72 
BRITISH GALLS 
nized by the brown tint around the orifice. The change 
of colour is perceptible within a few hours of the departure 
of the larva. Connold received these galls from Worksop 
in 1909, the first British record ; his photograph is here 
reproduced. 
The galls caused by Rhopalomyia millefolii on the Common 
Milfoil are probably not so uncommon as was at one time 
supposed. They are usually situated at the top of the root 
on a level with the earth, a position which makes them very 
inconspicuous (Plate XXXI.) ; sometimes the leaves are 
attacked, and even the flowers. They are usually coales- 
cent when growing at the base of the stem. This gall is an 
elegant structure about the size and shape of a hemp-seed, 
green at first, becoming reddish-purple, and finally black. 
The opening is stellate, with four or five lobes ; the interior 
contains a single yellow larva. To breed the flies, old galls 
should be obtained in May, and kept in test-tubes or glass- 
topped boxes. 
Galls caused by Cystiphora sonclii on the radical leaves of the 
Common Sow Thistle are shown in Plate XXXII. Sonchus 
arvensis is of wide distribution in Britain, but its gall is rare, 
being recorded only from the south of England. The 
pustules are evident on both surfaces of the leaf, but less so 
on the lower one. They are purple above, greenish below, 
and average about 4 mm. in diameter. Usually each gall 
contains a white larva, but Connold describes it as contain¬ 
ing two. The right-hand figure in the plate is the lower 
surface of a leaf with numerous galls; some of them show 
plainly the orifice from which the insect has emerged. 
Two species of Macrodiplosis attack Oak leaves, causing 
localized folding of the margin. The galls are very different, 
though not infrequently confused. They appear in June. 
Those depicted on Plate VIII., Figs. 9 and 10, are caused by 
M. dryobia. Some of the lobes are folded downwards until 
they touch the inferior surface ; between the folds one to six 
whitish larvae may be found. The surface above the fold is 
tinted red; thus, the gall is easily recognizable (Fig. 10). 
