of a species of fly called _Cynips_, which
is very apt to attack the oak. 'The female insect is armed with a sharp
weapon called an _ovipositor_, which she plunges into a leaf and makes
a wound. Here she lays her eggs; and when she has done so, she flies
away and we hear no more of her. But the wound she has made disturbs the
circulation of the sap. It flows round and round the eggs as though it
had met with some foreign body it would fain remove. Very soon the eggs
are in the midst of a ball-like and fleshy chamber--the most suitable
provision for them, and one which the parent-insect had provided by
means of puncturing the leaf. As the eggs are hatched the grubs will
find themselves safely housed and in the midst of an abundance
of food.'"

[Illustration: OAK-APPLES.]

"Well," exclaimed Malcolm, in great disgust, "_apple_ is a queer name
for a ball full of little flies!"

"It's a very pretty ball, though," said Miss Harson, "with a smooth skin
and tinged with red or yellow, like a ripe apple. If it is cut open, a
number of granules are seen, each containing a grub embedded in a
fruit-like substance. The grub undergoes its transformation, and in due
course emerges a perfect insect. These pretty pink-and-white apples used
to be gathered by English boys on the twenty-ninth of May, which was
called 'Oak-Apple Day.'"

"Did they eat 'em?" asked Edith.

"I do not see how they could, dear," was the reply; "they were probably
gathered just to look at. Yet 'May-apples,' which grow, you will
remember, on the wild azalea and the swamp honeysuckle, are often eaten,
and they are formed in the same way; so we will not be too positive
about the oak-apples."

"What are oak-_galls_, Miss Harson?" asked Malcolm. "Are they the same
as oak-apples?"

"Not quite the same," was the reply, although both are produced by the
same insect. This is what one of our English books says of them: 'When
the acorn itself is wounded, it becomes a kind of monstrosity, and
remains on the stalk like an irregularly-shaped ball. It is called a
"nut-gall," and is found principally on a small oak, a native of the
southern and central parts of Europe. All these oak-apples and nut-galls
are of importance, but the latter more especially, and they form an
important article of commerce. A substance called "gallic acid" resides
in the oak; and when the puncture is made by the cynips, it flows in
great abundance to the wound. Gallic acid is one of the ingredie