Galls by Gall- Flies. loi 



an ^gg has been inserted. Guided by these marks, it 

 is easy to find the eggs, but it is not until September 

 that the larva leaves the tgg, and then gall formation 

 begins. 



It would naturally be interesting to observe the 

 exact time when the larva quits the tgg and starts gall 

 formation ; but this is very difficult to do. Whether 

 the Qgg be enclosed within a bud or a leaf, it is 

 withdrawn from observation, and it is difficult to hit 

 off the exact time when the larva is in the act of 

 bursting the tgg. I have several times however been 

 successful in observing this stage in Neuroterus 

 laeviiisculus and Biorhiza aptera. The moment the 

 larva has broken through the ^gg covering, and has, 

 for the first time, wounded the surrounding cells with 

 its delicate mandibles, a rapid cell-growth begins. This 

 goes on so quickly, that, while the posterior part of the 

 larva is still within the egg-covering, a wall-like growth 

 of cells has already arisen in front. This rapid cell 

 increase can easily be explained, because the irritation 

 set up by the emerging larva is exerted upon highly 

 formative cells, which collectively possess every con- 

 dition for growth. The cells which are primarily 

 around the larva cannot be distinguished from the 

 parenchymatous cells from which they proceed. 



In every gall formation this holds good, and it will be 

 found that the first effect is only an increase of cell 

 formation in the part. A gall is not to be considered 

 as a parasitic growth in the surrounding tissue, but as 

 consisting of the same elements as that tissue, and as 

 substituting itself for them. Therefore the growth of 



