INTRODUCTION

A GALL is an abnormal growth of plant tissue produced by animal agency acting from within. All the natural orders of plants include species which are liable to be made use of by insects in this way. Each is visited by its own special gall-maker, which need not necessarily belong to the Cynipidae, for gall-makers are also found among the Coleoptera, Lepidoptera, Diptera, Nematoda1, and in other classes. Any organ of the plant may become the seat of this hyperplasia, but the form which the gall ultimately assumes is governed by the potentialities of growth in the part attacked, and by the nature of the animal excitation present. The rose and some Compositae produce well known galls, but the oak is the favourite home of the Cynipidae. In this monograph Dr. Adler has described those oak-galls and gall-flies most commonly found in this country, with the exception of Cynips Kollari, the Devonshire marble-gall, which does not occur in Germany north of the Elbe; as it is however one of the most familiar galls on English oaks, a description of it has been added in the appendix. Before Dr. Adler

1

H. Charlton Bastian, 'Monograph of the Anguillulidae,' Lin. Soc. Trans. vol. xxv, 1866.

had demonstrated the existence of cyclical propagation, many curious explanations were offered, in order to account for the lengthened interval that elapses between the death of one generation and the appearance of the next. The currant-gall, for example, appears on the male catkins of the oak in May, the fly quits the gall in June, lives for a few days only, and nothing more is seen until the male catkin appears again next May. What had become of the eggs in this long interval? Spontaneous generation of the insect, within a gall that had no external opening, had its advocates. Later it was believed that a form of metempsychosis took place, and galls were among the stepping-stones in the path of organic evolution, by which the vegetable passed into the animal soul. By some it was supposed that the eggs, found in the fly in June, reached the ground, whence they were drawn up, mingled with the sap, and arrested next spring in the leaves or flowering catkins, there to form the currant-galls again. Dr. Adler, by proving the existence of cyclical propagation, has shown that the interval between the appearance of the currant gall-fly in one spring, and its reappearance in the next, is occupied by another agamous generation. But while he showed that this rule holds good for the majority of species, he has also demonstrated that, in some at least, no sexual generation now exists.

Pliny' knew that a fly was often produced in a gall, but he did not associate it with the cause of gall-growth; on the contrary, he thought galls grew in a night, like fungi. Many early observers, however, considered them as insect productions and were aware that a variety of insects emerged from them; but the attention which some of these authors bestowed upon this subject was

1 Pliny, Nat. Hist. xvi. 9, 10; xxiv. 5.

not always due to its biological interest. Mathiolus, one of the best of the commentators of Dioscorides, and a believer in spontaneous generation, declared that weighty prognostications as to the events of the year could generally be deduced by observing whether galls contained spiders, worms, or flies. Most of the older writers describe gall-flies, which are now known to be agamous, as possessing males; but their descriptions are often perfectly clear, and the flies can be recognized as the males of one of the Torymidae, or of some other species of parasite. There is no reason to think that any males of agamous species were actually in existence at the time when these authors wrote.

The earliest systematic writer on galls was Marcellus Malpighi, Physician to Innocent XII. He was Professor of Medicine at Bologna, and afterwards at Messina, and his treatise1'De gallis,' published in 1686, gives an extremely accurate description of the galls then found in Italy and Sicily. Dr. Derham, Canon of Windsor, in the notes to his Boyle Lectures (1711-1712) compares Malpighi's account with the galls then found in England, and says, 'I find Italy and Sicily more luxuriant in such productions than England, at least than the parts about Upminster (where I live) are. For many, if not most, of the galls about us are taken notice of by him, and several others besides that I have never met with, although I have for many years as critically observed all the excrescences and other morbid tumours of vegetables as is almost possible, and do believe that few of them have escaped me ".' Malpighi's work does not appear to have been known either to Linnaeus or Fabricius; they include

1 Dioscorides, i. 146, Paris, 1549.

2 W. Derham, F.R.S., Physico-theology, iii. c. 6.

inquilines and parasites under the genus Cynips, indeed St. Hilaire, Latreille, and Olivier reserve the name Cynips for certain Chalcididae, and use Diplolepis for the true gall-maker, but with few exceptions subsequent writers have applied the general name Cynips to the gall-makers. It is well, however, to bear this change of nomenclature in mind, when the males of certain species are said to have been found 1. Réaumur has left excellent descriptions and drawings of many species of galls, but the first to bring order out of the confusion in which the Cynipidae still remained, was Theodor von Hartig of Brunswick. He greatly improved the existing classification of this family and carefully pointed out the true relationship which the various dwellers in galls bore to each other. He arranged gall inhabitants into three classes; first, the gall-makers which he compared to the actual householders; secondly, inquilines, guest-flies, cuckoo-flies or lodgers, who take up their quarters uninvited within the gall, and live on its food stores, but do not directly aim at the gall-maker's life; and thirdly, parasites who deposit their eggs on the larvae of their host or his lodgers, with the object of destroying them, and who are therefore murderers. Besides those living in a state of symbiosis, there are also true commensals in some galls.

It is the simultaneous presence of these various classes that frequently gives rise to confusion in carrying out breeding experiments. Synergi among inquilines resemble true gall-making species so closely, that caution

1 Westwood, Zoological Journal, No. 13; Cameron, P., British Phyt. Hymen. vol. iii. p. 140.

2 Réaumur, Mémoires pour servir à l'histoire des insectes, 1734-42.

Hartig, 'Ueber die Familien der Gallwespen,' Germar's Zeitschr. f.d. Ent. II. Heft i. p. 176, 1840; III. 322-358, 1841; IV. 395, 1843.

is always necessary to see that the gall-maker, and not a Synergus, has been obtained'. Both Malpighi and Canon Derham were aware of the attacks of parasites, and actually saw galls pierced by them. The latter says, 'I apprehend we see many vermicules, towards the outside of many oak-apples, which I guess were not what the primitive insects laid up in the gem from which the oak-apple had its rise, but from some supervenient additional insects, laid in after the apple was grown, and whilst it was tender and soft. Ratzeburg, a forester like Hartig, in his beautiful Forstinsekten ' corroborated Hartig's division of gall-dwellers. Giraud, Schenck, Reinhard, Reinhard, Taschenberg, Schlechtendal, Wachtl, Förster, and Lichtenstein, have since each advanced our knowledge of the Cynipidae, and the history of galls generally has been admirably written by Lacaze-Duthiers. The entomologists of America have not been behind those of Europe; Baron C. R. OstenSacken, before he quitted America in 1877, had discovered eighteen new species; Bassett, thirty species, and Walsh and Riley had each added much to our knowledge. Professor Gustav Mayr of Vienna has not only increased largely the work of previous observers, but has arranged all that is known of gall-makers and galldwellers in a series of admirable monographs* and has

1 See Walker, Ent. Mag. vii. p. 54.

2 Derham, Physico-theology, iii. p. 389.

3 Ratzeburg, Die Forstinsekten, vol. iii, Berlin, 1844.

Mayr, G., Die mitteleuropäischen Eichengallen in Wort und Bildern, Wien, 1870-71; Die Einmiethler der mitteleuropäischen Eichengallen, Wien, 1872; Die europäischen Cynipiden-Gallen mit Ausschluss der auf Eichen vorkommenden Arten, Wien, 1876; Die europäischen Torymiden, Wien, 1874; Encyrtiden, 1876; Olinx, 1877; Eurytoma, 1878; Telenomus, 1879; Die Genera der gallenbewohnenden Cynipiden, Wien, 1881; Die europäischen Arten der gallenbewohnenden Cynipiden, Wien, 1882.