GALL-INSECTS (from Lat. galls, gall-nut). Until about two hundred years ago galls were supposed to be purely of vegetable origin, and the maggots that grow within them were supposed to arise by spontaneous generation in the organic substances in the galls. Pliny knew that a fly came from galls, and thought they grew like fungi in the night. Malpighi, in the second half of the seventeenth century, was the first to record the fact that the production of galls followed puncture of vegetable tissue by insects, and he came to the conclusion that the insects inject a substance, which he called ichor, into the plant-tissue, and this substance produced a swelling similar to that which the sting of a bee causes in animal tissue. Reaumur held the theory that the gall is not the product of some specific irritating fluid, but is due to the irritation caused by the prick, and to the presence of the egg and developing larvae in the tissue. Some galls begin to develop as soon as the eggs are laid, but, unfortunately for the universal application of likaumur's theory, others do not begin to develop until after the eggs hatch, which may be months after they are deposited in the tissue.
Galls occur on a great many kinds of plants and are produced by a variety of insects, by mites, and by at least one nematode worm. Each species of insect confines its activities to one or, at the very most, to a very limited number of species of plants. The same kind of insect will produce different kinds of galls on different kinds of plants, and different kinds of insects will produce different kinds of galls on the same plant. Each species of gall-insect, however, in fests different parts of the plant, such as the leaf, flower, stem, or root, and that part alone; and it produces there galls with such precise qualities that it can be definitely stated, from the appearance of the gall, what sort of an insect has caused its development. In rearing galls, one cannot be certain from merely observing the emerging insects what species are the producers of the gall, for a number of different kinds of insects may develop within the same gall, some as guests, feeding on the tissue of the gall, and others as parasites on the larval of the true gall insect.
Nearly all the orders of insects have gall making representatives. In addition there are the galls of mites and nematodes. The galls made by mites, like those produced by plant-lice, have open mouths for the escape of the matured mites. An example of a gall produced by mites is the pear-leaf blister made by Phytoptus pyri. Nematodes of the genus Anguillula, which is al lied to the vinegar-worm, are the cause of smut in growing grain, particularly in wheat. The larvae of these insects have the most extraordi nary capacity of withstanding desiccation. The egg is laid by the parent in the growing ear, where the larvae develop and are set free by the dying grain. They then live in the moist earth until the young wheat begins to grow. They creep up the stem of the wheat, and when once lodged within the head they soon gain sexual maturity. In their wanderings in search of new,
growing grain, the larvae undergo great vicissi tudes. They may be compelled by drought to encyst a number of times, even on the very stem of the plant, and await moisture before they are able to reach their final destination. Accord ing to Spallanzani, they may retain their vitality for twenty years while awaiting their food-plant.
The family Cynipidn, of the order Hymenop tera, furnishes the greatest number of species of gall-producing insects. The majority of its spe cies (called gall-flies) infest some part of the oak, making closed galls. They are the best studied of all the galls, and a large amount of information concerning their life history has been gained by the painstaking studies of Adler, Riley, and others. Adler kept oak saplings, until from four to six years old, and on these he iso lated certain insects, and observed the resulting galls. Some of the species that Adler bred were so nearly alike that he could determine them with certainty only by their galls. Moreover, certain species that had been given different spe cific or even generic names he found to be the alternating generations of other described species. Some winged generations he found to be com posed entirely of females, and the next generation of both males and females. Thus the individuals of one generation do not resemble their parents, but their grandparents. (See ALTERNATION OF GENERATIONS.) Not only are the insects of these two generations very different, but the galls that they produce are likewise different. Other forms are believed to reproduce entirely parthe nogenetically without males ever appearing. Ad ler studied galls of the bud, leaf, bark, and root, and found that all of them are developed by abnormal activities of the cambium ring. The potentialities of the tissue-growth are always present at the spot pricked, and are merely called into activity by the prick or by the larva. He found that some of the galls are protected from attack by sweet juices, which attract guarding ants, and it is interesting to note that the honey making ants (q.v.) of the Southeastern United States gather honey from oak-galls. Other galls are provided with a sticky secretion on long hairs which entraps marauders; the spongy pa renchyma of some galls is so very thick that it acts as an effectual barrier against intruders. Other galls have an inner stony layer for the protection of the larva:; others, a large, hollow chamber in which it is difficult, for the enemy from without to locate the larva. The pine-cone like arrangement of scales in certain galls is a sufficient protection to the larva. Other galls are exempt frorri attack by virtue of their bitter tan nin or by their protective coloration. Insects, titmice, pheasants, and squirrels are the chief enemies of gall-insects, the birds and squirrels tearing them open in winter to get the larva within them.