ALTERNATING GENERATIONS 



IN 



OAK GALL-FLIES 



CHAPTER I. 



Introduction. — Earlier Views. — First Observations on Alter- 

 nation OF Generations. — Methods of Investigation. 



The curious fact that in many species of oak gall-flies 

 female specimens alone were found, long attracted 

 entomologists to a closer study of this interesting 

 family. But T. von Hartig ^ was the first to demonstrate 

 by experimental breeding, after having reared many 

 thousands of flies, that there are numerous species in 

 which none but females exist ; and that these when 

 they emerge from the gall have their ovaries filled with 

 perfectly developed eggs, which they at once proceed 

 to deposit. Although the occurrence of partheno- 

 genesis in these species had been placed beyond doubt 

 by these experiments, there was much in the life 



^ O'^ber die Familiend. Gallwespcn. Germar's Zeitschr. f. d. Entomol. 

 1840-43, vol. ii. pp. 176-209; vol.iii. pp. 322-358; vol. iv. pp. 395-423. 



B 



