GALLS CAUSED BY FLIES 71 



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 firinge 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 OUgotrofhus 

 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 191 1. 



An equally rare gall is caused by Oligotropkus Leemei on 

 leaves of the Wych Elm (Plate XXIIL). 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 b 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 tbAt the empty galls CQuld. be easily recog;- 



