I've found a lot of these really tiny caterpillars on calendula plants. As you can see from the photo they strip the flesh from the leaf, leaving only the upper layer. They are about 5mm long and there can be about 20 or so of them feeding on one leaf. They eat dahlia leaves as well.
Can anyone identify them for me please?
They are young larvae of the Gothic, Naenia typica. They are polyphagous (=eat lots of species) on herbaceous plants. If you look it up in most caterpillar books the illustration will look nothing like yours. This is because most only illustrate the final instar (=growth stage separated by a slough of the skin) of larvae. Most species have 5 instars which are often radically different in appearance.
You could try rearing some of them to see the changes in appearance, but you will have to keep them outdoors overwinter in a netted clay pot half full of compost as they don't complete growth and pupate (in the compost) until April. The moths emerge in June.
You can see pictures of the different instars on ukmoths.org.uk Put gothic in the search box, and make sure to click on the red "show detail" button for full details of larvae.
Thank you both for looking at this for me.
I had a look at the UK Moths site and there is a photo of the caterpillars at the same stage as mine. I've used that site to identify moths before but I find you need to have some idea of what you are looking for and with caterpillars I've got no idea at all! At least with the moths I can look at the thumbnails and try to find something that looks similar.