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. hetevobia; it may be known from the preceding 
Fig. 14—Stem of Yellow Bedstraw with Galls caused by the 
presence of the Larvae of Perrisia gcdii. (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 5 . tviandra , 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, 
