Many insects produce a good deal of irritation, and even dangerous sores, by biting and burrowing in the human skin, without secreting any active poison. Often they introduce microscopic germs of disease in this way from one animal to another, as, for instance, do gnats, tsetze-flies, and horse-flies, and as do some small kinds of tics. The bites of the flea, of midges, gnats, and bugs are comparatively harmless unless germs of disease are introduced by them, an occurrence which, though exceptional, is yet a great and terrible danger. We now know that it is in this way, and this way only, that malaria or ague, yellow fever, plague, sleeping-sickness, and some other diseases are carried from infected to healthy men. Various diseases of horses and cattle are propagated in the same way. The mere bites of insects may be treated with an application of carbolic acid dissolved in camphor. The pain caused by the acid stings of bees, wasps, ants, and nettles can be alleviated by dabbing the wound with weak ammonia (hartshorn). Insects which bury themselves in the skin, such as the jigger-flea of the West Indies and tropical Africa, should be dug out with a needle or fine blade. The minute creature, like a cheese-mite, which burrows and breeds in the skin of man and causes the affliction known as the itch must be poisoned by sulphurous acid—a result achieved by rubbing the skin freely with sulphur ointment on two or three successive days. A serious pest in the summer in many parts of England is a little animal known as the harvest-man. These are the young of a small red spider-like creature, called _Trombidium_. They get on to the feet of persons walking in the grass, and crawl up the legs and burrow into the tender skin. Benzine will keep them away if applied to the ankles or stockings when they are about, and will also destroy them once they have effected a lodgment. [Illustration: FIG. 15 _bis_.—A. Highly magnified drawing of a stinging hair of the common nettle. The hair is seen to be a single cell or capsule of large size, tapering to its extremity, but ending in a little knob. The hard case _e_ is filled with liquid _a_, and is lined with slimy granular “protoplasm” _b_, which extends in threads across the cavity to the “nucleus” _c_. The ordinary small cells of the nettle leaf are marked _d_. B shows the knobbed end of the stinging hair, and the way in which, owing to the thinness of its walls, it breaks off along the line _xy_ when pressed, leaving a sharp projecting edge, which penetrates the skin of an animal, whilst the protoplasm _p_, distended with poisonous liquid, is shown in C, issuing from the broken end. It would escape in this way when the sharp, freshly broken end had penetrated some animal’s skin.] FOOTNOTES