September 6, 2022

Gyrinus beetles darting over the surface of Hauser Lake.
Gyrinus beetles darting over the surface of Hauser Lake.

They ripple over the water in a shimmery silver school, hundreds of them darting in elegant curves across the smooth surface. The creatures in this school aren’t fish, though: they’re beetles. I’m at Hauser Lake this morning, on the south shore of the bay at Lakeside, where willows cast their shade over the water. I dip my hand into the cool surface, sending a ripple towards the beetles. They scatter in a panicked frenzy. They’re neither massive nor tiny; they’re about the size of our common ladybugs, but more streamlined. Coal black in the shade, the sun’s rays turn them into brilliant specks of silver, skittering across the water.

Who are these beetles? To learn more, I’ll need to collect one. Slowly, I wade out towards them. The lake bottom is rocky here. Where the beetles are resting, the water is a foot deep. As I approach, they scatter in an energetic outburst. A bit farther away, they regroup. Again, I inch closer. Plunging my hand into the surface film, I come up with several beetles in my palm.

Escape artists

But even in the hand, these beetles are nimble. They leap away in the blink of an eye, propelling themselves against my palm and escaping into the water. Only one remains, trapped between my fingers. Holding it carefully, I wade towards the shore, picking my way gingerly among the sharp rocks. I’ve almost reached land when this one squirms free and gets away, leaping into the water. 

The whirligig beetle in my hand.
The whirligig beetle in my hand.

Back in its preferred habitat, the beetle is an accomplished escape artist. It twists and changes direction in the surface film as I follow after it, always a few seconds behind. Mostly it stays at the surface. At times it dives reluctantly, never submerging more than an inch or two and always resurfacing quickly. 

Finally, I manage to catch it again, scooping it up from underneath. This time I’m more careful, and I get to shore with the beetle still detained between my fingers. Up close, it’s a remarkably black creature with translucent reddish legs. Its body is flattened like a skipping stone. Its back is covered with rows of tiny, recessed dots.

Along the shore

A raft of Gyrinus beetles on Hauser Lake.
A raft of Gyrinus beetles near the shore of Hauser Lake.

Now I start kayaking along the edge of the lake, continuing to watch for these beetles. They’re creatures of the near-shore. Twenty yards out, where the water gets deep and the waves are choppy, not a single beetle is swimming. But closer to the shore, where the water is calm and shallow, I find them in massive, silvery shoals, rippling into a frenzy whenever I disturb them. Others are more solitary, zipping and darting like tiny, silent speed boats.

A raft of whirligig beetles along a rocky shore, slightly sheltered by a small stump in the water.
A raft of whirligig beetles along a rocky shore, appearing like silvery specks, slightly sheltered by a small stump in the water.

How many are there? I try to count the beetles in one shoal, going by tens and then by hundreds. Just in this one group, I estimate that there are 500 beetles. But even close to the shore, these groups are extremely patchy. I paddle for two miles along the edge of the lake, watching carefully for them. Long stretches of lakeshore hold no beetles, while one sheltered bay contains over a thousand. Most are in areas that are sheltered from the wind. But I find one shoal up against a steep, rocky shore, congregating in the meager shelter of a gnarled stump that protrudes from the water.

Identifying the beetle

The back of the Gyrinus beetle, its scutellum highlighted in red.
The back of the Gyrinus beetle, its scutellum highlighted in red.

In the lab the next day, I learn more about my specimen. It’s a whirligig beetle (family Gyrinidae): a creature specialized for a life at the water’s surface. Like most aquatic beetles, its legs are flattened into massive paddles. More distinctively, its eyes are divided into four parts. Two eyes peer above the water, while the other two watch for enemies below the surface. Among the various families of aquatic beetles, only whirligig beetles have eyes like this, doubled like a reflection of the watery horizon.

It proves straightforward to identify this beetle to the genus, too. It’s a Gyrinus beetle. In Montana, these are the only whirligig beetles that have a visible scutellum: a small, triangular region on the back between the thorax and the wing covers.

From this point, identification becomes a matter for beetle specialists. To distinguish species of Gyrinus (there are 40 of them in the United States and Canada), one must dissect their genitalia and go through a daunting identification key (it’s available here if anyone is interested). This week, that’s more than I’m prepared to do.

Fortunately, recognizing these beetles as some species of Gyrinus tells us a lot about their lives. The same publication that gives a key to species also gives an excellent summary of these beetles’ biology. 

Gyrinus biology

The underside of the Gyrinus specimen, showing the strongly flattened, paddle-like legs.
The underside of the Gyrinus specimen, showing the strongly flattened, paddle-like legs.

Gyrinus beetles are predators and scavengers of the surface film, well-adapted to maneuver rapidly through this interface of air and water. Besides snatching dead and living insects, they will also eat plant matter.

The adult beetles secrete distasteful substances, which might be toxic. Fish and birds only rarely eat them. And in the late summer, they typically form these large schools on calm waters. Why? Most researchers explain this as a defensive adaptation: perhaps the silvery flashes of a disturbed school may confuse would-be predators.

Surprisingly, these rafts of beetles often contain multiple species. One researcher found seven different species of Gyrinus in a large school on a lake in Massachusetts! Is this the case here on Hauser Lake? And why do multiple species aggregate like this? If I ever get an ambition to learn beetle dissection, these would be interesting questions to explore.

Adult Gyrinus overwinter, though many don’t survive the cold season. As the water warms again the following spring and summer, they mate. The larvae are aquatic predators. But they’re also predated, eaten in large numbers by a variety of fish.

Once the larvae are full-grown, they move from the water to the shore. Climbing the stem of a grass, rush, or cattail, they build a pupal case by gluing together bits of rock, wood, or vegetation. At this stage, they’re especially vulnerable to attack: several ichneumonid wasps attack Gyrinus as they’re pupating.

Once the beetles finish their late-summer pupation, the adults emerge and re-enter the water. There, they form these conspicuous, late-summer schools. On Hauser Lake, I’ve started noticing these rafts of beetles within the last week or two. Presumably, they’ll be present here for the rest of the fall.

A taste of Gyrinus

Side view of the Gyrinus specimen, showing its streamlined, flattened body.
Side view of the Gyrinus specimen, showing its streamlined, flattened body.

So many questions remain. What do the pupal cases of our local Gyrinus look like? How can so many closely-related species coexist without one outcompeting the others? 

But for this week, only one more question seems easy to investigate. Do Hauser Lake’s Gyrinus taste bad, as the literature suggests?

It’s September 8 now, and I’m back along the shady south shore of the bay with a dip net. A school of whirligig beetles is in the same spot where I found them two days ago. It’s easier to catch them with a net than with my hand. I plunge the net into the water and bring it up with dozens of struggling beetles inside.

The beetles all appear similar, sleek and black with reddish legs. I check a handful of them, and I can see the tiny triangle of the scutellum on each one – they’re all Gyrinus. And if there are multiple species present here, the differences between them are too subtle for me to notice them at a glance.

The beetles have a definite odor in my hands: they’re sour and musty, reminding me of certain ants I’ve found in the past. Bracing myself, I pick one up, put it in my mouth, and chew. The flavor is surprisingly mild, vaguely reminiscent of the smell but less obvious. At the same time, I notice a slightly chalky, dry sensation in my mouth. Is the beetle causing this? I could sample more to know for sure, but I decide I’d rather not.

If you find yourself at the edge of a lake this fall, keep your eyes open for whirligig beetles, darting in silvery schools over the shallows. Enjoy their dance across the water, make note of the habitats where you find them, and let me know what you’re seeing. But if you find yourself getting hungry, I’d suggest that you look for something else to snack on.

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