"The tsetse-fly," said the Doctor, "is scientifically known as _Glossina morsitans_; it is about the size of the common house-fly, and is of a brownish color. It is very quick in its movements, and will evade the most dexterous attempts to capture it with the hand except in cool mornings or evenings, when it is less agile than at other times. Its bite causes the death of the ox, the horse, and the dog. A remarkable feature about it is that, while it is fatal to oxen and cows, it is perfectly harmless to calves until after they are weaned." "How is it that men can travel where this fly abounds, if its bite is so deadly?" one of the boys asked. "Because," was the reply, "its bite is no more injurious to man than is that of a mosquito. It causes a slight itching, just as does the sting of a mosquito, and that is all. "The horse is the most ready victim of the tsetse-fly. It comes in swarms sometimes, and lights on the back of a horse by hundreds. There is no known remedy for its bite, and in less than a week the animal will die. "The sting is not a sensible one to the horse or ox, and very often an animal may be bitten without being aware of it. In a few days there are symptoms of a cold; the eyes and nose throw off quantities of mucus, swellings appear under the jaws and elsewhere, the poor beast begins to grow thin, as if starving, and finally dies of exhaustion. "The period for the disease to perform its work varies from one week to eight or ten weeks, and sometimes longer. It is a singular circumstance that, while oxen, dogs, and horses are killed by it, the donkey and mule escape altogether. Sheep perish from the bite of the tsetse, while the goat is unharmed; and it does not seem at all injurious to any of the wild animals of Africa. In the valley of the Zambesi there are whole districts where the natives can keep no domestic animals except goats; and it sometimes happens that, while one bank of a river is infested by the deadly fly, the other will be wholly free from it, and cattle may graze there in safety. "When the natives are about to pass with their cattle through a tsetse region they endeavor to do so on a cold night, when the fly is quiet; and they take the precaution of smearing their animals with a paste made from ashes and other substances, so that the tsetse cannot bite through it. Inoculation does no good; and, until some remedy is discovered, many portions of Africa will be uninhabitable to those who desire to keep oxen and horses." It turned out that the bite on the horse was not that of the dreaded tsetse, and so the animals were safe for the present at least. It was getting late, and the conference came to an end with the dissertation on the deadly flies of Africa. The Doctor closed by saying that there was an insect in the western part of Africa whose bite has somewhat the same effect on man as the tsetse on the ox, though not likely to be as fatal.