This is a very bright-colored syrphid fly often seen soaring in shadowy places, but what he is doing we do not know. He stays poised in the air and is one of the most beautiful flies we have. The larvæ of some of the smaller syrphid flies feed upon the larvæ of other insects, aphids in particular; but the larva of this one has never been seen, at least it has never been recognized. [Illustration] NOT A HOUSE FLY (_Archytas aterrima_, Des.) This portrait of one of the many species of fly, not a house fly, however, is as different as it is possible to be from the maggot from which it grew. The eggs of the mother fly, deposited in some decaying animal matter, hatch in a few days, and out of these eggs come maggots with rudimentary legs and looking like beasts from another world entirely. In a few days more they reach the limit of their growth, and stop, the tissues break down to a mush and out of this mush-like substance are formed flies with wings and sucking, trunklike mouths just like their mothers. The maggots have no sexual organs, and yet, out of the creamy mass of cells, the sexual organs of the flies are formed as though directed by a force as certain in its effects as the law of gravitation. We have been so intent on killing the fly and so afraid of it as the great carrier of human diseases that we have lost sight of one phase of its character, so to speak. Think of having under our eyes animals like these dipteras from which you can breed a new generation in twelve days! And would it not be strange if, from studying the fly, we should learn the meaning of heredity and sexuality, for this is one of the places where the scientists of the day are at work on the problem of inheritance, that problem which, when elucidated, is likely to make more changes in the world of humankind than almost anything which has so far been discovered. The bearing of the fly on the welfare of the world is one of the most spectacular developments of modern times and a tribute to the value of knowing the minutest details of the world in which we live. [Illustration] THE HORSE FLY (_Tabanus atratus_, Forst.) The head of the horse fly appears to be all eyes, and it is no wonder that we can so seldom take it by surprise. Below the oblong, compound eyes are the sharp mouthparts, which in the female are provided with lancets, which enable her to puncture the skin of warm-blooded animals and suck their blood. It is curious that the female should have such habits, while the males are content to lap up nectar from the flowers. This jet black, loud-buzzing creature flew into my laboratory and made so much noise that I was forced to kill her. This photograph of her is nine times her real diameter. She belongs to a large and important family of flies, whose females make the lives of men and animals miserable in many parts of the world by their bites, which form most annoying wounds. [Illustration] A GREEN-HEADED HORSE FLY (_Tabanus punctifer_, O. S.)