VEGETABLE GALLS 



759 



"BEDEGUAR," OR "ROBINS PINCUSHION" 

 OF ROSE. 



the most perplexing of all insects from 

 the standpoint of the student. Quite a 

 number of kinds, especially of the 

 numerous species which are associated 

 with the oak tree, have two alternating 

 generations in the course of each tweh'e 

 months — each generation differing from 

 the one which preceded it, and emerging 

 from a different kind of gall. Moreover, 

 one of these generations is agamic [i.e. 

 made up of solitary females capable of 

 virgin reproduction), while the 

 other is sexual, comprising 

 both males and females. These 

 facts were not known to the 

 early naturalists, who gave the 

 insect which emerged from each 

 kind of gall a different name. 

 For example, the wingless 

 agamic female which is bred in 

 the small root galls of the oak 

 was called Biorhiz.i aptsra, while 

 her direct offspring (the flies 

 which emerge from the '■ oak 

 apple") had the name Biorhiza 

 t^rminalis given to them. In 

 order to avoid hea])ing confu- 

 sion upon confusion, these 

 separate names are still retained 

 by most writers ; but until the 



tyro fiiUv grasps the facts of the case. 

 this use of one name for the " mother " 

 and another name for the " cliild " is apt 

 to be very perplexing. Further, the fact 

 that the phenomenon of alternating 

 generations is not exhibited by all Gall- 

 flies docs not tend to simplify matters. 

 Quite a number of species have only one 

 generation, or brood, in the course of each 

 year, and in such cases the life cycle is 

 usual!}' quite in accord with that of an 

 ordinary insect. Both sexes are repre- 

 sented, the offspring resemble their 

 parents, and only one kind of gall is pro- 

 duced from year to year. 



As examples of these " one generation " 

 Gall-flies I ma}' mention three kinds 

 whose galls are commonly found upon the 

 leaves of wild-rose bushes. These galls 

 differ much in appearance, and this is 

 probably because the grubs attack the 

 cell tissue in distinct ways. First there 

 is the familiar " bedegiiar " or " robin's 

 pincushion," formed by an insect known 

 as Rhodites rosa. This gall is a kind of 

 community, being made up of a number of 

 cells, each of which contains a grub. 

 It often appears to grow from a twig or 

 stem, but this is apparent only, and 

 consequent upon its great development. 

 Actually, it invariably originates in a leaf, 

 or in leaves — the insect ovipositing in a 

 leaf-bud. The number of cells in these 

 galls varies greatly. Sometimes there 

 are as few as three ; while in the case of 

 a very large specimen there may be as 

 manv as fortv or fift\'. The mass of 



S^XTION 

 ROSE 



THKOUOII A SMAI.l. " BKDKGU AR " GAII. OV 

 BUSH. SHOWING GKIBS IN THEIR CHAMBERS. 

 (Madnjfiid ) 



