24 THE FUNCTIONS OF A LOCAL NATURAL HISTORY SOCIETY. 
produced by the puncture and irritation of some member of 
creation, for I may say, in passing, that all pathological swellings 
on plants are not galls. This irritation gives rise to a local over- 
growth of the cellular tissue of the plant, and this over-growth 
assumes, in each case, a form distinctive of the species which 
produced it. Into this puncture an egg is laid, and when hatched 
the larva feeds on the abnormal growth. These galls may assume 
all sorts of forms, as rounded, in the common oak-apple; flattened, 
in the oak spangle ; stalked, like a fruit from the axil of a leaf, as 
in a specimen which I found in Switzerland, in which the shoots 
of a sallow seemed to be bearing a currant-like fruit from each leaf; 
or feathered, like the beautiful bedeguar of the rose. In all these 
forms the galls look as if they were only attached to the plant, 
but in a vast number of cases the gall is a local thickening, as in 
the knobs produced by the Phylloxera on the root, by some of 
the longicorn beetles on the stem, and by the Rhodites in the 
irregular thickenings of the edges of the leaves in the Burnet 
rose. The colours of some of the galls are very brilliant, and 
where this is the case, the colour is generally some shade of red, 
while in others the normal colour of the chlorophyll of the plant 
is not affected. 
Some of the gall-makers are of vast economic importance, as 
the nut-gall of the oak, from which writing ink is manufactured, 
and which is also of medicinal value, and, on the other 
hand, some are almost unrivalled in their powers of destruction, 
as the Phylloxera of the vine, which has almost destroyed many of 
the French vineyards. 
What are the gall-makers? ‘The vast majority are insects, but 
galls are also produced by minute worms, and even by mites. 
The galls produced by mites differ considerably in their structure 
from those of insects; those of insects having a more or less 
solid structure, which, when cut open, shows the larva living ina 
closed cavity, while in those produced by mites, which all belong 
to the family of the Phytoptide, there is always an opening 
connecting the interior of the gall with the external world. For 
example :—To take two galls very much alike externally, viz., the 
