Read it through once

The stinging hairs are hollow and shaped rather like a narrow bulb or flask; the tip is slightly bent over and rounded (not sharp); the hairs contain formic acid. If one grasps the nettle or strokes it in a particular way (from below upwards) the hairs are pressed flat against the stem or broken, so that no wound is made by them in the skin and consequently they do no harm. But if the point of the hair pierces the skin, the well-known irritation is set up. That is because formic acid is poured into the wound. Besides the stinging hairs which keep off all the larger animals (including man) there are others, shorter and thickly set, which do not sting at all, but are intended to keep off snails.[96]