268
OAK GALLS AND GALL INSECTS
stalks of the cabbage, belongs to the order Coleoptera (beetles).
In the order Diptera (true flies) gall-making species are found
chiefly, but not entirely, in the family Cecidomyidae, an example
being the hairy nail-gall, Hormomyia piliger a, found on the upper
sides of the leaves of the beech. In the order Hemiptera we have
the family Aphidae including such species as Chermes abietis, which
causes the common but remarkable cone-like gall on the Norway
spruce, and Schizoneura ulmi which forms a gall on the elm.
The order Hymenoptera includes the family Tenthredinidae
(sawflies) some of which form well known galls on the leaves of
various willows. To this order also belong the Cynipidae,
which produce galls on the rose and other plants, but chiefly
upon the oak. There are many other gall-makers besides those
which I have enumerated, but in the following record I am deal-
ing only with those Cynipids which form galls upon the oak.
Cause of the formation of Oak-Galk.
A gall is a peculiar growth of the tissue of a plant due'to
irritation set up by some internal animal agency. This irritation
acts upon the cambium layer of the plant in which the gall-
maker lays its egg. The abnormal formation is not due to any
poison injected by the insect at the time of oviposition, but the
egg must be laid in the cambium ring, or growing meristamatic
tissue of the plant, in order to form a gall. Morbid growth
results either from irritation caused by the swelling of the egg
after opposition and the subsequent hatching of the larva, or
more commonly by the latter alone, in which case the gall does
not commence to form until the larvae have emerged. Growth
of a gall only occurs while the juices of the plant are active, no
growth taking place in the winter.
The gall-makers themselves are usually small and insignifi-
cant looking insects, but the galls which they form differ greatly
both in size, shape, colour and texture. The position in which
they appear on the tree also varies as they may arise from the
buds, leaves, stems, or roots. In the centre of each gall
there is a hollow cavity in which the larva feeds and grows,
subsisting on the juices of the surrounding tissue; here it
pupates, the perfect insect subsequently boring its way out through
the side of the gall, the hole thus formed being in many cases
exceedingly small compared with the size of the insect. The hole