My progress with the strimmer was halted as I realised that the nettle beds (Urtica dioica) were full of caterpillars. The masses of cobweb strewn over the heads of the plants weren't as I first thought the work of spiders but were in fact nests of young caterpillars at different stages of growth. The caterpillars in the most recently hatched batches, each of perhaps 50 caterpillars, were tiny with their heads disproportionate in scale but the older ones were clumped in groups at the top of plants consuming their way down the stems. As I approached they reared up as one, perhaps trying to put off a potential predator with their confidence perhaps and, alerted to the caterpillars in my own garden I saw another on School Hill which might suggest that across the village there are other groups all collectively protected from predation by their sheer numbers. I ended up strimming my way around the caterpillars leaving 6 distinct clumps of nettle plants for the caterpillars to feed on, reminded of the importance of wild patches of growth in a garden.
I think these are the caterpillars of the small tortoiseshell butterfly (Aglais urticae)