Wading for Bugs: Exploring Streams with the Experts

17. The Phantom Midges of Silver Lake

94 17 The Phantom Midges of Silver Lake Michael C. Swift It was mid May, and everyone was returning for the summer season at Silver Lake. I’d been sewing nets in preparation for sampling when I saw the twins, Sue and Mark, running down the dock toward me. The Franklin family had the cabin next to mine, and I always looked forward to their arrival. The twins should be about ten years old now, I mused. “Can you take us out on the lake?” the children said together. I could see that they were ignoring their mother’s calls for help unpacking the car. “We want to go fishing, but Grandpa says there aren’t any fish in Silver Lake anymore.” “Hold on, hold on,” I said, laughing. “First you need to get moved in, and I need to finish mending this net and organize my sampling gear. How about going out tomorrow, after lunch?” The next day, after I loaded a few supplies and my sampling equipment, the kids climbed into my boat, and we set off for the middle of the lake. “Let’s see what critters we can find with this plankton tow,” I explained, handing them the gear. “We’ll start with a vertical haul from the bottom of the lake to the surface. Sue, you lower the net until it’s fifteen meters deep; that will put it just above the bottom. See the marks on the rope? Mark, you watch as she lowers the rope and call out the depths.” Sue began to lower the net. “Eleven, twelve, thirteen, fourteen, STOP!” called out Mark. “Now, slowly and gently pull the net up,” I instructed. “When it reaches the surface, we’ll rinse it and then empty the plankton The Phantom Midges of Silver Lake 95 into a jar.” They both performed like pros. In no time we had a jar full of plankton. “What do you see?” “There are little critters zipping all around in the jar,” Sue said as she tightened the lid. “What are they?” asked Mark. I held the jar up to the sky so we all could see the plankton against a light background. “It looks like copepods and cladocerans for sure. I’m also looking for rotifers, which are too small to see without a microscope, and transparent Chaoborus larvae. But maybe it’s too early for them.” “What’s a koberus?” asked Sue, quizzically. “Ka-ob-er-us, spelled C-H-A-O-B-O-R-U-S, are fly larvae that live in the mud at the bottom of the lake,” I explained. “They migrate up into the water column and come almost to the surface every night to feed. They’ll catch tiny rotifers, and the bigger zooplankton like the ones you see in the jar. Once they feed, they migrate back down to the mud where they spend the daylight hours.” “Well,” said Mark, “If they only come up at night, why would we catch any in the afternoon?” “Very good question. Let’s look at this plankton sample in the lab to see whether we caught any.” After preserving the sample we returned to the dock and went straight to the small laboratory in my cabin. Under the microscope I could show Mark and Sue the diversity in their sample. We had collected lots of red, teardrop-shaped copepods, each with a single eye and either long or short antennae. “Those with the long antennae are Diaptomus and the ones with shorter antennae are Cyclops,” I told them. “Fats stored in their bodies have red pigments that give them their color. And fats are used for making eggs. See how many of them carry clusters of eggs?” “What are these clear, fatter critters with long arms, one big black eye, and a spine?” “Those are Daphnia, Sue. Their antennae, those things that look like arms to you, are used for swimming, and they have legs to filter algae. Their outer protection is a carapace shaped like a folded taco shell. Cladocerans like Daphnia migrate up and down the water column, but maybe not as far as Chaoborus.” 96 Truly Flies “Now I see something different from Daphnia or the copepods,” said Sue. “They are long and skinny. Look, Mark.” Mark looked into the microscope. “It’s really transparent, but has little black dots near both ends and strange projections at one...