392 INSECT ARCHITECTURE. 



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 ob- 

 served, about Midsummer, a small snow-white tuft 

 of downy-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 sin- 

 gular object is a four-winged fly (Eriosonia 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 inanimate substance 

 floating about on the wind, 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 punctures 

 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 re- 

 mains 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 harder than the latter. In this snug little cham- 

 ber, secure from the intrusion of lady-birds and the 

 grubs of aphidivorous flies (Syrpki), she brings 

 forth her numerous brood of young ones, who imme- 

 diately assist in enlarging the extent of their dwelling, 

 by puncturing the walls. In one respect, however, 

 the galls thus formed differ from those of the moun- 

 tain-ash just described, — those of the poplar having 

 always an opening left into some part of the cell, and 

 usually in that portion of it which is elongated into 

 an obtuse beak. From this opening the young, when 

 arrived at the winged state, make their exit, to form 



