GALLS CAUSED BY FLIES 67 



swelling. The bark ultimately falls away, exposing the 

 little holes which had been occupied by the larvae. They 

 must not be confused with the galls caused by R. salicis, 

 which are pronounced fusiform or rounded swellings, usually 

 on one-year twigs (Plate IX., Fig. 10) of various Willows ; 

 the larvae bore into the pith (Fig. 11). There are excellent 

 illustrations of them in Connold's "Vegetable Galls," 

 Plates XXVI., XXVII., XXVIII. ; unfortunately, XXVI. 

 and XXVII. are erroneously attributed to R. saliciperda. 



The familiar rosette galls, consisting of bunches of leaves 

 at the apex of shoots of many Willows, are caused by 

 Rhabdophaga rosaria. Another rosette gall of Willows is 

 caused by R. heteroUa ; it may be known from the preceding 



»4 15 



Fig. 14 — Stem of Yellow Bedstraw with Galls caused by the 



PRESENCE OF THE Larvae OF Ptrrisia golii. (1/2.) 

 Fig. 15 — Section of a Gall, (i/i.) 



by the white pubescence on the leaves. This gnat also 

 attacks the male catkins of S. triandra, causing the filaments 

 of the stamens and the scales -to become thickened and 

 covered with a white woolly pilosity. Theobald observes 

 that the Willow flies live only twenty-four hours, and are 

 attacked by several parasites. 



The attacks of Perrisia galii cause large hypertrophies on 

 the flower stalks and terminal part of the stem of the 

 Yellow or Ladies Bedstraw. Sometimes the diameter of a 

 gall is ten or twelve times that of the stem on which it is 

 produced. Each gall may contain but a single cell ; usually 

 three or four are present. The larvae are orange-yellow, 



