An accidental capture

So, while we were capturing the Gammarus for last week’s entry, we scooped up some water from the stream into a jar to put them in. In the process, since the stream was so shallow, we got some debris off of the bottom into the jar, too. Then, when we got it back to the house and started looking at what we had, S_ noticed that one of the bits of debris was oddly symmetrical and had a hole in one end[1]. So, we put it into the petri dish along with the amphipods, and waited to see if anything came out. Something did:

This is a caddisfly nymph, order Trichoptera. Given that we are in the north, it’s likely to be a Northern Caddisfly, family Limnephilidae. Caddisflies have aquatic nymphs, and most caddisfly species build these protective tubes around their bodies out of bits of debris glued together with silk. This one built its tube out of sand. Apparently most species have a characteristic material that they use for their armor, other species might use bits of wood, snail shell fragments, or a simple case made out of silk.

I was photographing this one while the Gammarus were swirling briskly about, and after a while one rammed into the caddisfly and rolled it over[2], so I could see the underside:

The poor caddisfly couldn’t roll back over, because the tube had a more or less square cross-section and was flat on the back, so I got to keep photographing while it lay there, waving its little feet impotently in the water:

The feet look to me like they are adapted to hang onto a rock or stick while the current flows past carrying food. The tube-building caddisflies are “shredders”, they catch bits of organic debris to eat (they basically have the same diet as the Gammarus amphipods do, so it is not too surprising that we found them in the same place). There are also some predatory species that don’t build cases, but this isn’t one of them. Once they mature, the nymphs molt to produce a winged adult that looks a bit like a small moth, which flies off to eat nectar or plant juices, mate, lay eggs, and die.

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[1] The fact that we caught this one completely by accident suggests pretty strongly that our little stream is absolutely lousy with caddisfly nymphs. Next time we go down, I’m just going to shovel some sediment into a jar at random and see what we turn up.

[2] “Gammarus versus the Caddisfly” sounds like a Japanese monster movie. Just imagine these being about 60 feet tall and fighting it out in Tokyo.