From left to right: The spiny oak-gall.   The pointed bullet-gall on oak twigs.   A cluster of galls on midrib of oak leaf.   The acorn plum-gall.  Photo by L. H. Weld.

The Gall-Dwellers

LTHOUGH Aladdin is out of fashion, we still have houses of magic that are even more wonderful than that produced by his resourceful lamp. These houses are built through an occult partnership between insects and plant tissues; and no one understands just how they are made, although we are beginning to understand a little concerning the reasons for the growth. These houses are called galls and are thus well named, since they grow because of an irritation to the plant caused by the insect.

There are many forms of these gall-dwellings, and they may grow upon the root, branch, leaf, blossom, or fruit. The miraculous thing about them is that each kind of insect builds its magical house on a certain part of a certain species of tree or plant; and the house is always of a certain definite form on the outside and of a certain particular pattern within. Many widely differing species of insects are gall-makers; and he who is skilled in gall lore knows, when he looks at the outside of the house, just what insect dwells within it.

We may take the history of the common oak apple, as an example. A little, four-winged, fly-like creature lays its eggs, early in the season, on the leaf of the scarlet oak. As soon as the larva hatches, it begins to eat into the substance of one of the leaf veins. As it eats, it discharges through its mouth into the tissues of the leaf, a substance which is secreted from glands within its body. Immediately the building of the house commences; out around the little creature grow radiating vegetable fibers, showing by their position plainly that the grub is the center of all of this new growth; meanwhile, a smooth, thin covering completely encloses the globular house; larger and larger grows the house until we are accustomed to call it an oak apple, so large is it. The little chap inside is surely content and happy, for it is protected from the sight of all of its enemies, and it finds the walls of its house the best of food. It is comparable to a boy living in the middle of a giant sponge cake, and who when hungry would naturally eat out a larger cave in the heart of the cake. After the inmate of the oak apple completes its growth, it changes to a pupa and finally comes out into the world a tiny four-winged fly, scarcely a quarter of an inch in length.

Oak-apple, showing the larva of the gall insect.  Comstock's Manual.

The story of the willow cone-gall is quite different. A little gnat lays her eggs on the tip of the bud of a twig; as soon as the grub hatches and begins to eat, the growth of the twig is arrested, the leaves are stunted until they are mere scales and are obliged to overlap in rows around the little inmate, thus making for it a cone-shaped house which is very thoroughly shingled. The inhabitant of this gall is a hospitable little fellow, and his house shelters and feeds many other insect guests. He does not pay any attention to them, being a recluse in his own cell, but he civilly allows them to take care of themselves in his domain, and feed upon the walls of his house. He stays in his snug home all winter and comes out in the spring a tiny, two-winged fly.

Willow cone-galls.  Photo by Verne Morton.

There are two galls common on the stems of goldenrod. The more numerous is spherical in form and is made by a fat and prosperous looking little grub which later develops into a fly. But although it is a fly that makes the globular gall in the stem of goldenrod, the spindle-shaped gall often seen on the same stem has quite another story. A little brown and gray mottled moth, about three-fourths of an inch long, lays her egg on the stem of the young goldenrod. The caterpillar, when it hatches, lives inside the stem, which accommodatingly enlarges into an oblong room. The caterpillar feeds upon the substance of the stem until it attains its growth, and then seems to dimly realize something about its future needs. At least it cuts, with its sharp jaws, a little oval door at the upper end of its house and makes an even bevel by widening the opening toward the outside. It then makes a little plug of debris which completely fills the door; but because of the bevel, no intrusive beetle or ant can push it in. Thus the caterpillar changes to a helpless pupa in entire safety; and when the little moth issues from the pupa skin, all it has to do is to push its head against the door, and out it falls, and the recluse is now a creature of the outside world.

Spherical gall of goldenrod, opened, showing its prosperous looking owner.  Photo by M. V. Slingerland.

Many galls are compound, that is, they are made up of a community of larvæ, each in its own cell. The mossy rose-gall is an instance of this. The galls made by mites and aphids are open either below or above the surface of the leaf; the little conical galls on witch-hazel are examples of these. In fact, each gall has its own particular history, which proves a most interesting story if we seek to read it with our own eyes.

Mossy rose-gall.  Comstock's Manual.

Method—Ask the pupils to bring in as many of these galls as possible. Note that some have open doors and some are entirely closed. Cut open a gall and see what sorts of insects are found within it. Place each kind of gall in a tumbler or jar covered with cheesecloth and place where they may be under observation for perhaps several months; note what sort of winged insect comes from each.

2. What is the shape of the little house? What is its color? Its size? Is it smooth or wrinkled on the outside? Is it covered with fuzz or with spines?

Porcupine gall on leaf of white oak. Section of same showing cells. Female gall-fly laying eggs in oak bud.  Photo by C. J. Triggerson.

3. Open the gall; is there an insect within it? If so, where is it and how does it look? What is the appearance of the inside of the gall?

4. Is there a cell for the insect at the very center of the gall, or are there many such cells?

5. Has the house an open door? If so, does the door open above or below? Are there more than one insect in the galls with open doors? What sort of insect makes this kind of house?

6. Do you find any insects besides the original gall-maker within it? If so, what are they doing?

7. Of what use are these houses to their little inmates? How do they protect them from enemies? How do they furnish them with food?

8. Do the gall insects live all their lives within the galls or do they change to winged insects and come out into the world? If so, how do they get out?

9. How many kinds of galls can you find upon oaks? Upon goldenrod? Upon witch-hazel? Upon willow?

Supplementary reading—Outdoor Studies, Needham, pages 18 and 37; "Houses of Oak," in Insect Stories, Kellogg; Manual for the Study of Insects.

Stem of golden-rod, showing the spherical gall above, made by larva of a fly; and the spindle-shaped gall below, made by the caterpillar of a moth.