| Fig. 1.—a, Aleppo “blue” gall; b, ditto in section, showing central cavity for grub; c, Aleppo “white” gall, perforated by insect; d, the same in section (natural size). |
The gall-making Hymenoptera include, besides the Cynipidae proper, certain species of the genus Eurytoma (Isosoma, Walsh) and family Chalcididae, e.g. E. hordei, the “joint-worm” of the United States, which produces galls on the stalks of wheat;[20] also various members of the family Tenthredinidae, or saw-flies. The larvae of the latter usually vacate their galls, to spin their cocoons in the earth, or, as in the case of Athalia abdominalis, Klg., of the clematis, may emerge from their shelter to feed for some days on the leaves of the gall-bearing plant.
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. the ash, maple, horn-beam, oak,[21] grape-vine,[22] alder, gooseberry, blackberry, pine, juniper, thistle, fennel, meadowsweet,[23] common cabbage and cereals. In the northern United States, in May, “legions of these delicate minute flies fill the air at twilight, hovering over wheat-fields and shrubbery. A strong north-west wind, at such times, is of incalculable value to the farmer.”[24] Other gall-making dipterous flies are members of the family Trypetidae, which disfigure the seed-heads of plants, and of the family Mycetophilidae, such as the species Sciara tilicola,[25] Löw, the cause of the oblong or rounded green and red galls of the young shoots and leaves of the lime.
Galls are formed also by hemipterous and homopterous insects of the families Tingidae, Psyllidae, Coccidae and Aphidae. Coccus pinicorticis causes the growth of patches of white flocculent and downy matter on the smooth bark of young trees of the white pine in America.[26] The galls of examples of the last family are common objects on lime-leaves, and on the petioles of the poplar. An American Aphid of the genus Pemphigus produces black, ragged, leathery and cut-shaped excrescences on the young branches of the hickory.
The Chinese galls of commerce (Woo-pei-tsze) are stated to be produced by Aphis Chinensis, Bell, on Rhus semialata, Murr. (R. Bucki-amela, Roxb.), an Anacardiaceous tree indigenous to N. India, China and Japan. They are hollow, brittle, irregularly pyriform, tuberculated or branched vesicles, with thin walls, covered externally with a grey down, and internally with a white chalk-like matter, and insect-remains (see fig. 2). 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.[27]
Chinese galls examined by Viedt[28] yielded 72% of tannin, and less mucilage than Aleppo galls. Several other varieties of galls are produced by Aphides on species of Pistacia.
M.J. Lichtenstein has established the fact that from the egg of the Aphis of Pistachio galls, Anopleura lentisci, is hatched an apterous insect (the gall-founder), which gives birth to young Aphides (emigrants), and that these, having acquired wings, fly to the roots of certain grasses (Bromus sterilis and Hordeum vulgare), and by budding underground give rise to several generations of apterous insects, whence finally comes a winged brood (the pupifera). These last issuing from the ground fly to the Pistachio, and on it deposit their pupae. From the pupae, again, are developed sexual individuals, the females of which lay fecundated eggs productive of gall-founders, thus recommencing the biological cycle (see Compt. rend., Nov. 18, 1878, p. 782, quoted in Ann. and Mag. Nat. Hist., 1879, p. 174).
| Fig. 2.—a, Chinese gall (abt. ½ natural size); b, ditto broken, showing thin-walled cavity; c, Japanese gall (natural size). |
Of other insects which have been recognized as gall-makers there are, among the Coleoptera, certain Curculionids (gall-weevils), and species of the exotic Sagridae and Lamiadae and an American beetle, Saperda inornata (Cerambycidae), which forms the pseudo-galls of Salix longifolia and Populus angulata, or cottonwood. Among the Lepidoptera are gall-forming species belonging to the Tineidae, Aegeriidae, Tortricidae and Pterophoridae. The larva of a New Zealand moth, Morova subfasciata, Walk. (Cacoëcia gallicolens), of the family Drepanulidae, causes the stem of a creeping plant, on the pith of which it apparently subsists, to swell up into a fusiform gall.[29]
Mite-galls, or acarocecidia, are abnormal growths of the leaves of plants, produced by microscopic Acaridea of the genus Phytoptus (gall-mites), and consist of little tufts of hairs, or of thickened portions of the leaves, usually most hypertrophied on the upper surface, so that the lower is drawn up into the interior, producing a bursiform cavity. Mite-galls occur on the sycamore, pear, plum, ash, alder, vine, mulberry and many other plants; and formerly, e.g. the gall known as Erineum quercinum, on the leaves of Quercus Cerris, were taken for cryptogamic structures. The lime-leaf “nail-galls” of Phytoptus tiliae closely resemble the “trumpet-galls” formed on American vines by a species of Cecidomyia.[30] Certain minute Nematoid worms, as Anguillula scandens, which infests the ears of wheat, also give rise to galls.