These strange swellings on leaves and leaf petioles of stinging nettle are the work of a gall midge called Dasyneura urticae, whose larvae develop inside the pouches. The midge lays its eggs on the leaf upper surface and when the larvae hatch they irritate the plant tissue on either side of the the groove in the leaf ribs, which grows over them forming a protective pouch lined with nutritive tissue. When the larvae are fully developed they escape through a slit in the gall wall and fall to the ground, pupating in the soil.
You can find some excellent photos of the midge and its larvae at this web site
These are gall midge galls of another species of the same genus, Dasyneura ulmaria, on the leaf of meadowsweet Filipendula ulmaria. The galls and life cycle of the insect are broadly similar but in this case the midge lays its eggs on the underside of the leaf and there is only one generation each year.
All photos taken at Romaldkirk in Teesdale on 25th. September.