66 BRITISH OAK GALLS. 
other are developing, or have arrived at maturity, that 
the twigs give evidence of their presence. 
Solitary galls, or several scattered along an ordinary 
slender twig, are difficult to locate. This may be 
understood by referring to A, Plate XII. 
The galls are much more abundant in twigs than in 
leaf petioles. 
This is a most obscure gall, and difficult for the 
uninitiated gall collector to find. It may usually be 
found by peeling off the bark of shoots near where 
galls of Aphilothrix radicis have grown. It occurs 
only in shoots of the year, or in‘a leaf petiole. 
Its situation is beneath the bark, more or less 
embedded in the xylem, and is therefore completely 
hidden from view. Its presence in a shoot can only 
be conjectured by a very slight swelling of the bark 
immediately above where the gall is situated. 
The gregarious habit does not furnish any additional - 
indication. In a leaf petiole, however, considerable 
thickening is produced, and the galls are usually more 
numerous there than in a shoot. ° 
Mayr says: “On Quercus pubescens the swelling 
occasioned by this gall is still less perceptible on 
account of the tomentum on the surface, and sometimes 
it (the swelling) is entirely absent.” 
After the imago has emerged the swollen part of the 
bark subsides, but the minute circular exit-hole betrays 
the position of the empty gall. 
I have found that this gall occurs more frequently 
on stub-oaks, young trées, and bushes in hedges than 
on large trees. 
The gall is widely distributed throughout Britain, 
and in places very plentiful, a fact accounted for by 
the vast number of imagines from an average-sized 
gall of A. radicis. 
