Some aquatic insects can survive being encased in ice – water boatmen, for example, or dragonfly larvae. But what happens if they are frozen near the surface of a pond and the ice around them melts (or sublimates), leaving them exposed to the air when they thaw out? This is what I was wondering last weekend, as I poked around the icy wetland at our family prairie.
As I wandered around our wetland, I found several dragonfly larvae and a couple other aquatic insects frozen at or near the surface of the ice. I’m still trying to puzzle out how they got there. My best guess is that they must have been swimming near the surface as the water around them neared its freezing point. Maybe they got cold enough they couldn’t swim back down before the water around them froze? Regardless, there they were, right at the surface. In some cases, they were partially exposed to the air as the ice was melting and/or sublimating from around them.
Dragonfly larvae breathe through gills, which I assume means they can’t survive for long out of water. They can apparently survive being frozen, at least for a while, but I assume they only survive if they thaw out underwater where they can breathe. If they thaw out on top of a frozen pond, that seems like a really bad outcome… If so, the larvae I was seeing were either already dead or doomed to be so.
I’m still not sure why the larvae would have been swimming near the water’s surface as it froze, or if that’s what actually happened. It’s not an isolated incident – I find insects near the frozen surface of wetlands and ponds pretty frequently. Anyone have a great explanation for what’s happening?