1907-1908.] Galls, Gall-makers, and Cuckoo Flies. 47 



formed by the female or agamic generation ; the oak-spangles 

 by the sexual generation of the same Cynipid, Neuroterus 

 baccarum. The large, woody, many-chambered gall on the 

 root of the oak is formed by Andricus radicis, the sexual form 

 of which earlier in the season inhabits swellings on the leaves 

 and twigs. The artichoke and a small oval gall on the 

 catkins both belong to A. gemmae. Biorhiza aptera, which 

 forms the many- chambered oak-apple, also occasions one- 

 chambered galls occurring in clusters on the rootlets of the 

 tree. The cherry, silk-button, and several other oak-galls are 

 also dimorphic. 



Only double-brooded gall-flies are known to possess males ; 

 these in some cases seem to be developed in separate galls 

 from the females, and they rarely survive the winter. Single- 

 brooded species are described as agamic ; they have no known 

 males, and it is doubtful if any exist ; the sexual generation, 

 if there ever was one, has ceased to be produced. There is, 

 for example, no spring form corresponding to the marble gall. 

 Among 12,000 specimens of Cynips Kollari, reared from one 

 and a half bushels of these galls, not a single male could be 

 discovered. This insect is perpetuated from year to year 

 without males, and it is tolerably certain that the species 

 consists entirely of females. We have thus an interesting 

 parallel to the phenomenon of apogamy observed in certain 

 ferns, where the prothallus or egg- bearing generation is 

 dropped out of the plants' life-history. 



Dimorphism, so far as is known, is confined to the Cyni- 

 pidse ; parthenogenesis, or the production of offspring by virgin 

 mothers, is, however, fairly common in several groups of in- 

 sects. Most of them are incapable of reproduction until the 

 perfect or winged condition is attained : the Cecidomyias are, 

 however, an exception ; the larva has the power of multiplying 

 asexually by a process termed psedogenesis. Several new 

 individuals appear in the interior of the grub as the result of 

 internal budding, and are developed at the expense of their 

 parent. Hence it may come about that from a gall which 

 originally contained but one egg, five or six gall-midges may 

 ultimately emerge. 



Alternate generations are also characteristic of the Aphides ; 

 in this case six or seven agamic generations may intervene 



