278
OAK GALLS AND GALL INSECTS
as the cambium ring is only represented by a thin layer at the
base of the bud ; consequently in many cases no gall is formed
because the eggs have not been placed exactly in this growing
zone. The cell growth after the emergence of the larva is some-
times very rapid, in some cases it commences before the larva
is quite free from the egg. In the case of Trigonaspis renum a
period of months elapses after the egg has been laid before gall
formation commences. The probable cause of the differences in
size and form, etc., existing between various galls has already
been referred to, but besides having some property in their saliva
which acts on the surrounding tissue, the different ways in
which the larvae feed may also have some important bearing on
the case.
The general characteristics of the Family Cynipidae.
Antennae twelve to fifteen jointed, never elbowed.
Legs have the trochanters biarticulate.
Wings without a stigma. One radial cellule. One to three
cubital cellules. Submarginal nervure absent.
Prothorax reaching at the sides to the insertion of the wings.
Abdomen pedunculated and compressed.
No cocoon spun by the larvae, which are apodal.
Eggs stalked.
The Cynipidae are divided into five Sub-Families :—
(1.) The Ibalina, containing a single genus Ibalia including
one species, which is parasitic on Sirex.
(2.) Allotrina, containing two genera, Phaenoglyphis and
Allotria, parasitic on Aphides or on the parasites of Aphides.
(3.) Encoelina, containing genera which include species
parasitic on larvae of various insects, especially Diptera, such
as Musca, Eristalis, Tachina, etc.
(4.) Figitina. This sub-family is divided into three
sections, Anacharides, Onychides and Figitides. The Onychides
are chiefly parasitic on Beetle and Syrphus larvae, while the
Figitides comprises genera (such as Figites) which contain species
parasitic chiefly upon Diptera, mostly dung-flies of the family
Sarcophagidae.
I have mentioned the above sub-families of the Cynipidae
merely to show how very generally parasitic the members of the