The existence of natural death in the animal world cannot be denied, but it is very rare. The best example is that of the curious insects known universally as ephemerids (Fig. 18). Swarms of these delicate and graceful insects are to be seen in the summer months round lights. The perfect insects emerge from water, in which the six-legged larvæ feed on the organic débris contained in fresh water. The larvæ are not predaceous, and escape from their numerous and hungry foes by agility. They are long-lived, some of them passing two or three years in the mud of streams, and in the end become winged insects after a rapid metamorphosis. Near Paris, anglers have a popular name (_manne_, manna) for one species (_Palingenia virgo_) which emerges in swarms after sundown from the waters of the Seine and Marne. The swarms fly in huge numbers, like heavy snow-flakes, for a very short time, and then fall into the water (Fig. 19). The flight of these insects lasts only an hour or two, and then, in an enfeebled condition, they fall down in vast numbers. They are attracted by the lanterns lighted by fishermen, and are collected to be used as bait. The life in the winged condition is truly ephemeral and lasts no more than a few hours. The structure of the insect is adapted to this short life. The larvæ have powerful jaws, used in the mastication of food; the winged insects possess only vestiges of jaws. They are unable to feed, and so are adapted only for the briefest existence. Their hour of aerial life is devoted to love. As soon as they emerge the males and females unite, and the packets of eggs, which are deposited at once, fall into the water, and in a few weeks the young larvæ hatch out. The mode of life and the organisation of the adult ephemerids show plainly that they are adapted to natural death. Death comes to them not because they are without food, or because the environment fails to provide something necessary to life, but merely because they emerge from the larval state in a non-viable condition, without the organs necessary to the maintenance of life. [Illustration: FIG. 19.—Swarms of _Palingenia virgo_. ]