OF EPPING FOREST 271
Gall variation.
As before mentioned galls of different generations of the
same species often occur on different parts of the oak besides
varying in shape and colour, etc. Galls of the agamic genera-
tion, Dryophania folti, the common Cherry-galls of the oak, occur
on the back of the leaf while the corresponding galls of the
sexual generation, D. taschenbergi, usually arise from adventitious
buds situated on the trunk of the tree. The former when
mature are large, round, yellowish galls, often with a red cheek,
the latter are very small velvety galls, purplish in colour, hardly
noticeable against the bark.
The great difference between the galls as they occur on the
oak, and the similarity existing between the insects which
form them, is very marked. The larvae are fleshy, legless grubs,
usually while or yellowish in colour. The perfect insect is, as a
rule, brown or black, and it is almost impossible to find sufficient
variation to differentiate between the species in some cases.
Those agamic forms having only one generation a year, such as
Andricus quadrilineatus and A. seminationis, are practically identical,
and yet the galls are very different. This may be accounted for
by the fact that the larva is surrounded by the gall tissue
throughout its whole life, and is never exposed to the light, so
cannot be acted upon in any way by external influences, and
consequently varies but little. The perfect insects live only a
short time and die as soon as the eggs are laid. The galls on
the other hand serve to protect the larvae from parasites, and
are, therefore, greatly modified by external conditions.
Inquilines and Parasites.
The number of parasites and inquilines preying on the gall-
makers and living in their galls is enormous. They are of three
kinds:—
Inquilines or "guest-flies" which live in the thickening of
the gall substance, doing no harm to the hosts.
Inquilines which do not directly prey upon the host but
starve it out and take up the extra room thus obtained for their
own benefit (including certain Synergidas in the galls of Cynips
kollari.)
Direct parasites preying on the larvae, such as Chalcididae,
Ichneumonidae, and Braconidae.