Francis Henry Butler

“ Often the gall bears no visible resemblance to the structures out of which it is developed; commonly, however, outside the larval chamber, or gall proper, and giving to the gall its distinctive form, are to be detected certain more or less modified special organs of the plant. The gall of Cecidomyia strobilina, formed from willow-buds, is mainly a rosette of leaves the stalks of which have had their growth arrested. ”

“ 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 ”

“ Galls are formed by insects of several orders. Among the Hymenoptera are the gall-wasps (Cynips and its allies) , which infect the various species of oak. They are small insects, having straight antennae, and a compressed, usually very short abdomen with the second or second and third segments greatly developed, and the rest imbricated, and concealing the partially coiled ovipositor. ”

“ 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. ”

“ 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. ”

“ The escape of the insect takes place on the spontaneous bursting of the walls of the vesicle, probably when, after viviparous (thelytokous) reproduction for several generations, male winged insects are developed. The galls are gathered before the frosts set in, and are exposed to steam to kill the insects. ”

“ 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. ”

“ The dipterous gall-formers include the gall-midges, or gall-gnats (Cecidomyidae) , minute slender-bodied insects, with bodies usually covered with long hairs, and the wings folded over the back. Some of them build cocoons within their galls, others descend to the ground or become pupae. The true willow-galls are the work either of these or of saw-flies. Their galls are to be met with on a great variety of plants of widely distinct genera, e.g. ”

“ 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. ”

“ Their formation from buds which normally would have yielded leaves and shoots is explained by Parfitt as the outcome of an effort at fructification induced by oviposition, such as has been found to result in several plants from injury by insect-agency or otherwise. [6] Galls vary remarkably in size and shape according to the species of their makers. The polythalamous gall of Aphilothrix radicis, found on the roots of old oak-trees, may attain the size of a man’s fist ”