January i, 1886.] THE TROPICAL AGRICULTURIST. 



487 



their name by their appearance; while the "artichoke 

 giill,'' resembling a miniature artichoke, takes its 

 risi: from a linii. ' I-e-^s kr.own forms grow ou the 

 roots anil bark. Of foreign oak-galls the most 

 important are those known to commerce as " Aleppo 

 galls," which grow ou a species of oak common in 

 the Levant. Galls are importcil into Britiiiu in 

 considerable (luaatitics, the imports for ISf^;! having 

 amounted to 39,.50l) cwt.. Valued at £108,ti00. It 

 ought to be stated, however, that more than half of 

 that <iuautity came from China, and that the Chinese 

 gall of commerce is the work of a ])lant-louse on a 

 dillerint tree from the oak. Galls are used in the 

 manufacture of uik, that useful material being a 

 mixture usually of green vitriol, a decoction of 

 oak-galls and some gum arabic. Their value for 

 this purpose is due to the abuuilaut tannin they 

 contain— as much sometimes as 77 per cent. The 

 British forms are comparatively poor in this constituent, 

 although the "Devonshire marble gall," now one 

 of the must abundant in Eritain. yields 17 per 

 cent of tannin, and is said to make excellent ink. 

 The Dead Sea Fruit or .Vpples of Sodom, described 

 by .Tosephus and others as lovely to the eye, but 

 uau6eou8 to the palate as bitter ashes, arc generally 

 identified with the larger JIec:a galls which grow- 

 on a species of oak in the neighbourhood of the 

 Dead Sea. Among the most carious of oak-galls are 

 those known in California as " flea-seed." They 

 grow on the leaves of an oak, from which in August 

 they get detached and fall to the ground. So ex- 

 ceedingly lively, in this instance, is the living kernel, 

 that its jerking movements within the thiu-walled 

 cavity cause the entire gall to skip .ibout. 



What is the history of the growth of these veget- 

 able excrescences ? The gall-tlies are small insects 

 with bodies seblom bigger than garden ants, but 

 wiih four large clear wings. They belong to the 

 same order as bees and wa.sps, and when the time 

 has come for depositing their eggs, the female 

 eelect-s a spot on leaf, bud. or bark of the oak, and 

 pierces it with her ovipositor. This is a slender 

 organ projecting from the creature's abdomen, and 

 is but a slightly modified form of the wasp's sting. 

 After piercing a hole, the blades of the ovipositor 

 open, and au egg jiasses down betw'een them into 

 the ])!ace prepared for it. There is then injected 

 into the aperture a small quantity of an irritating 

 fluid, and the work of the parent insect is completed. 

 This fluid, which is at least suggestive of the poison 

 of the hyrocnopterous sting, has a remarkable effect 

 on the tissues of the plant, causing them to swell 

 into tliose globose knobs known as galls. In due 

 time the egg of the insect becomes a grub, which 

 finds its proper nourishment in the abundant juices 

 of the gall. When it.s transformations are over and its 

 natural term of imprisonment expired, the gall-llj tunnels 

 its way through the enclosing tissues and emerges 

 into daylight. How such puny insects can make 

 their way, for example, through a Hungarian gall, 

 which is as hard as the most seasoned oak, is al- 

 together mysterious. That they do tunnel their way 

 through is evidenced by the little hole on the 

 surface which marks their points of exit. "Where no 

 such aperture occurs in a sulficiently mature gall, 

 the insect has failed to make its way out ; and 

 thus, no doubt, the strongest gall-flies are " naturally 

 selected'' before they ever see the light. Sometimes, 

 however, the strongest may fail, as in the case 

 mentioned by the Kev. J. G. Wood in which two 

 galls had united. The insect in the one gall escaped, 

 but that in thf other unfortunately bored into the 

 jjart joining the two, and being unable to timnel 

 farther than the distance which under ordinary 

 circumstances would have brought it to the out.side, 

 it died. In order to secure the' insect reared within 

 th» gall, it is necessary to jilace the branch in a 

 bottle of water and cover it over with gauze. 

