GALLS CAUSED BY FLIES 
7 i 
5 mm. in diameter—absence of hairs, and the thick and very 
hard walls. They are of rare occurrence in Britain. On the 
inferior surface of the leaf there is a circular pustule 
surrounded with a fringe of hairs. These galls have been 
detected on fossil leaves of Fagus pliocenica Saporta. 
Plate XXX. depicts galls on leaves of the Mealy Guelder 
Rose, caused by the presence of the larvae of Oligotrophus 
Solmsii. The pustules are lenticular, more or less thickly 
scattered over the leaf (never truly coalescent), and are 
about 5 mm. in diameter. Green at first, they soon become 
red, and are dark purple at maturity. The gall shows on 
the lower surface of the leaf as a thin circular whitish disc 
covered with minute hairs (see the lowest leaves in the 
illustration). Each gall contains a yellowish larva, which 
pupates in the earth. It is rare in Britain ; its first record 
dates from June 12, 1904, when I found it on the Downs 
near Maidstone. Miss Spittal informs me that it was very 
plentiful about Winchester in the summer of 1911. 
An equally rare gall is caused by Oligotrophus Leemei on 
leaves of the Wych Elm (Plate XXIII.). It occurred in 
great abundance in the large wood above Weston-super- 
Mare in June, 1910. Usually the midrib and the larger 
lateral veins bear the galls, but the petiole is occasionally 
involved, becoming greatly hypertrophied. Not infrequently 
the leaf is pressed backwards against the twig from which 
it grows. Much distortion results when young and unfolding 
leaves are attacked (Plate XXIII,. c ). On the lateral veins 
the gall usually consists of a rounded yellowish swelling 
below, with the aperture above (at h is shown the upper 
surface of a leaf studded with little pin-like holes, which are 
the orifices from which the larvae have emerged). When 
the midrib is attacked, the openings are for the most part 
on the under surface of the leaf, and are lateral ( c ). The gall 
is very hard. The larval chambers may be easily made 
out with a pocket-lens; each contains an active yellowish 
larva. On June 26 I noticed that many larvae had left 
to pupate, and that the empty galls could be easily recog- 
