Friday 5: Caddisfly Case Diversity

Writing my post Monday reminded me of how much I like caddisflies – and how little I write about them here on my blog.  Today I’m going to write about them again.  And because it’s Friday, it’s time for another Friday 5!

Caddisflies are incredibly important aquatic insects for many reasons.  They’re an essential part of many food webs, processing whole leaves into smaller particles that other organisms can consume, breaking down dead material, and acting as a food source for many larger aquatic animals such as fish and frogs.  From a water quality standpoint, they’re sensitive indicators of pollution and valuable when assessing the quality of streams.  They’re also a very diverse group of insects.  In some places you find several different species living together, each building its own style of case and performing various roles within the stream.  In one tiny area of a single stream, you might find big cases and little cases, cases made from leaves or wood or pine needles or rocks, sometimes even snail shells.  The variety can be quite impressive!

To demonstrate this diversity, I’d like to share 5 photos of caddisfly cases from a single river, the Little Colorado River of Arizona’s White Mountains.  These caddisflies were all collected on a single day from a small section of the river near Sheep’s Crossing, a pine wooded area (or at least until the Wallow Fire – not sure what it looks like now!) at fairly high elevation (over 9200 feet).  The water flows fast and cold, though there has been a lot of beaver activity the last few years that’s been transforming the area.  There could be even more – or different – caddisflies there now!

Rock Cases

Many species of caddisfly make cases out of rocks.  This is one example:

Glossosomatid caddisfly

Glossosomatid caddisfly

Glossosomatid caddisflies are known as the saddle case making caddisflies because they make cases that are reminiscent of saddles.  The cases are constructed using silk with rocks attached.  On the top and sides, the caddisfly places large rocks from head to tail.  On the bottom, it uses smaller rocks and makes a much narrower band that wraps under the abdomen.  Think of how a saddle fits on a horse – the mental image is pretty good.  I find these out in the high flow areas of the stream, essentially glued into place on a rock.  Out in this part of the stream they can scrape the delicious algae off the rocks to their heart’s content!  There’s something about this family that I just love and they’ve become my favorite caddisfly group.

Limnephilidae is a very diverse family of caddisflies, but they tend to be rather large and often build their cases from rocks.  An example:

Limnephilid caddisfly pupa

Limnephilid caddisfly pupa

This case is sealed at both ends because the caddisfly has pupated inside.  It will dig its way out of the case when the adult emerges, and then leave the stream to spend its adult life on land.  I usually find these on the bottom of the stream in sandy or gravely areas within a few feet of the banks where they can chew up leaves and break them down into smaller pieces.  Their case is just a tube of silk with moderately sized pebbles attached.  Pretty simple really.  But some cases n this family are much more complex.  Case in point…

Mixed Media Cases

This case is from another limnephilid species:

Limnephilid caddisfly

Limnephilid caddisfly

Rather than using all rocks, these caddisflies build a rock case and then wrap at least the bottom half with leaves.  This is a much more complex case design as the caddisflies have to find both the right size of rocks and stitch them all together, but then they have to find a second completely different material to finish the job.  Pretty cool!  I think these look a little like a sushi handroll.  :)

Wooden Cases

Several species of caddisflies make cases that incorporate pieces of wood.  Some even make cases entirely from wood like this one:

Lepidostomatid caddisfly case

Lepidostomatid caddisfly case

This case is from a caddisfly in the family Lepidostomatidae, a family with only two genera in North America.  I find these larvae, often in very great numbers, near the banks where they break down stream detritus including dead leaves and dead fish.  This species is among nature’s recyclers and plays a very important role in the stream.  The fact that they wander around in fashionable wooden jackets makes them extra awesome.

Vegetable Cases

Many caddisflies make their cases entirely from non-woody plant material.  One example is this brachycentrid larva:

Brachycentrid caddisfly case

Brachycentrid caddisfly case

I just love these little caddisfly neat freaks.  Check out the carefully arranged geometrical case!  Amazing.  If you hold the case with the opening toward you, those little bits of grass form a perfect little square.  These larvae tend to be tucked up in the crevices along the banks where they collect and eat little food particles floating around in the water (diatoms, bits of algae, etc).  I find a lot of them in very small sections of the bank too.  One scoop with my hand dandy soup strainer will often haul up 30 or 40 of these caddisflies!  I usually take two or three and put the rest back.  Otherwise my collection would be overrun with brachycentrids!

There are far more than 5 species in this river, but most of them make cases from these same types of materials because that’s what it available.  If you travel downstream just a mile, the beaver activity is much less and the deeper water flows faster, so you get several other species there, some that build very different cases than any of these.  I think it’s impressive that there are so many varieties of caddisfly in a single mile of river!  This is a seriously diverse group.

