18 THE BOOK OF NATURE STUDY 



parents, and eventually produce males and females of a different 

 appearance. In some species, however, no males have ever yet 

 been discovered. 



If galls are gathered when fully formed, and kept in a box, 

 or if they be enclosed in fine muslin bags while still attached 



B 



FIG. 8. Oak apple gall-fly. A, male ; B, female ; C, wingless female of second generation. 



to the plant, the gall-flies will in due course be obtained. But 

 in addition to the rightful tenants there will also emerge from 

 the gall a number of other insects. Of these some are parasites 

 upon the gall-flies, and others are thieves whose parents have 

 taken advantage of the rich succulent character of the gall and 

 have chosen to deposit their eggs there. Hence care is needed 

 to discriminate between the genuine provoker of the gall and 

 the intruders. 



BIBLIOGRAPHY. Adler and Straton, Oak Galls and Gall-Flies ; Connold, 

 British Vegetable Galls ; Cambridge Natural History, vols. v. and vi. ; Cameron, 

 British Phytophagous Hymenoptera, 4 vols., Ray Soc. ; Cameron, " On the Origin of 

 the Forms of Galls," Trans. Nat. Hist. Soc. Glasgow, 1885 ; Fitch, " The Galls of 

 Essex," Trans. Essex Field Club, ii. ; Huxley, Trans. Linn. Soc., xxii., 1858, and 

 numerous papers by various authors in the Entomologists' Monthly Magazine and 

 Entomologist. 



