24 FOOD PLANTS. 



How we are to account for the origin of the saw- 

 fly galls is a question difficult of explanation. There 

 can be here only three factors : (1) the wound made 

 by the saw ; (2) the growth — the swelling up — of the 

 egg ; and (3) the venom ejected by the fly. Beyerinck, 

 it may be added, adopts the latter theory for the saw- 

 fly galls. 



So far as my observations go, I do not find any 

 marked difference in the mode of oviposition of the 

 gall and non-gall making saw-flies. I have noticed 

 with some of the latter incipient gall-formation fol- 

 lowing oviposition. No doubt the distinction between 

 the two lies in the fact that the former brings its eggs 

 in contact with the cambium layer, the latter not. 



The larval irritation theory may further be sup- 

 ported by the observation of the distortion and enlarge- 

 ments caused in galls by the inquiline flies. 



Further information on this matter may be had in 

 the above-mentioned memoir of Lacaze-Duthiers, 

 Recherches pour servir a 1 'histoire des galles, 1. c. ; 

 by M. W. Beyerinck, Beitrage tot de Morphologie 

 der Plantengallen, 1877, and in his Beobachtungen 

 iiber die ersten Entwickelungsphasen einiger Cynipi- 

 dengallen, 1882; in Frank's Die Krankheiten der 

 Pflanzen, Breslau, 1881; Meyer, Pflanzen-Pathologie, 

 1841 ; Courchet, Etude sur les Galles causees par 

 les Aphidiens, Acad. d. Sciences et Lettres de 

 Montpellier, 1881 ; and Hollis, Trans. Linn. Soc, 

 1875, who gives an admirable historical resume of the 

 subject. 



Food Plants. 



Apart from the oak, the food plants of the European 

 Cynipidse are — 



Rosa canina, eglanteria, rubiginosa, spinosissima, &c, 



attacked by the various 

 species of Rhodites. 



Rubus fruticosus, &c, by Diastrophus rubi. 



