Who has not found his rose bushes swarming with minute green bodies, the Aphidæ? Brush them off at night and in a few hours as many more are seen, due to the marvelous rapidity of their increase. The eggs are laid in the autumn, and hatch in the early spring, the young then appearing as wingless little creatures which in turn produce not eggs but winged or wingless Aphidæ (Fig. 216). These appear in such numbers and so quickly that in a single summer a pair of plant lice will produce one quintillion of young ones. Can we wonder that it is difficult to keep the rose bushes free from such a swarm? The story of the development of these insects is but merely touched upon, but it is among the most remarkable of all the strange and unexplainable transformations we find in animal life. [Illustration: FIG. 216.--Aphis.] Here we may glance at the countless scaly insects which infest fruit of various kinds. The black, red, and cottony scale are common in California, and have to be fought with all the cunning and intelligence that man can invoke. In 1886 the orange groves of southern California were almost ruined by the cottony scale. I have seen trees that looked as though the limbs were covered with snow. But an enemy of the scale, a little spotted lady bug, was imported from Australia, and in a few months the scale had disappeared. The black and red scale and several others are pests which devastate the groves, stopping the growth of the trees and operating against the fruit grower, who is obliged to spray the trees with poisonous washes to destroy them. XXVI. FLIES AND MOSQUITOES [Illustration: FIG. 217.--A fly, natural size and magnified.] The flies and mosquitoes are among the greatest pests and dangers to man. Both are conveyers of disease, and the former, as an agent of destruction, deposits its eggs in meat of all kinds, making it impossible to keep meat in some countries. On the other hand, it should be remembered that the flies are valuable scavengers, hastening the destruction of dead matter which might contaminate the air. [Illustration: FIG. 218.--Tongue of a fly.] [Illustration: FIG. 219.--Eyes of a fly.]