November 6, 2022

A light snowfall blanketed Missoula overnight, and this morning my feet are feeling the approach of winter. They’re screaming gradually louder at me from within waders that I may not have dried sufficiently since my last stream adventure. But if the aquatic insects dislike the cold, they aren’t telling us. In just a few minutes of wading and netting, we’ve caught over 300 of them in this single section of stream. It’s an impressive diversity of mayflies, stoneflies, caddisflies, and more.

Today, I’ve joined the Watershed Education Network (WEN) to volunteer for their stream science on Rattlesnake Creek. We’re near the Lincolnwood Trailhead, where Rattlesnake Creek flows through a mature forest of ponderosa pine, Douglas-fir, and black cottonwood. It’s a beautiful section of stream, flowing over large, mossy boulders, with rafts of fallen cottonwood leaves stacking up in the eddies. And unlike the Rattlesnake Creek Dam site, which we visited last week, here the stream has been a stream for much longer than two years.

Aquatic insects are just one part of the sampling that WEN’s Stream Team is doing today. We’re also measuring the stream cross-section here and testing the water chemistry. But when I had the chance to decide which activity I would help with, the insects were an easy choice.

Aquatic insects

Al Pak is leading the insect exploration this morning. Lauren Sampson and Grace Spella complete our team of four. Today’s plan is simple. We’ve begun by catching aquatic invertebrates using dip nets, sampling across the width of the stream. Now we’ll sort these creatures into groups we can identify in the field – such as mayflies, stoneflies, and midges – and tally how many of each we’ve found. 

Certain groups of invertebrates, such as mayflies and stoneflies, are indicators of good water quality and stream health. Others, like midges, tend to be more tolerant of polluted, damaged streams. So by looking at a sample, we can get a good idea of how healthy this stream section is. What’s more, it’s a lot of fun – and it’s a great opportunity to get to know this hidden community of tiny creatures.

How to catch mayflies

Before we got started this morning, Al instructed us in our netting technique. First, we would wade out into the creek and face downstream with our nets planted in front of us in the water. Then, he told us, we would gently shuffle the rocks of the stream bottom with our feet, dislodging small creatures into our nets.

In practice, it wasn’t quite as easy as it seemed. I immediately managed to dislodge a grapefruit-sized rock from the bottom, and the current swept it into my net. I fished the rock back out and continued stomping the bottom. After a few minutes, there was something in my net – an assortment of leaves, small bits of wood, and tiny pebbles. With luck, I hoped, there would be some insects, too.

Sorting the catch

Now our stream wading is done and we’ve all returned to the picnic table with our net samples. We rinse them into two plastic tubs filled with stream water. It’s time for the really fun part: sorting. Our goal is to pick out 300 creatures and figure out which group each one belongs to. An ice cube tray turns out to be an excellent tool for sorting, providing small wells where we can place these tiny animals while they’re waiting to be released again.

Al reminds us how to distinguish the major groups, and then we get busy sorting. Stoneflies (order Plecoptera) have two “tails” at the tip of the abdomen. Their gills, if visible, are in their “armpits,” at the base of their legs. Mayflies (order Ephemeroptera) can look rather similar, but most of them have three “tails.” Their leaf-like gills are located along the sides of the abdomen, not in their armpits. 

Mayflies and caddisflies

The diversity is impressive. There are lots of mayflies here, camouflaged in subtle tans, their three tails splayed widely. They differ in size and shape, and it’s no wonder: this is a complex group. In fact, there are over 600 species of mayflies known from North America.

Among the bewildering assortment of tan mayflies, Al shows me one that looks strikingly different. It’s small and blocky, with a reddish head and a surprising dark band across its whitish back. Al tells me that this is a spiny crawler mayfly (family Ephemerellidae). This family is rather sensitive to pollution, so it’s exciting to find them here.

We pick out several different types of caddisflies (order Trichoptera). They look more wormlike than the mayflies, with small legs and obviously segmented abdomens. At the tip of the abdomen, each caddisfly has a pair of short, hooked appendages. Some of them are surprisingly colorful, with notably orange heads.

