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On closer inspection they look hairless and with the web,maybe a Ermine moth caterpillar,type would be determined if we know the plant which looks like a defoliated shrub,sapling,what's your thought's LordMuzzy.
Hi, the early instar of the peacock caterpillar start off small hairless and brown and develop into the larger black spikey/hairy late instar phase before pupation.
They were hairless and brown. The plant is/was a nettle. I'll go back and have another look tomorrow and see if I can get an updated image
I'm sure those are not Peacock caterpillars, but neither are they 'horrible maggot things'!!!
Have just been back: they have now moved down the nettle plant to a lower leaf and have become black spiky caterpillars, so I think the ID is correct :)
Lat/Lng: 50.7921, -3.7481
OS grid ref: SS768006
In the corner of a barley field, amongst a patch of nettles