At first I thought they were caterpillars. On closer inspection I could see that they were galls or growths of some sort.
Once home, I Googled, as one does, and found out they were caused by nettle rust fungus, Puccinia urticata.
Rusts are obligate parasites of plants, i.e. they must find a host in order to complete their life-cycle. Their proper name is Pucciniales and there are thousands of them.
To say that rust fungi have intricate life-cycles is a massive understatement. Their life-cycles are monstrously complex. I have always found fungal reproductive biology rather abstruse, but rusts seem to take reproductive intricacy to extremes. For a start, they can produce up to 5 different types of spore (best to read about them here on Wikipedia). And there are 2 types of rust;
1. Autoecious rusts complete their life-cycle on one host.
2. Heteroecious rusts have 2 different species as hosts (these hosts are not related taxonomically).
Nettle rust is heteroecious, so has 2 hosts; nettle (obviously) and a sedge. Its life-cycle (which I won't embarrass myself by trying to explain) is similar to wheat rust. (In the case of wheat rust, barberry (Berberis vulgaris) is the rust co-host with wheat, which explains why barberry (a once common shrub in hedges) was uprooted with some considerable fervour last century.)
Seeing the nettle rust reminded me of another rust I'd seen at Holme Fen NNR in late May (this time the rust was identified by those patient people on ISpot). It is dock rust, Puccinia phragmitis.
If rusts don't float your boat, and I am not entirely sure they float mine, whilst I was at Holme Fen I managed to find a few smart insects, despite the rather dull weather. The photos below were taken in late May this year.
Black-tailed skimmer Orthetrum cancellatum (I believe the lovely creature below is a female). By the way, cancellatum means latticed (linkie). If you scroll down a bit on that link you'll find that Orthetrum means, err, 'straight tool', which refers to the anal appendages or claspers which male dragonflies use to clasp females round the head during mating.
And a lesser stag beetle was also seen strutting his stuff.
There is an excellent post on the Wild about Britain forum by one, Alan Silverside, which explains something of the ecology of the nettle rust fungus. It's worth a gander, here, if you're interested.