So, have I convinced you that caddisflies are amazing yet?  :)

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Unless otherwise stated, all text, images, and video are copyright © TheDragonflyWoman.com

The Purpose of Caddisfly Case Extensions: A Case Study

Well, Science Sunday ended up getting pushed to Monday, but that’s okay.  It happens!  But today I’m going to share a fascinatingly simple study with you about caddisflies, so I hope it was worth the wait.

I haven’t talked about caddisflies all that much on my blog yet, but I really should.  They are incredibly interesting little insects and hugely diverse as far as aquatic insects go.  Caddisflies fly around on land as adults and look like little bland moths:

caddisflies

Caddisfly adults

Their order name, Trichoptera, means hairy wings, and if you look closely at the wings you’ll understand why: rather than scales, as you see in their close relatives the butterflies and moths, caddisfly wings are covered in hair.  Their common name, caddisfly, is based on a peculiar structure that the aquatic larvae and pupae use, the case.  Not all caddisflies build cases, but those that do build them using silk that they produce and materials they gather from the stream such as algae, pebbles/sand, leaf bits, pine needles, small pieces of wood, etc.  There are a huge variety of cases, many of which are species specific so that you can identify the species based solely on the case.  Others are less distinct, but regardless of the structure of the case, they tend to be big, bulky, heavy things that are much larger than the larvae carrying them.

Over the years, researchers have proposed a variety of purposes for caddisfly cases.  Some are likely helping the caddisfly breathe by stirring the water around the larva, allowing it to collect oxygen from the water via gills that run along the abdomen.  Cases may protect some species from predation as fish and other aquatic predators are less likely to eat something that looks like a pile of rocks or leaves than a soft, squishy insect.  Still other caddisflies may use their cases to weigh them down, causing them to sink to the bottom so that they can move about fast flowing streams with less risk of being swept downstream.  Each species may use its case for a slightly different purpose, or even more than one.

One species of caddisfly, Dicosmoecus  gilvipes, builds a case from silk and plant bits, adding small pebbles as they get older.  The first through fourth instars also attach needles from Douglas fir to their cases (the fifth and last instar does not), attaching them near the top of the case so that they stick far out to either side:

Dicosmoecus gilvipes

Dicosmoecus gilvipes larva. Redrawn from Limm and Power 2011.

This is a rather peculiar arrangement of materials, so researchers Michael Limm and Mary Power wanted to figure out why those fir needle wings were so important.  They considered two hypotheses.  First, the wings might protect the larvae from predation.  Even if the little rocks didn’t discourage fish from eating the larvae, perhaps the pointy spikes sticking off the sides would.  Alternatively, the extensions could help stabilize the larva so that the larva would be less likely to tip over in areas of high flow in a stream.

To test these hypotheses, they did two simple experiments.  In the first, they released caddisfly larvae at the site where a stream flowed into a deep pooled area containing steelhead trout.  One person hid behind a boulder and released the larvae, which were swept into the pool.  A second person observed the fish and counted how many times each larva was approached by the fish, were “mouthed” by the fish, or eaten.  They did three treatments: caddisflies with the case intact, caddisflies with the douglas fir needles clipped off, and naked caddisflies that had been removed from the case prior to release.  In the second experiment, the researchers constructed a large rocking water tank that would roll the larvae over.  They placed a larva on the bottom of the tank and turned the machine on, then counted how many times each larvae rolled before it recovered its footing and how long this took.  They then compared the number of rolls and the time to recovery between caddisflies with cases intact and with the fir needles clipped off.  The team also measured the cases to determine how the width, length, mass, and center of mass changed with the addition of the fir needles.

The reseachers learned that the fir needles increased the width of the case by 410%, the length by 36%, and the weight by 24% and shifted the center of mass upward off the streambed.  They also learned that, while the steelhead readily consumed naked caddisflies, there was no difference in the number of approaches or the number of times the caddisflies were mouthed between the larvae with the extensions and those without.  Clearly the case alone was sufficient to prevent predation regardless of whether the extensions were present or not.

The results of the rocking tank were interesting though.  Larvae with the fir needle extensions rolled three times less than larvae without the extensions.  They also regained their footing more than three times faster with the extensions than without.  The cases might be providing other benefits to the larvae, but Limm and Power concluded that the function of the fir needles is to stabilize the larvae in areas of high flow.  Apparently it’s worth the extra effort of finding the fir needles and carrying around a much more unwieldy case if it means that you are more stable in the stream.