Most of the caddisflies we’re seeing today are naked, without the intricate cases of wood or stone that certain members of this group carry with them. But then Al finds a saddle case-maker caddis (family Glossosomatidae) that still has its home with it, a fragile structure built of pebbles. 

“These are the ones that adhere to larger rocks in the stream,” he tells us. We watch under my macro lens as the caddisfly tentatively extends its head outside of its case.

“Okay, this is way too fun,” says Grace.

I have to agree: streams are fascinating to begin with, but it’s amazing to find this miniature world of creatures unfolding in front of our eyes. 

Stoneflies and riffle beetles

“I’m really happy with the number of stoneflies,” Al says as we progress with the sorting. In the section where the Stream Team sampled last weekend, the invertebrate community was heavily dominated by mayflies. But this week, we’re getting to see lots of diversity: not just mayflies, but many other creatures as well.

Several of the stoneflies are impressively large, nearly filling one of the plastic spoons we’re using to transfer creatures to the ice cube trays. And with the macro lens, we’re amazed by the intricate patterns of marbling and chevrons that cover their heads and thoraxes.

Lauren has been carefully searching the other plastic tub for new creatures. Now she shows us an adult riffle beetle (family Elmidae) she’s just found. It’s tiny, just two or three millimeters long. It would have been easy to overlook, but Lauren is sharp-eyed and patient. With the macro lens, I can see that the beetle has a magnificent orange and black pattern on its wing covers. I show the photo to Lauren and her face lights up.

“Oh my gosh, that’s so cool!” she says.

An insect mystery

Al quizzes me with a mysterious creature he’s found. Its body is elongated, segmented, and translucent orange. I grab some photos with my lens and try to imagine what it might be. It has a pair of protuberances at the tip of the abdomen, and another pair of miniature legs just behind the head. What could it be?

Is it a caddisfly? I ask him. No. 

I rack my brain. I’m somewhat familiar with terrestrial insects, but aquatic ones are a whole new world. Is it some kind of fly, order Diptera? I ask. Yes!

But flies are one of those insect groups that are extremely diverse: there are roughly 17,000 species known in North America. So to really understand this creature, just calling it a generic “fly” doesn’t get us very far. 

I try to think of the families of aquatic flies that I know. Is it a deer fly, family Tabanidae? I hazard a guess. No.

“It’s a very common one,” Al tells me. 

Could it be a midge, family Chironomidae? I wonder. Yes! The single pair of prolegs just behind the head is a good clue for this family, Al says. 

The chironomid midges – extremely common, non-biting relatives of mosquitoes – can be found almost anywhere there’s water. Unlike most of our mayflies, stoneflies, and caddisflies, many midges can tolerate polluted waters. But they’ll also live in cold, clear, high-quality streams. And because they’re so abundant, they’re important food sources for a variety of creatures, including fish and birds.

An underwater world

It seems like we must be getting close to our benchmark of 300 insects now. And I’m not the only one whose toes are starting to congeal from the cold. We each count the creatures we’ve sorted into the ice cube trays, and Grace tallies them all up. There are lots and lots of mayflies, as well as a respectable assortment of stoneflies and caddisflies. We’ve also picked out some other creatures that I haven’t had space to mention here. We’ve found black flies, a crane fly, flatworms, and some other aquatic worms. And our tally, astonishingly, comes out to exactly 300 of them!

We’re done for the day, and it’s a good thing for the sake of my frigid toes. But this has been such a fun morning: an introduction to an entire world. There’s a lot more to learn, and I look forward to getting out in the field with Stream Team again next year. 

What’s more, this underwater world isn’t unique to Rattlesnake Creek. The details differ from place to place, but every healthy stream is full of creatures like these.

It’s a world full of complexity and surprises. A person could study stream insects for their entire life without getting bored. But it’s also a world that anyone, regardless of age or experience, can get to know. (For an entertaining introduction to the life of a stream that is accessible to people of all ages, check out Montana-based ecologist Sara Owen’s excellent book, Seth and Mattie’s Big River Adventure.)

We hadn’t finished sorting through the mayflies yet when Al summed it all up.

“This is the coolest part, I think, is the bugs,” he said. I have to agree.

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