90 EPPING FOREST
insects is a fascinating subject, and one that opens
out into quite a new field of observation for the
entomological microscopist.
Still lingering with the Hymenoptera, we may
well suppose that the whole range of zoology hardly
affords a more wonderful subject for study than
the life-histories of some of our common Oak-gall
Flies (Cynipidae). Galls are common enough in
the forest, and the appearance of an oak tree with
its leaves covered with the handsome cherry-like
galls of Dryophanta scutellaris, or later in the
summer with " Oak-spangles " (Neuroterus lenticu-
laris) and " Silky-button " galls (N. numismatus),
is sure to attract the attention of even the careless
visitor. The " Oak Apple " gall (Teras terminalis)
is sometimes extremely abundant, giving the oaks
in May or June the appearance of trees in full
fruit, and near Loughton the remarkable woolly
galls of Andricus ramuli are often to be seen,
looking like tufts of cotton caught on the twigs.
Oak-galls result from the deposition of an egg
or eggs by the parent fly in the tissues of the
developing bud or leaf. As the young grub grows,
possibly some irritating secretion acts upon the
tissues of the plant exciting to abnormal growth
and excessive cell-division; but how mysterious is
it that each particular species or generation of fly
should on the same tree produce its own particular
gall, often differing widely from those of allied
species. And a still more wonderful tale anent gall-
flies is that told by entomologists under the title
of "Alternation of Generations." This principle
will be best explained by an instance of it. The
" Oak-spangle " gall (known as Neuroterus lenticu-
laris) produces, in April, flies which are all females ;
these lay eggs in the buds of oaks, and, as the
result, in May are to be seen the common currant-
like galls either on the pendulous catkins or on
the lower surface of the leaves. The flies from
these latter galls are of two sexes, and they and
the galls are known as Spathegaster baccarum.
The female lays eggs in the oak leaves by piercing
the cuticle with her ovipositor, so that in July is