So, why does this matter?  According to calculations the authors did, the drag force required to tip a larva over is more than four times greater when the larva has the extensions than when it does not.  This means that the larvae can safely walk out into areas of the stream with flow up to two times faster without getting tipped over and washed downstream.  The extensions also help the larvae orient themselves so that they’re positioned parallel to the flow.  This decreases the chance of being swept away.  All of these benefits combined likely allow the larvae to wander out into areas of the stream where they would not otherwise be able to go.  Food limits the number of Dicosmoecus  gilvipes that can live in any particular stream, so by increasing their stability by the simple addition of a few Douglas fir needles, the larvae increase the area where they can forage for food in the stream, allowing more individuals to survive in any given area.  Pretty darned cool!

This is yet another example of how an insect can make a very simple, small change that provides huge benefits – just another example of why insects are such amazing creatures!

Literature Cited:

Limm, M., & Power, M. (2011). The caddisfly Dicosmoecus gilvipes: making a case for a functional role Journal of the North American Benthological Society, 30 (2), 485-492 DOI: 10.1899/10-028.1

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Unless otherwise stated, all text, images, and video are copyright © TheDragonflyWoman.com

Notes From NABS Day 5

meeting logoPhysical, Chemical, and Biological Changes Along the Continuum of an Agricultural Stream: Influence of a Small Terrestrial Preserve

Well, I didn’t quite get this post done the day I meant to, but my day ended up being quite busy.  However, the NABS/ASLO joint meeting of 2010 is now officially over!  That means this will be my last NABS post until I go to another conference.  It’s nice to be back home!  Meetings are exhausting and melt your brain after a while.  They’re fun, but they’re intense – and I never get enough sleep.  No matter how much I enjoy a meeting, I’m always happy to get back home and sleep.  And, if I come away from the experience without any new communicable diseases, I’ll consider it a success.

I’m going to skip the Things I Leaned section for today and jump right into the last talk.  Today, my focus is a talk by Dr. David Houghton that was given on Day 3 of the conference.  Dr. Houghton is an entomologist and Associate Professor at Hillsdale College.  Hillsdale is a small liberal arts college (even smaller than the one I went to as an undergrad!) and Dr. Houghton is in the department of biology there.  Sadly, there aren’t all that many people from these sorts of schools at most of the meetings I go to.  This is doubly sad because Dr. Houghton’s presentation was really interesting and made some excellent points.

In the area in southern Michigan where Dr. Houghton completed his study, the streams used to be surrounded by a wide strip of dense vegetation (the riparian zone).  The area is now an agricultural region.  This means that, rather than large trees and plants that require a lot of water filling the space adjacent the streams, the native plants have been removed and the agricultural fields go right up to the banks.  This has several implications.  The lack of trees means the water is warmer than it was before the trees were removed because there isn’t as much shade on the water.  A lot of chemicals such as pesticides and fertilizers end up in the water any time water flows over the fields and into the streams (i.e. during rains, heavy irrigation, etc).  Those chemicals decrease the water quality, which in turn impacts the plants and animals that live in the water.  Overall, the water quality decreases and with it the number of species that can live in the river.

In general, this situation isn’t good for the stream or any of its biota.  The river needs the forested areas for everything to work properly.  Removing the riparian area means that things in the river change and the “health” of the river goes down.  Stream health is a somewhat vague concept that I don’t want to get into here, but it is essentially a measure of how close to naturalistic conditions an ecosystem is.  Dr. Houghton’s talk began with this introduction.  Then he asked a question: are the small forested areas that are still available along southern Michigan’s streams capable of improving the water downstream so that the area downstream more closely resembles conditions without the influence of agriculture?  This has important implications for conservation of aquatic species.

Dr. Houghton’s study was conducted in the St. Joseph River in southern Michigan.  Like other rivers in the region, the St. Joseph has agricultural fields along the majority of its length with small forested areas near the headwaters.  In particular, Dr. Houghton was interested in one section of the river that had a small terrestrial preserve where the riparian area remained intact.  The river running through the preserve looked better than the area upstream, so he thought the water flowing through the area might be improved such that insects downstream of the preserve would fare better than the insects above the preserve.

