OAK APPLES. 37 



awakened by this tiny gall, that we crush in 

 our fingers, or pass unheeded in 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 prob- 

 ably lacks the poison organ for such an opera- 

 tion. 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 nu- 

 merous 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 



