I collect many of the giant water bugs I use in my research from a spring fed stream in southern Arizona. It’s a gorgeous place – tons of giant cottonwoods, watercress and duckweed covering the water, and mint lines a good part of the bank so it smells minty as you walk. The stream contains a lot of giant water bugs, so I am always sure that I can get them at this location whenever I need them. Even better, it’s almost always cooler at this stream than it is in Tucson, so it’s a nice place to go to escape the heat in the summer!
I often collect other insects at the stream for classes that I teach. There are a wide assortment of dragonfly and damselfly nymphs, several mayfly and beetle species, sideswimmers (also known as scuds or amphipods), and tons little riffle bugs that skittle about on the surface of the water. There are also leeches, though these are supposedly not parasitic on humans – and I have yet to get any on me, which makes this seem more likely to be true. Although the stream bottom is very muddy and it’s common for me to trip and fall into the water or over-top my hip waders, the water’s clean and cool and it’s a really great place to collect aquatic insects.
On my last trip, I was collecting insects for my insect behavior class and pulled out a bunch of wormy looking things that I thought were leeches. Rather than picking them out and dumping them back into the stream, I brought a few of them back to the lab with me to show my students. Leeches are the most fantastically disgusting things to watch swim, so I find them fascinating. However, I went to check the containers the bugs were in a few days later and learned that the things I had thought were leeches were not in fact leeches. They were something much more exciting: flatworms!
If you had a good high school biology class like mine (or a good intro biology class in college), you may have encountered these animals before. We were told they were called planarians, which is the common name for several genera and species of flatworms in the family Planariidae. You can do some pretty amazing experiments with these very simple creatures. Those little white spots on the head that look like eyes are not exactly eyes, but they do sense light (and make planarians very cute!). Because they have light sensing organs, you can train planarians to become attracted to or to avoid certain types of light. You can also cut them in half and watch them regenerate! If you cut them lengthwise, you’ll get 2 whole new worms. If you cut down the middle of their heads, you’ll end up with a flatworm with two heads. If you cut them across the middle though, you just get two halves of a dead worm. We chopped our planarians up in one particularly memorable lab in high school, so I know this first-hand. If you ever have a chance to slice a planarian down the middle, I highly recommend you try it, if only to prove to yourself that it works.
(I can’t even imagine giving a bunch of high schoolers flatworms and a handful of razor blades and letting them loose, but there weren’t any major incidences in my class.)
One of the things I find the most interesting about these worms is how they move. They are among the most graceful animals I’ve ever seen. They simply glide through the water. Even more amazing is their ability to glide along the underside of the surface of the water. I took some video of the flatworms in my lab a few weeks ago, so take a look. This video (which I recommend blowing up so that you can see it more easily) shows a planarian gliding along the Rubbermaid container it is housed in, then onto the underside of the surface of the water:
Isn’t that amazing? I think planarians are incredibly elegant animals, and this is saying a lot for something that is a step away from being a parasite.
If you look down the middle of the worm as it glides on the underside of the water’s surface, you’ll see the digestive tract. Planarians are predatory, which means they eat other animals. They use their mouth to secrete chemicals that begin digestion, then suck food into their bodies where they complete digestion. What you can’t see, however, are the little hairs called cilia that they use to move. There are tons of these tiny hairs along the undersurface of the worms, which they back and forth like oars to propel themselves through the water. Because they are moving the cilia and not their bodies, the flatworms appear to be gliding.
You can find planarians in many different kinds of water. The planarian in the video was collected from that spring feed stream in the desert I mentioned at the beginning of this post. My high school biology teacher collected them from a storm sewer on the west side of Colorado Spring, Colorado. All planarians need is a location that stays moist most of the time. I encourage you to look out for these fabulous little animals. If nothing else, just watch them for a few minutes as they serenely move about and feel your stress melt away! But don’t let them trick you into thinking they’re innocent. They are, after all, looking for animals to devour.
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2 thoughts on “Flatworms!”
are these the species that use penis fencing to reproduce.
I have no idea. But those guys are SO cool! I didn’t keep any of them to ID, so I can’t even go back to look…