GALL-BEETLES. 391 



surface. The mountain-ash (Pyrus auciiparia) has 

 its leaves and young shoots frequently affected in 

 this way, and sometimes exhibits galls larger than a 

 walnut, or even than a man's fist: at other times 

 they do not grow larger than a filbert. Upon open- 

 ing one of these, they are found to be filled with the 

 aphides sorbi. If taken at an early stage of their 

 growth, they are found open on the under side of the 

 leaf, and inhabited only by a single female aphis, 

 pregnant with a numerous family of young. In a 

 short time, the aperture becomes closed, in conse- 

 quence of the insect making repeated punctures 

 round its edge, from which sap is exuded, and 

 forms an additional portion of the walls of the cell. 



■^^. 



A Plant Louse fAphisj, magnified. 



In this early stage of its growth, however, the gall 

 does not, like the galls of the cynips, increase very 

 much in dimensions. It is after the increase of the 

 inhabitants by the young brood that it grows with 

 considerable rapidity; for each additional insect in 

 order to procure food, has to puncture the wall of the 

 chamber and suck the juices, and from the punctures 

 thus made the sap exudes, and enlarges the walls. 

 As those galls are closed all round in the more ad- 

 vanced state, it does not appear how the insects can 

 ever effect an exit from their imprisonment. 



A much more common production, allied to the 



