Rusty nettles

The kids were searching the nettles in the hope of finding caterpillars, or other interesting bugs. Suddenly there was a yell, "I've found an orange caterpillar!" However, on closer inspection this wasn't a caterpillar, it looked like some sort of growth in the vein of the nettle leaf. After a bit of googling (what did we do before the internet?) we found the answer; it was a gall caused by a parasitic rust fungus, Puccinia urticata.