GALLS CAUSED BY FLIES 
6 3 
Mel. T. Cooke, who makes a special study of the galls, 
and he advises me as follows: ‘ There is but one species of 
gall reported on Taxodium distichum , and I have specimens 
of that species. It is entirely different from the one you 
send me. The gall which you send is of insect origin 
without doubt, and apparently belongs to the genus 
Cecidomyia.’ ” 
One of the commonest of British galls is caused by the 
presence of the larvae of Perrisia ulmariae on the leaves of 
the Meadow Sweet. Wherever this plant occurs—it ranges 
throughout the British Isles and Europe—the observer will 
find the galls in abundance during the summer. The fly 
attacks the leaf in late spring, and the galls appear soon 
after as small, glabrous, light green, umbonate pustules on 
the upper surface, with whitish projections on the lower 
one. They are usually situated on the midrib or the larger 
lateral veins, and are often densely gregarious, causing the 
leaf to pucker, but not otherwise producing very marked 
distortion. Over 200 galls have been counted on one leaf. 
Later, the gall assumes a reddish-brown or pinkish tint 
on the upper surface, the lower remaining greenish-white. 
The umbo vanishes from the upper part, and the lower 
assumes the form of an elongated cone covered with a felt 
of whitish hairs. The cavity is somewhat triangular. The 
larva is yellowish-orange. It pupates in the point of the 
cone. A circular separating line is formed in the tissue, to 
enable the cap to come away easily when the fly emerges. 
This is easily demonstrated; by seizing the point of the cone 
with forceps it comes away quickly, and always in exactly 
the same way. The larva frequently bears an external 
parasite, a minute, active, hyaline, worm-like creature. 
Perrisia ulmariae also attacks Spiraea filipendula. It is 
noteworthy that the galls on this plant differ very markedly 
from those on S. ulmariae; the truncated cone opens on to 
the upper surface of the leaf, the pustule being on the 
inferior one. As a rule the galls caused by any species of 
insect on closely allied plants present very slight differences 