To study this, Dr. Houghton chose six sites in the St. Joseph River from which he collected water and insect samples.  Two sites were above the preserve, two were within the preserve, and two were further downstream.  He measured several parameters of the water itself, including the temperature, dissolved oxygen, pH, and conductivity (effectively a measure of the amount of salt compounds in the water).  He also measured the insect populations by collecting adult caddisflies at light traps near the river.  Measuring the water parameters would tell him whether the water running through the preserve or downstream of the preserve was better than the water upstream of the preserve.  Because caddisflies are aquatic as larvae and live in the water for most of their lives, they are strongly impacted by water quality and are excellent indicator species.  Counting the number of individual adults and the number of species (also known as species richness) that came to the light traps would tell Dr. Houghton something about how “healthy” the river is.

His results were interesting.  There was no difference in any of the measurements of water quality Dr. Houghton collected above and below the preserve.  This meant that the river is an agricultural stream for its entire length and the preserve did not improve the water quality downstream.  There were two water parameters that improved within the preserve: the temperature (it went down) and the amount of dissolved oxygen (it went up).  These two changes can likely both be attributed to the amount of shade the river receives in the preserve versus the areas outside.  Shading the water causes the temperature to go down because less sun hits the water.  This in turn causes the dissolved oxygen to go up because cooler water holds more oxygen than warmer water.  However, once the water flowed back out of the preserve, the temperature and the dissolved oxygen went back to the levels seen above the preserve.  The preserve did not appear to be improving the water quality in the river.

Similar results were found using the insect samples.  Dr. Houghton found that 7 species of caddisflies made up 90% of all of the specimens coming to the light traps both within and outside the preserve.  These 7 species all feed in similar ways (they are collector-gatherers and they eat things that are floating in the water, like leaf particles and floating algae that are of the appropriate size) and have the same level of tolerance to pollution.  So, it appears that the majority of the caddisflies in the river were about the same throughout, again suggesting that the preserve didn’t do much to improve the quality of the river.

However, Dr. Houghton did detect one important difference between the caddisflies in the preserve compared to those outside: there were more species of caddisflies inside the preserve, so the species richness improved.  22 species of caddisflies were found only in area of the river where it flowed through the preserve.  Most of these caddisflies fed in a similar way (they are shredders, or insects that tear leaves and algaes into pieces small enough to eat – an important component of decomposition in aquatic systems) and the remaining species were ones that required cooler waters than those found outside the preserve.  None of these species were very abundant and in fact a few of them were represented by only a single specimen, but the species richness was definitely improved within the preserve compared to outside.

Dr. Houghton ended his talk with a question: is the river “healthier” because of the presence of the preserve?  He suggests that the answer to this question depends on what measure of health you are using.  The preserve clearly didn’t change the water quality so that the section downstream of the preserve was different from the area above.  If your measure of river health is whether the water quality and caddisfly populations downstream of the preserve are better than those above, then the preserve does not have any effect.  This could give some policy makers the idea that it’s okay to rip those last few preserves out, making space for more agricultural fields.  However, if your measure of river health is species richness, the presence of the preserve had a huge impact.  The river above and below the preserve had many fewer species of caddisflies than the area within the preserve.  Clearly the preserve is acting as a refuge for species that are unable to live in the more harsh conditions outside of the preserve.  Thus, if your goal is to maintain diversity in the stream, the preserve is very important.  In fact, building new forested areas along the water might further improve the diversity of the river even further.

I thought this talk was excellent.  It was a simple project, but it did everything it needed to accomplish.  Dr. Houghton’s talk also highlighted a couple of important points.  First, when looking for the biological impacts of a system on a species, you need to identify which measurements of health you want to use.  Second, it is good to consider multiple measurements of health within a system.  It would be tragic for any study to say that forested areas near a stream aren’t necessary because they don’t improve the water quality downstream.  I think what makes Dr. Houghton’s study great is the fact that he identified the changes in the species richness of the forested preserve, which showed that the preserve really did have an impact on the river system, if only in the area within the preserve.  It wasn’t exactly the one he might have expected or hoped for, but it does suggest that forested preserves are valuable to river systems and should be protected so that species diversity within the river is maintained.

And that wraps up the Notes from NABS series!  I hope you all enjoyed the glimpse into the research that is currently happening in the aquatic sciences and learned some new things.  Scientific conferences are an excellent place to gain new insights, think about things in new ways, or learn about things you’ve never even considered.  Hopefully I have passed some of these qualities on as they’re just too good to keep to myself.

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Posts in this series:
Day 0 - Introduction to the Series
Day 1 - Invasive Crayfish
Day 2 - Giant Water Bug Dispersal
Day 3 - Dragonfly Captive Rearing
Day 4 - Integrating Service-Learning Programs into College Courses
Day 5 - Impact of a Small Preserve on Stream Health

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Unless otherwise stated, all text, images, and video are copyright © 2010 DragonflyWoman.wordpress.com.