THE ENTOMOLOGIST IN THE FOREST 91
reproduced the " Oak-spangle " gall from which we
started, and the cycle is complete. This alterna-
tion of asexual with sexual generations, and of
strikingly different gall-formation, has already been
traced in twenty or thirty species—our common
Oak-apple gall (flies, male and female) alternates
with a hard root-gall called Biorhiza aptera (females
only) ; and the pretty cherry-like gall of the oak-leaf
(alluded to above) is the agamous form of a little
gall which is found in April on the undeveloped
buds of the oak (known as Spathegaster taschenbergi
(two sexes). A few gall-flies are only known in
the agamous stage; for instance the Marble-gall
of the oak (Cynips kollari), so common in the
forest. Tens of thousands of the flies have been
bred from these galls, but all females; no one has
yet recorded a male !
We hope that some of our readers will take up
the study of galls; the observation of their life-
histories will provide an admirable lesson in
biology; it is intended shortly to place in the
Forest Museum at Chingford a series illustrative
of most of the forest forms, for the information of
students.
But the Cynipids are not the only gall-makers;
in fact all orders of insects contain species which
produce galls, and many more are formed by gall-
mites (Phytoptidae, Order Arachnida) which are
not insects in the zoological sense. One of these
is very common on the sloe, thickening and dis-
torting the margins of the leaves {Phytoptus
attenuatus); another occurs on Field Maple in the
hedgerows, producing numerous little red pimples
on the leaves (P. myriadenum); and the curious
"Witches' Broom," so common on the hornbeams
and birches, is an abnormal growth initiated
by the attacks on the buds of a species of
Phytoptus. The Diptera (two-winged flies) furnish
several gall-making genera, particularly in the
family Cecidomyidae or Gall-gnats—tiny creatures,
and yet a destructive race, for to the family
belong the dreaded wheat-midge, and many
species which distort leaves and flowers. A