66 
BRITISH GALLS 
filicina. The roll is cigar-shaped and shining black at 
maturity; it contains a single pale orange-yellow larva, 
which pupates in the earth. This gall has a wide range 
in Europe and the British Isles. An allied species, Pervisia 
pteridicola KiefFer, causes a similar gall on Bracken in 
Germany and Central Europe, but has not as yet been 
observed in this country. The hypertrophy is feebler; the 
larvae are gregarious and colourless. 
The larvae of Pervisia tenninalis, perhaps better known to 
English dipterists under the name of Dasyneura tenninalis , 
cause swollen brown galls in the apices of the shoots of the 
White Willow and the Crack Willow (Plate VIII., Fig. 4). 
As many as thirty of the reddish larvae may be found in a 
single gall, which is formed by the rolling together of the 
terminal leaves, which also become thickened and brown. 
The larvae pupate either in the gall or in the earth. 
Fig. 6 gives a magnified view of a pupa from the gall 
shown above it, and Fig. 5 the elegant little fly, highly 
magnified, the expanded wings of the insect measuring only 
inch from tip to tip. This gall is frequent wherever 
Salix fvagilis occurs. The illustrations are from specimens 
I gathered in a garden in Gower Street, London, in July, 
1910. 
Pervisia marginem-tovquens causes the margins of the leaves 
of the Osier and other Willows to become more or less 
tightly rolled inwards. The roll often extends the entire 
length of the leaf, and consists of an aggregation of little 
yellow or reddish pockets, each about 3 mm. long, and each 
containing a single larva. The gall of P. Inchbaldiana is 
similar, but is bent like a bow and smaller at each end. 
Though usually gregarious, these galls are rarely coalescent, 
and the margin is never continuously rolled. 
Other well-known galls on Willows are caused by gall- 
gnats of the genus Rhabdophaga. R. saliciperda causes 
hypertrophy of woody tissue on branches up to 4 inches in 
diameter. When numerous, as they often are, these galls 
collectively form an elongated spindle-shaped (fusiform) 
