Three classes of hymenopterous insects may be reared from one and the same gall. (1) Psenids, or true gall-flies, which lay their eggs in the tissue of the plant; many of these species cause those subsequent modifications in the de velopment of the plant-tissue that we call galls. (2) Inquilines, or guests, which lay their eggs and develop in the galls caused by the true gall makers. (3) Parasites, which prey on the larvae of the true gall-makers or their guests. Accord ing to Adler, Riley, and others, the growth of the gall probably depends upon the activity of the larvae, and is the result of some secretion or excretion thrown out by the larvae.
The rate of growth of the gall will depend on that of the meristem, those that are formed on catkins and young leaves growing rapidly, while those on roots and bark require perhaps months to gain full size.
Some of the gall larva of the Diptera (especial ly the minute flies of the family Cecidomyiida) transform in the plant tissue and others in the ground. The larva are maggot-like and without anal opening. The goldenrod gall, a round ball produced in the stem of the plant by a fly (Try p6ta solidaginis), and the pine-cone galls on the heart-leaved willow (Salix cordata) are formed by dipterous insects. The Hessian fly of wheat, which stings the base of the leaf, and the wheat midge, which stings the flower, are also classed as gall-insects. The Hemiptera have gall-producing
representatives among the plant-lice (aphids) of the Coccidte and of Phylloxera. The galls pro duced by plant-lice have open mouths for the escape of the developed lice. Reproduction may take place within the gall. The cockscomb elm galls, on the upper side of elm-leaves, are pro duced by a plant-louse (Colopha ulmicola). The destructive grape-vine phylloxera makes galls on the under side of the grape-leaf and on the roots of the vine. The elongated galls on the golden rod stems are produced by a tineid moth ( Gelechia gallcesolidaginis). In Australia several plants are infested by gall-producing thrips, and galls are also said to be caused by beetles.
Consult: Osten Sacken, "On the Cynipidte of North American Oaks and Their Galls," in Pro ceedings of the Entomological Society of Philadel phia, vol. i., pp. 47 to 72, 241 to 259; and vol. ii., pp. 33 to 49; vol. iv., pp. 331 to 380 (Phila delphia, 1861-64) ; Cameron, Monograph of the British Phytophagous Hymenoptera. (London, 1882-93) ; Rothama, "On the /Etiology and Life History of Some Vegetal Galls," in Natural Sci ence, vol. iii. (London, 1893). Beutenmtiller, "Catalogue of Gall-Producing Insects Found Within Fifty Miles of New York," in Bulletin, of the American, Museum. of Natural History, vol. iv. (New York, 1892—) . See PHYLLOXERA; APIIID.