ON BRITISH OAKS. 29 
greenish-yellow colour, with tufts of red-brown hairs, and some- 
times cover the entire underside of the leaf. They were very 
numerous in 1886. ‘The insects lie dormant during the winter, of 
course falling with the leaf. They appear in the winged form in 
April or May. 
The Button Gall (Spathegaster vesivatrix) is formed by a small 
species of about one-tenth of an inch in length, both sexes of 
which are produced, The female deposits her eggs on the under- 
side of the leaves in June, and the galls are about one-tenth of an 
inch in diameter. They resemble a small button covered with 
fine threads, which under the microscope are very beautiful. The 
flies do not emerge from the galls until spring, when only females 
are produced. These attack the undersides of the leaves, which 
causes small galls, somewhat resembling the oak spangles. 
The Currant Gall (Neuroterus lenticularis).—Vhis fly is about 
one-eighth of an inch long, and of a reddish-brown colour. The 
females deposit their eggs in the buds containing the male flowers, 
and also on the undersides of the leaves. The galls when mature 
are perfectly globular, of a transparent green, speckled with red, 
They are of a soft consistency, with a considerable hollow space 
in the centre, in which is the grub. When formed on the male 
flowers they resemble a bunch of currants. The flies emerge from 
the galls in June. 
Such is an account of the more common of the galls to be found 
on the oak in Britain; but what the effect is on the growth of the 
tree it would be difficult to say, as they have occurred in most 
years for ages past, with perhaps the exception of the marble gall, 
which is stated to be a more recent introduction. 
We now come to a more serious class of insect pests, namely, 
the caterpillars, chief among which is the Tortrix viridana, or 
oak-leaf roller. This caterpillar attacks the leaves, and has 
assumed alarming proportions at intervals of years apart, such as 
in 1831, 1848, 1864, and 1881. This gives a period of about 
seventeen years between each attack. I particularly noticed the 
latter years, but in 1888 their ravages were the most disastrous of 
any. On the lst of June in that year, a bright sunny day, I 
entered an oak wood, and, after proceeding a short distance, became 
conscious of being enveloped in cobwebs. On discovering the cause, 
so thick were the caterpillars hanging by their threads all around 
me, that I determined to return, and although the distance was but 
short, I emerged literally covered with them. During that month 
