Besides the larva of the gall-maker, or the householder, galls usually contain inquilines or lodgers, the larvae of what are termed guest-flies or cuckoo-flies. Thus the galls of Cynips and its allies are inhabited by members of other cynipideous genera, as Synergus, Amblynotus and Synophrus; and the pine-cone-like gall of Salix strobiloides, as Walsh has shown,[30] is made by a large species of Cecidomyia, which inhabits the heart of the mass, the numerous smaller cecidomyidous larvae in its outer part being mere inquilines. In many instances the lodgers are not of the same order of insects as the gall-makers. Some saw-flies, for example, are inquilinous in the galls of gall-gnats and some gall-gnats in the galls of saw-flies. Again, galls may afford harbour to insects which are not essentially gall-feeders, as in the case of the Curculio beetle Conotrachelius nenuphar, Hbst., of which one brood eats the fleshy part of the plum and peach, and another lives in the “black knot” of the plum-tree, regarded by Walsh as probably a true cecidomyidous gall. The same authority (loc. cit. p. 550) mentions a willow-gall which provides no less than sixteen insects with food and protection; these are preyed upon by about eight others, so that altogether some twenty-four insects, representing eight orders, are dependent for their existence on what to the common observer appears to be nothing but “an unmeaning mass of leaves.” Among the numerous insects parasitic on the inhabitants of galls are hymenopterous flies of the family Proctotrypidae, and of the family Chalcididae, e.g. Callimome regius, the larva of which preys on the larvae of both Cynips glutinosa and its lodger Synergus facialis. The oak-apple often contains the larvae of Braconidae and Ichneumonidae, which Von Schlechtendal (loc. sup. cit. p. 33) considers to be parasites not on the owner of the gall, Andricus terminalis, but on inquilinous Tortricidae. Birds are to be included among the enemies of gall-insects. Oak-galls, for example, are broken open by the titmouse in order to obtain the grub within, and the “button-galls” of Neuroterus numismatis, Oliv., are eaten by pheasants.
A great variety of deformations and growths produced by insects and mites as well as by fungi have been described. They are in some cases very slight, and in others form remarkably large and definite structures. The whole are now included under the term Cecidia; a prefix gives the name of the organism to which the attacks are due, e.g. Phytoptocecidia are the galls formed by Phytoptid mites. Simple galls are those that arise when only one member of a plant is involved; compound galls are the result of attacks on buds. Amongst the most remarkable galls recently discovered we may mention those found on Eucalyptus, Casuarina and other trees and plants in Australia. They are remarkable for their variety, and are due to small scale-insects of the peculiar sub-family Brachyscelinae. As regards the mode of production of galls, the most important distinction is between galls that result from the introduction of an egg, or other matter, into the interior of the plant, and those that are due to an agent acting externally, the gall in the latter case frequently growing in such a manner as ultimately to enclose its producers. The form and nature of the gall are the result of the powers of growth possessed by the plant. It has long been known, and is now generally recognized, that a gall can only be produced when the tissue of a plant is interfered with during, or prior to, the actual development of the tissue. Little more than this is known. The power that gall-producers possess of influencing by direct interference the growth of the cells of the plant that affords them the means of subsistence is an art that appears to be widely spread among animals, but is at the same time one of which we have little knowledge. The views of Adler as to the alternation of generations of numerous gall-flies have been fully confirmed, it having been ascertained by direct observation that the galls and the insects produced from them in one generation are entirely different from the next generation; and it has also been rendered certain that frequently one of the alternate generations is parthenogenetic, no males being produced. It is supposed that these remarkable phenomena have gradually been evoked by difference in the nutrition of the alternating generations. When two different generations are produced in one year on the same kind of tree it is clear the properties of the sap and tissues of the tree must be diverse so that the two generations are adapted to different conditions. In some cases the alternating generations are produced on different species of trees, and even on different parts of the two species.
On galls and their makers and inhabitants see further—J.T.C. Ratzeburg, Die Forst-Insecten, Teil iii. pp. 53 seq. (Berlin, 1844); T.W. Harris, Insects injurious to Vegetation (Boston, U.S., 2nd ed., 1852); C.L. Koch, Die Pflanzenläuse Aphiden (Nuremberg, 1854); T. Hartig, Die Familien der Blattwespen und Holzwespen (Berlin, 1860); Walsh, “On the Insects, Coleopterous, Hymenopterous and Dipterous, inhabiting the Galls of certain species of Willow,” Proc. Ent. Soc. Philadelphia, iii. (1863-1864), pp. 543-644, and vi. (1866-1867), pp. 223-288; T.A. Marshall, “On some British Cynipidae,” Ent. Month. Mag. iv. pp. 6-8, &c.; H.W. Kidd and Albert Müller, “A List of Gall-bearing British Plants,” ib. v. pp. 118 and 216; G.L. Mayr, Die mitteleuropäischen Eichengallen in Wort und Bild (Vienna, 1870-1871), and the translation of that work, with notes, in the Entomologist, vols. vii. seq.; also, by the same author, “Die Einmiethler der mitteleuropäischen Eichengallen,” Verhandl. d. zoolog.-bot. Ges. in Wien, xxii. pp. 669-726; and “Die europäischen Torymiden,” ib. xxiv. pp. 53-142 (abstracted in Cistula entomologica, i., London, 1869-1876); F. Löw, “Beiträge zur Kenntnis der Gallmücken,” ib. pp. 143-162, and 321-328; J.E. von Bergenstamm and P. Löw, “Synopsis Cecidomyidarum,” ib. xxvi. pp. 1-104; Perris, Ann. Soc. Entom. de France, 4th ser. vol. x. pp. 176-185; R. Osten-Sacken, “On the North American Cecidomyidae,” Smithsonian Miscellaneous Collections, vol. vi. (1867), p. 173; E.L. Taschenberg, Entomologie für Gärtner und Gartenfreunde (Leipzig, 1871); J.W.H. Traill, “Scottish Galls,” Scottish Naturalist, i. (1871), pp. 123, &c.; Albert Müller, “British Gall Insects,” The Entomologist’s Annual for 1872, pp. 1-22; B. Altum, Forstzoologie, iii. “Insecten,” pp. 250 seq. (Berlin, 1874); J.H. Kaltenbach, Die Pflanzenfeinde aus der Classe der Insecten (Stuttgart, 1874); A. d’Arbois de Jubainville and J. Vesque, Les Maladies des plantes cultivées, pp. 98-105 (Paris, 1878).
(F. H. B.)
[1] Quoted in Zoological Record, iv. (1867), p. 192.
[2] P. Cameron, Scottish Naturalist, ii. pp. 11-15.
[3] Entomologist, vii. p. 47.
[4] See in Proc. Entom. Soc. of London for the Year 1873, p. xvi.
[5] See A. Müller, Gardener’s Chronicle (1871), pp. 1162 and 1518; and E.A. Fitch, Entomologist, xi. p. 129.