 A diflicultj', however, occurs here from the fact that 

 a c<uisiderHble variety of insect species usually emerge 

 from a single gall. Some of those are clo.sely allied 

 forms that do not grow galls for themselves, but 



which, like the cuckoo among birds, drop their eggs 

 into the nests of others. Thes : are lodgers or guests 

 of the true gall-dies. Others, again, are parasitic ou 

 the latter, and the.se, feeding on the juices of 

 its luxly, ultimately destroy it. ll is only the en- 

 tomologist who has specially devoted himself to the 

 study cf this family of insects that can readily 

 identify the fly that has emerged froni a particular 

 gall. It might be suppo-sed that, in seeming this 

 fly, the observer ha<l got an insect similar in every 

 respect — supposing it to be a fem.Tle — to the one 

 which had placed its egg in the juices of thc^ oak- 

 leaf. In some species at least this is not the case, 

 and the probability now is that it is seldom, if ever, so. 

 The r.:cent researches of Mr. ]!. Walsh in America, 

 and still more of Dr. .Vdler in (4crmany — au excellent 

 summary of which is given by I'rof. Trail, of 

 Abenlee's in a paper recently communicated to the 

 Perthshire Natural History Society— show that the 

 gall-flies exhibit IJ/iHO/'/'///.-;j«, or " alternation of gener- 

 ations." Dimorphism, according to i'rofessor Trail, 

 consists e-sentially in thi.s, " that there are in the 

 full life histnry of any species in which it occurs 

 two forms of the inject, or, as it is at times expressed, 

 two gi-neration, of which the one possesses both males 

 and t'» ma e.s and produces offspring in the usual 

 way. while the other consists of so-called females 

 aliiic, an.l reproduction is effected by what we may 

 regard as a piocess of budding from a part of tht; 

 body of the parent, not by the tru<: sexual method." 

 Before dimorphism was thought of in the case of 

 gall-flius, there were mysteiies in the life histiry of 

 these insects which no fellow ^of the ICntomoIogical 

 Soci*-ty) could understand. For example, there is no 

 reason to be'ieve that the life of the particular fly 

 which emerges from the " curr-ant-gall ''of the oak 

 in June is prolonged beyond the autumn of the 

 same year. How comes it, then, that in the following 

 spiing a new crop of those oak-galls appears on the 

 young leaves and male catkins, seeing that the 

 gall-makers have been all dead months before the 

 appearance either of leaves or catkins y The answer, 

 according to Dr. Adier, is, that the fly producing 

 the " currant-gall " has existeil through the winter 

 in a totally different oak-gall ; that the fly itself 

 differs so entirely in form from the insect into 

 which its eggs develop — i. r., its own offspring — a 

 to have been hitherto regarded as belonging to a 

 totally distinct genus, but that these ort'spiiug deposit 

 eggs which, after surviving the winter as grubs 

 emerge in spiing in the likeness of their grand jjarents. 

 This cycle of generations will appear more clearly 

 if we trace in detail the lite history of this currant- 

 gall fly. As already stated, it emerges from its gall 

 in the month of .Tune, and the brood contains both 

 males and females. In July the impregnated eggs 

 of the female are deposited usually on the lower 

 surfaces of oak leaves, and soon after they give rise 

 to what are knowu as " spangle-galls." Of a russet 

 >)rown colovu- and flat lens-like from, the.v are totally 

 utdike the " currant-galls." The spangle-galls" fall 

 to the ground slun-tly before the leaves, and there 

 lie concealed among the fallen foliage during winter. 

 In this position they enlarge and bccouto much more 

 convex, while within the insect is undegoing its 

 metamorphoses. In spring it emerges, but the form 

 that now ajipears is never male, but always what 

 Professor Ti-ail terms u:tt\rt(ttl. It is what the bov 

 said his canary was when askeil by a possible jiur- 

 cha.ser as to i*s sex. Not wishing to commit himself 

 before he knew what was wanted, he replierl, " It 's 

 a kind o' she, and not a she either." 'The spangle- 

 gall fly is a kind of female in so far as it deposits 

 eggs, but inasuuich as these eggs are not i)roducetl by 

 the true sexual methoil, the creature is in reality 

 sexless. It is these unimpregnated eggs deposited by 

 the spangle-gall fly in spring that give rise to the 

 " currant-galls " in May, and to the male and female 

 currant-gall flies in .June. This dimorphism has been 

 traced through a considerable number of species ; 

 and in the list of Scottish gall-flies given by Professor 

 Trail, nine species, belonging to four genera, are 



