GALLS AND Galls are unnatural plant-growths caused by various forms of parasitic animals or plants, more partic ularly by the hymenopterous family of gall-flies (Cynipidee). Gall-gnats (Cccidomyiidee),— (minute two-winged midges or flies), many species of mites, certain aphids, nematode worms, some forms of caterpillars, and the larva of weevils and other beetles also cause galls. Among plant gall parasites, the most important are bacteria, slime molds and certain alga. The formation of a gall is due to the puncturing of any portion of a plant, the surface of the leaf, stem, roots or bark and the deposition of an egg in the cavity formed, or by the presence of larva subsequently hatched from it. Within these excrescences the larva feed and grow, and either eat their way out while still grubs or remain till the pupa stage is past and emerge as adolescent insects. A gall may contain a single egg and larva or many, and both external form and internal structure vary widely. Each gall-fly has its favorite or exclusive host, and usually restricts its egg-laying to some special part of the plant. While most produce true galls, some members of the family utilize galls already formed.
The process of gall formation is but imper fectly understood. It is not known whether the gall-forming stimulus is mechanical or chemical in nature, nor whether it consists in the oviposition of the female or the growth and movements of the larva. The extreme complexity and specialization of the gall adds greatly to the difficulty of explanation. A cynipid oak gall may consist of an outer shell, with radiating fibres supporting a larval cham ber with well developed food-layers, Not only is there a considerable increase in the number of cells, but their size is also often immensely greater than normal. The gall is often full of waste products, such as resins and tannic acid — whence comes their commercial value in tanning and in the manufacture of ink. Galls of closely related species may maintain a con stant difference in structure. Many galls are hairy or spiny, while the plant is naturally smooth.
The reproductive relations of gall-flies are very interesting. In many cases parthenogene sis undoubtedly occur; in some species, for ex ample, of Rkodites, no males have ever been found ; in other forms the males when they occur are very few in proportion to the females.
It must be emphasized that many gall-wasps distinguished by entomologists as separate species or even referred to different genera have turned out to be the parthenogenetic and the sexual forms of one species. A common life history is as follows: (a) Out of a sum mer-gall male and female forms emerge; (b) the females lay their fertilized eggs and give origin to winter-galls in so doing; (c) from these winter-galls there arise parthenogenetic females which in their egg-laying produce the summer-galls from which we started.
Galls vary greatly in shape, and may be solid or spongy, and contain one or several cavities, in each of which a larva is lodged. Though galls are very generally distributed, they occur in commerce chiefly as Levantine ar ticles of trade. The Aleppo nut-galls are spher ical and tubercular; blue, black and white varieties are recognized, the two former being picked before the escape of the larva, the lat ter after its exit. They are produced by a gall-fly (Cynips gallcetinctoricr) on twigs of an oak (Quercus infectoria). The galls made on oak by the common British "ash-fly* (C. quer cifolia), or by the hundred or more American species of Cynips, might serve the purpose of ink making, tanning, etc., just as well; the 70 to 80 per cent of tannin they contain is the principal element of value. Dead Sea Apples, or Mecca or Bussorah galls, or Apples of Sodom (mcda insana, or C. q. infectorie in sana), are varieties of this vegetable product. The artichoke or strobile galls consist of sev eral pieces, and resemble the fruit (strobilus) of the hop. They are exemplified by the gall Produced on the willow by a Cecidomyia. The hairy galls, or bedeguars, or rose sponges, are chiefly found on Rosa rubiginosa; they are Produced by Rhodites rose. A peculiar fact about many galls is that they contain insects other than those which have caused them. These are called inquilines. They are often, but not always, closely related to true gall forming insects. Consult Adler, H.,