xxni GALL-FLIES 531 



mother, and was previously known to entomologists under the 

 name of Chilaspis nitida. Only females of it occur, and these 

 parthenogenetic individuals lay their eggs in the young buds of the 

 oak that are already present in the autumn, and in the following 

 spring, when the buds open and the leaves develop, those that 

 have had an egg laid in them produce a gall from which Chilaspis 

 lowii emerges in April or May. In this case therefore the cycle 

 of the two generations extends over two years, the generation 

 that takes the greater part of the time for its production con- 

 sisting only of females. Adler's observations showed that, though 

 in some species this alternation of generations was accompanied 

 by parthenogenesis in one part of the cycle, yet in other species 

 this was not the case. He found, for instance, that some gall- 

 flies of the genus Aphilotlirix produced a series of generations 

 the individuals of which were similar to one another, and were 

 all females and parthenogenetic. In some species of the old 

 genus Cynips 110 males are even yet known to occur. A very 

 curious observation was made by the American, "Walsh, viz. that 

 of galls gathered by him quite similar to one another, some pro- 

 duced speedily a number of both sexes of Cynips spongifica, while 

 much later on in the season the remainder of the galls gave rise 

 to females only of an Insect called Cynips aciculata. It is be- 

 lieved that the galls gathered by Walsh x were really all one 

 species ; so that parts of the same generation emerge at different 

 times and in two distinct forms, one of them parthenogenetic, the 

 other consisting of two sexes. It has, however, been suggested 

 that Cynips spongifica and C. aciculata may be two distinct species, 

 producing quite similar galls. 



Turning now to the questions connected with inquiline or 

 guest-flies, we may commence with drawing attention to the 

 great practical difficulties that surround the investigation of 

 this subject. If we open a number of specimens of any kind of 

 gall it is probable that several kinds of larvae will be found. 

 In Fig. 350 we represent four kinds of larvae that were taken 

 out of a few bedeguar galls gathered on one day in a lane near 

 Cambridge. It is pretty certain that No. 1 in this figure repre- 

 sents the larva of Rliodites rosae, and that Nos. 2 and 3 are 

 larvae of inquilines, possibly of Synergus, or of a parasite ; while 

 No. 4, which was engaged in feeding on No. 3 in the position 



1 P. cntom. Soc. Philadelphia, ii. 1864, pp. 447, etc. 



