Among the many thousands of different kinds of insects which inhabit the Earth we have to regard some as our enemies, especially those which attack us by biting or stinging. In some cases this is a serious matter, for biting insects may carry disease. The way they spread the infection is as follows: The insect first bites and sucks up the blood of a person suffering from some particular disease, and later, while biting a healthy person injects some of its saliva into his blood and so infects him with the disease germs. Examples of these are the Anopheles mosquito, a kind of mosquito found in swampy areas, chiefly in tropical countries, which conveys malaria; the tsetse fly of equatorial Africa, which transmits the terrible disease called sleeping sickness; the louse, which conveys the virus of typhus; and the rat-flea, which carries the bacillus of bubonic plague. Fortunately, ordinary fleas do not infest rats and so do not carry the disease.
There is a difference between stinging and biting insects. The wasp and the bee carry a sting at the tip of the tail. This is connected with a gland containing poison and is a weapon, having nothing to do with the insect’s feeding. In action it works exactly like the doctor’s hypodermic syringe; the sting, like a little hollow needle, pierces the skin and some poison is injected.
Unlike that of a wasp, the sting of a bee is barbed and can only with difficulty be pulled out of the victim’s skin. For this reason bees often leave their stings behind them in the wound they have made, but the injury resulting from tearing of the sting out of the bee is so severe that it soon dies.
A mosquito does not sting but ‘bites’; that is to say, it pierces the skin with its beak, which is like a tiny hollow needle. Its object is not to attack you, or defend itself, but to feed on your blood. Before it sucks up the blood it injects some of its saliva in order to keep the blood from clotting in the very fine tube through which it is being sucked. It is this saliva which causes the irritation of a mosquito bite and which conveys the germs if the insect is carrying disease.
Other insects which bite in this way are biting flies, such as horse-flies, midges and the tsetse fly; bugs, like the bed-bug; and fleas. Only biting insects carry and transmit disease, never stinging ones.