OAK APPLES. 37
awakened by this tiny gall, that we crush iii our fingers, or pass unheeded iii our walks abroad.
We are not yet at the end of our story. If we collect a dozen or more of these oak apples, and keep them in boxes till the flies appear — and that may be over winter in some instances — we will be quite sure to find that there are at least two species, looking very much alike to untrained eyes. One of these is a gall-fly, the other is a guest-fly — he is an uninvited guest of the owners of the gall. He is the offspring of a mother that could drill a hole and lay her eggs in a gall after it was started, but had no power to begin one on her own account. She probably lacks the poison organ for such an operation. Here, then, is a creature equipped with a fine arrangement to take advantage of the co-operation of the oak-tree and gall-fly, and make them provide for her brood. There seems to be a moral disorder in nature : there is no sense of fair play ; pirates and plunderers and parasites abound everywhere. These guest-flies are a numerous race, living always as young ones on the ready-made galls, or often grubs of the gall-makers.
Not without some service to mankind are these oak galls. They have long been used in