Aphilotrix seminationis. 87 



In one generation only females occur, and reproduction 

 is wholly by parthenogenesis, in the other generation 

 on the contrary both sexes exist, and parthenogenetic 

 reproduction is unknown. This alternation of genera- 

 tions, as it occurs in these species, appeared to me at 

 first to be the natural and universal rule ; and I believed 

 that the problem of the position of the so-called agamous 

 gall-flies would be solved by regarding an agamous and 

 a sexual generation as forming a connected generation- 

 cycle. Further observation has however shown me that 

 the existence of alternating generations is not an absolute 

 rule applying to all species of oak gall-flies. 



Several agamous species of gall-flies still remain to be 

 described which are not linked to any alternate 

 generation. 



These species are few in number, and propagate them- 

 selves in an unbroken succession of generations in the 

 female sex. I add the following descriptions of them 

 because it is interesting to compare them closely with 

 the other species. They belong entirely to the genus 

 Aphilotrix. 



20. Aphilotrix seminationis. Gir.' 

 Gall. Spindle shaped, stalked or sessile, with sharp 

 or scarcely visible longitudinal ridges ; green, often 

 tinged with red ; at first hairy, especially on the apex, 

 generally smooth later. The gall occurs on leaves as 

 well as on the stems of the flowering catkins. When 

 galls are formed on leaves these undergo striking 



[' Cynips seminaitonis, Giraud. Cynips inflorcscentiae, Schltdl. 

 Andriciis semittationis, Mayr, Cameron considers the fly indis- 

 tinguishable from A. quadnlineata^ 



