Initially the trees look just like any other tree in summer - covered in leaves. Then the eggs are laid on the trees, and hatch into caterpillars. The caterpillars are there in their thousands and eat all the leaves completely away, so that not a single fragment of leaf is left.
In the distance the trees just look white.
In close-up we can see the thick white webs on the twigs of the tree, and the caterpillars swarming all over them.
At the base of the tree, the webs fan out and the caterpillars can be seen marching away towards, what they hope is the next tree. They usually have to dodge round a bunch of curious children and lots of adults with cameras.
In the photo below you can see a grey area on the base of the trunk. This is a big pile of caterpillars. There are thousands of them on each tree.
The caterpillars do this for maybe a month or two each summer, and then they all vanish away to get on with their lives, and all of the trees' leaves grow back. A few weeks later, there is no sign at all that the caterpillars were ever there.