Galls by Gall-Flics. 99 



times by Lubbock \ among others; but as early as 

 1873 Professor Thomas, of Ohrdruflf, objected to this 

 explanation of it, and after numerous experiments, 

 I am convinced that the simple prick of the gall-fly 

 does not set up gall formation ; this, I hold, only begins 

 when the larva emerges from the ^gg. 



But it is to be observed that in stating this view 

 I limit it to oak gall-flies ; there do exist gall-producing 

 flies in another class of hymenoptera, which produce 

 their galls in the way just mentioned. I have carefully 

 observed this in Nematus Vallisnierii. This fly, which 

 is armed with a finely serrated terebra, cuts into the 

 tender leaves of the end shoot of the Salix amygdalina, 

 and inserts her ^gg into the open wound, frequently 

 placing several in the same leaf. At the same time 

 some glandular secretion flows into the wounded leaf. 

 A few hours after this injury the leaf surface presents 

 an altered appearance, and new cell formation begins 

 freely, leading to a thickening of the surrounding 

 leaf surface. After the lapse of about fourteen days 

 the green and red bean-shaped gall is fully grown. If 

 it be now opened the egg can still be seen lying 

 within the cavity. The embryonic development is yet 

 unfinished, and three weeks elapse before the larva 

 emerges from the egg, to find around it the material 

 prepared for its nutriment. In this case the wound 

 caused by the fly is the immediate exciting cause of 

 cell activity, and leads to gall formation. In the case 

 of Cecidomyidae, belonging to another division of gall- 



' Sir J. Lubbock. Origin and Metamorphosis of Insects, p. 8, 

 1876. 



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