A dichotomous key is pretty easy to use; all you have to do is answer the questions that
appear in the order they're given.
There are only two ways to answer any given question - that's where the name "dichotomous"
comes from.
"Di-" means "two," just like "tri-' means "three," and "quad-" means "four."
There are four legs on a quadruped, three wheels on a tricycle, and two choices in a
dichotomous key.
Let's work through an example.
Here we have a macroinvertebrate that we just caught in the Chesapeake Bay.
What is it?
Here's where a dichotomous key comes in handy.
We click on the key to begin.
The first question asks whether it has legs.
Yes, it does, so we're going to click on the second answer and go to number three.
This asks if it has six legs.
I don't know about you, but I don't remember how many legs it has, so we click on "Look
at catch" to examine it.
So here's our catch.
It's got one, two, three, four, five, six, seven, eight, nine...
It's definitely got more than six legs.
Click on the back arrow, and we're going to answer "No, it has more."
It tells us to go to number four.
This one asks if it has a tail.
We can check to make sure.
I don't know about you, but I can't really tell.
I'm going to go ahead and say that it doesn't have a tail for now.
So, we're going to go back and select "No."
Here it warns, "Are we sure?"
So we're probably wrong.
We're going to go back now.
We can look at our catch again to make sure.
I guess I can kind of see how that's a tail.
So we click back and select, "Yes."
Does it have pinchers on its front legs?
No, it doesn't.
So we're going to click, "No," and it takes us to the spine-backed scud page.
It's got the name at the top, our organism in the middle, and then a couple of facts
down at the bottom.
So there you have it.
That's how you use a dichotomous key.