From the menus, choose Insert > New Symbol.

Name it caterpillar and make it a movie clip type.

Start by drawing a circle with no stroke and green fill. Right click and convert it to a symbol.

Name it body, and choose graphic type.

Now goto the library, and click and drag out more body parts, overlapping them until you have built a caterpillar.

You will notice that a new layer has appeared name Armature. On this layer, right click on frame 50 and choose Insert Pose.

Now right click on frame 25 and choose Insert Pose.

While still on frame 25, and using the pointer tool, drag the bones into a crawling position.

Press Enter to see the motion. When you are happy with it, return to Scene 1 by clicking the Tab at the top of the window.

At this point you should see nothing but a blank scene because we have just created a caterpillar library object.

Open the library and drag the caterpillar onto one edge of the stage.

Right click on frame 1 and choose Create Motion Tween.

While the playhead is still on frame 25, drag the caterpillar to the other side of the stage. Place it so that its tail is just on the edge of the stage.

If you press Ctrl-Enter you will see your caterpillar race across the stage!

To slow it down, we need to extend the length of the tween. Sit your mouse over the end of the tween until it turns into a two headed arrow.

Then click and drag the tween out to about 250 frames.

Press Ctrl-Enter to see your movie now.