can look for. The anal claw is described as “comb-shaped” or having many teeth. Additionally, the first two segments of the thorax will be covered with large sclerites (hard, plate-like coverings), and the third segment will have several smaller sclerites. The larva will also tend to stay in a curled shape from when it was in a case.
This family contains only two genera; Helicopsyche is widespread on every continent except Antarctica, and Rakiura is only found in New Zealand. Helicopsyche contains several hundred species, but most of them occur in tropical areas. Two species have really adapted to northern climates and populated widespread ranges: H. borealis in North America and H. sperata in Europe. My invertebrate collections are predominantly in the northern Midwest states, so only H. borealis occurs in this area. A few other species are known in the United States with limited ranges.
H. limnella in the Ozark and Ouachita Mountains of Arkansas, Missouri, and Oklahoma
H. mexicana in Arizona, California, New Mexico, Texas and Utah
H. paralimnella in North and South Carolina
H. piroa in Arkansas, Kansas, Louisiana, Oklahoma, and Texas
H. sinuata in California
In the areas where these species overlap with H. borealis, I recommend finding a regional key to distinguish them from each other.
I want to give a 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 caddisfly posts.
Sources
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Hinchliffe, R. and A.R. Palmer. 2010. Curious chiral cases of caddisfly larvae: handed behavior, asymmetric forms, evolutionary history. Integrative and comparative biology 50(4):606-618.
Houghton, D.C. 2012. Biological diversity of Minnesota caddisflies. ZooKeys Special Issues 189: 1–389.
Lea, I. 1834. Observations on the Naiades, and descriptions of new species of that and other families. Transactions of the American Philosophical Society 4:63–121.
Merritt, R.W., K.W. Cummins, and M.B. Berg. 2017. Trophic relationships of macroinvertebrates. Pages 413-433 in F.R. Hauer and G.A. Lamberti (editors). Methods in stream ecology: Volume 1: Ecosystem structure. Third edition. Academic Press, Cambridge, Massachusetts.
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, Iowa.
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