Caterpillars on peas?

We seem to be plagued with caterpillars or similar on our pea plants in the garden. What is the best way to go about eliminating/dissuading them?

 

Never thought of photo. Will work on that. Thanks.

 

I use butterfly netting from these guys http://www.wmjames.co.uk/fruit-cages.htm
to keep the caterpillars away as I don't like using sprays on them