Very well known are the stinging-hairs of the Urticaceæ, long hairs (Fig. 20) with an elastic base, but with glass-like, brittle, rounded heads, which break off at the lightest touch, whereupon the sharp point of the broken hair penetrates the skin of the creature which has touched it, and the poisonous contents of the hair are poured into the wound. Even our large stinging-nettle (_Urtica dioica_) can cause intense irritation, and evoke the 'nettle-rash,' named after it, on the human skin; but there are many tropical species of nettle, e.g. _Urtica stimulata_ in Java, and others, which have an effect similar to that of snake-poison and produce tetanoid spasms, and so on. In addition to formic acid these hairs contain an undefined ferment, a so-called Enzyme. It need scarcely be said that these stinging-hairs must have much more severe effects on the mucous membrane of the mouth of grazing animals than on the human skin, and that they are therefore an excellent protection for the plants. As a matter of fact we never find our nettle patches eaten away, and even the donkey, which eats thistles freely, turns away from the stinging-nettle. But even these stinging-hairs, like all other protective devices, do not afford an _absolute_ protection. The caterpillars of several of our diurnal butterflies feed exclusively on the stinging-nettle, and they eat up the leaves, stinging-hairs and all. This is the case with five species of the genus _Vanessa_, namely: _Vanessa io_, the 'peacock,' _Vanessa urticæ_, the small tortoiseshell, _Vanessa prorsa_, _Vanessa C. album_, the C. butterfly, and _Vanessa atalanta_, the admiral. [Illustration: FIG. 19. Barbed bristles of _Opuntia rafinesquii_; enlarged.] [Illustration: FIG. 20. Vertical section through a piece of a leaf of the Stinging-nettle _(Urtica dioica_), bearing two stinging-hairs; magnified 85 times; adapted from Kerner and Haberlandt.] We are all familiar with our mulleins (_Verbascum_), those beautiful flower-spikes with the thick, soft felt leaves, which grow on stony or sandy soil. Harmless as they look, they are much disliked by animals as food, for the thick hairy felt which covers them breaks up in the mouth, and sticks in the folds of the mucous membrane, causing burning sensations and other discomforts. They, too, are therefore spared by grazing animals, but they have smaller enemies, like the caterpillars of the genus _Cucullia_, which, however, never completely destroy them, but only eat large holes in their leaves.