GALL-BEETLES. 341 



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. 



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 

 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 small snow-white tuft of downj^-looking substance floating 

 about on the wind, as if animated. Those tufts of snow- 

 white down are never seen in numbers at the same time, 

 but generally single, though some dozens of them may be 

 observed in the course of one day. This singular object is 

 a four- winged fly (^Eriosoma populi, Leach), whose body is 

 thickly covered with long down — a covering which seems 

 to impede its flight, and make it appear more like an inani- 

 mate substance floating about on the widd, than impelled 

 by the volition of a living animal. This pretty fly feeds 

 upon the fresh juices of the black poplar, preferring that of 

 the leaves and leaf-stalks, which it j)unctures for this 

 purpose with its beak. It fixes itself with this design to a 

 suitable place upon the principal nervure of the leaf, or 

 upon the leaf-stalk, and remains in the same spot till the 

 sap, exuding through the punctures, and thickening by 

 contact with the air, surrounds it with a thick fleshy wall of 

 living vegetable substance, intermediate in texture between 

 the wood and the leaf, being softer than the former and 



