Web Caterpillars

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This may seem a stupid question, but I've been trying to search for an answer for over a week.

Last week a friend called me to ask about a caterpillar in their damson tree, I went and had a look and it turned out to be a Lackey - which as far as I was concerned was nothing to worry about. Since then I've seen two other web making caterpillars - Brown-tails which I know can be a nuisance because of the hairs and a Small Eggar which I believe is quite rare. I also know of the Ermines which have sometimes caused mass defoliation on some plants.

What worries me is that I often hear people talking about the webs and saying that you need to remove them before they cause damage, before they actually check out what the caterpillar is.

What I would like to know is how many web forming caterpillars are there? Can anyone give me an idea of any other species?