Gall-flies

(Super-family Cynipoidea.)

To this group belong the true gall-flies, as well as certain insects which are inquilines, or gall-fly guests, and also certain forms which are true parasites. Although we call the Cynipoids the true gall-flies, there are many other insects which make galls, such as some of the two-winged flies of the families Cecidomyiidae and Trypetidae, certain caterpillars, a few Chalcis flies, and a few beetles, as well as certain mites and scale insects. Certain of the oak galls formed by Cynipoids were early used in the manufacture of ink, and later for tannin, so that when one speaks of gall-flies these creatures are always thought of. They are small dark colored four-winged flies, at once distinguished from their close allies by the venation of the wings and the structure of the thorax, as pointed out in the analytical table.

Those which make galls lay their eggs in the tissues of the growing plant, and the larvae when hatched feed upon the plant cells and their contents. A very slight gall deformation may result, but in the majority of cases there is a rapid growth of plant-cells and a curious enlargement of variable shape which is called a gall.

The nature of the gall has long been a disputed point. It was at first thought that it was a purely vegetable growth and that the little grubs found within it were the result of spontaneous generation. Later it was supposed that galls were caused by the punctures of insects and the injection of a poisonous liquid. With the true gall-flies, however, the gall apparently does not commence to form until after the egg hatches. It is supposed that the larva secretes a liquid which causes the abnormal growth of the plant, the plant cells which are most active in growth and subdivision being directly affected.

The egg of the gallfly is slender, and has a very long petiole which is six to ten times the length of the egg body, and this is inserted by means of a very long curiously formed ovipositor. A good account of the method of oviposition reported by Riley from observations made by Pergande will be found in the Proceedings of the Entomological Society of Washington (Vol. III. pp. 260-263).

Most of the Cynipoids make galls upon oak. Others, however, are found upon rose bushes. All parts of the plant are affected--roots, stems, twigs and leaves, as well as leaf petioles. In some galls but a single larva develops, while in others very many develop. The oak galls of commerce are European galls, but some of our native galls would undoubtedly be found to possess commercial value through the quantities of tannin they possess.

The origin of tannin in galls has been the subject of investigation by Kraemer. He finds that gallic acid is formed at the expense of the starch during the chrysalis stage of the insect. With the maturing of the winged insect, the gallic acid is changed to tannic acid. The transformation of gallic acid into tannin is accomplished by the condensation of two molecules of the former with the loss of one molecule of water.

Most of these insects are single-brooded and develop but one generation in the year. Under unfavorable circumstances, however, this period may be greatly lengthened, and circumstances are on record where the flies have emerged only after two or three years. Certain moisture conditions favor the proper issuing, and when these are lacking the development is retarded.

Some very curious alternations of generations occur among these insects. Of certain species, and indeed of certain genera, for years only the female sex was known, and it was afterward discovered, first by Riley and afterward by Adler, that what had been considered two entirely distinct forms were really the same species, but that one generation included both sexes while the alternate generation comprised only females which reproduced parthenogenetically.

One of the most peculiar facts connected with the gall-flies is that a particular part of the plant is always affected by the same species, and that each species of the same generation always produces a deformation or gall of exactly the same character, so that the gall alone identifies the species of insect, and in fact for a long time generic and specific names were given to the galls before the insects were named, the name subsequently being applied to the insect itself.

The full development of none of the American gall-making Cynipoids has been studied with the care which this subject should have and doubtless there are many interesting and important facts yet to be discovered.

The guest gall-flies closely resemble the true gall-flies, but lay their eggs in galls already formed by the true gall-flies, their larvae living upon the plant growth produced by the true gallfly larvae.

The parasitic gall-flies live as true internal parasites in other insects, mainly plant lice and the larvae of dipterous insects.

About fifteen hundred species of this super-family have been described.