416 Insect Architecture. 



others are in form of an inflated vesicle, witli a narrow 

 opening on the under side of a leaf, and expanding (for the 

 most part irregularly) into a rounded knob on its upper 

 surface. The mountain-ash (Pyrus aucuparia) has its leaves 

 and young shoots frequently affected in this way, and some- 

 times 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 opening 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 consequence 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 (Aphis), magnified. 



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

 not, liks 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 suek 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 

 advanced state, it does not appear how the insects can ever 

 effect an exit from their imprisonment. 



A much more common production, allied to the one just 

 described, may be found on the poplar in June and July. 

 Most of our readers may have observed, about midsummer, a 



