68 
BRITISH GALLS 
and pupate in the earth. The Yellow Bedstraw occurs 
throughout Britain, but Perrisia galii is by no means widely 
distributed, apparently occurring chiefly in the south and 
east. It attacks various species of Bedstraw on the Con¬ 
tinent, but I am not acquainted with British records. 
The inflorescence and the young shoots of the Common 
Lime are frequently attacked by Contavinia tilianm. The 
gall takes the form of an elongated or rounded tumour, 
sometimes io mm. in diameter, green at first, becoming 
reddish (Plate VIII., Fig. 7). It contains numerous larvae 
of a sulphur-yellow hue (see magnified section, Fig. 8). 
Other members of the genus attack various plants. There 
are fifteen recorded British species of gall-causing Contavinia , 
exactly one-quarter of the number known in Europe. 
A much higher percentage of the members of the genus 
Oligotvophus are gall-causers in this country. Houard 
records nineteen Continental species. Eleven occur in 
Britain, causing galls of very attractive appearance and of 
great interest. Perhaps the commonest of all is the 
hypertrophied bud of the Yew, resulting from the presence 
of the orange-coloured larvae of Oligotvophus taxi. The gall 
consists of a mass of adpressed leaves surrounding a whitish, 
fleshy central part. It is usually terminal. Late in May 
many of the galls contain pupae; these are orange- 
coloured at first, but gradually become darker. The leaves 
are then erect and nearly straight. Early in June the fly 
emerges. (It is an orange-coloured creature, 5 mm. 
long, and not more than 9 mm. in expanse of wing.) 
The apical leaves then curve and twist, so perceptibly 
altering the appearance of the gall that it is then quite 
easy to distinguish those containing pupae from the empty 
ones. It is doubtful if this change is always delayed until 
the fly has emerged, for I have found flies crushed between 
the leaves at the mouth of the gall. They are so minute 
and delicate that it is not to be marvelled at that many 
meet an untimely death in their compulsory journey to the 
outer world through this dangerous channel of moving leaves. 
