July 1. 1898 ] 



KNOWLEDGE 



127 



Fig. 3. — Sections of Marble 

 A, showing larva of Gall Fly 



cell ; B, showing 

 Gall Flies. 



cells of parasitic 



direct route. The gall does not necessarily drop at this 

 juncture, and, as most of the flies onierf,'e in the autumn, 

 the majority of the galls found during the winter on the 

 bare trees will have no inhabitant within, or rather no 

 Iq/itimatf inhabitant, since, as we shall see presently, they 

 may contain other kinds of insect life. But the presence 

 of the circular hole caused by the escape of the tiy will 

 always distinguish empty galls from those which are still 

 inhabited by the true owner. Sometimes the galls may 

 be seen with a large excavation in one side ; this is the 

 work of the tomtits, which delight to feed on the gall-fly 

 grubs. 



Such in general is the history of a Kollari gall, but it is 

 often complicated by the presence of parasites. These 

 may be chiefly of two kinds, firstly, vegetalile-feeding ones, 

 i.e., other kinds of gall-flies which take advantage of the 

 large store of food contained in a Kollari gall, and save 

 themselves the trouble of making galls of their own ; and 

 secondly, insect-eating ones, belonging to a family of 

 parasitic Hymenoptera which are noted for their gorgeous 

 metallic green and golden coloration — these will be parasitic 

 on the gall-flies themselves, both the legitimate owners 



and the cuckoo-like in- 

 vaders. On cutting open 

 a Kollari gaU, we some- 

 times see not merely the 

 central cavity, which is 

 appropriated to the true 

 owner, but a number 

 of other similar, if 

 smaller, ones, scattered 

 about in the substance 

 of the gall (Fig. 3, b). 

 These are the homes 

 of parasitic gall-flies, 

 (.('., species of Cynipidie which do not appear to have 

 the poner of producing by means of the puncture of 

 their ovipositors the abnormal vegetable growth which 

 constitutes a gall, and therefore lay their eggs in galls 

 already formed by other species. The mother fly in such 

 cases is not satisfied with introducing a single egg, but a 

 dozen, or perhaps a score, will be inserted into the one gall. 

 Mr. B. A. Fitch records rearing twenty-three specimens 

 of a parasitic gall-fly from only a portion of a double gall, 

 i.e., one in which two galls that had been formed in close 

 proximity had coalesced. 



These parasites, or rather inquilines, as they are more 

 appropriately called, develop early, while the gall is still 

 growing, and it can be easily seen that in consequence of 

 their numbers they absorb the sap of the gall as fast as 

 it is produced, and hence arrest its growth ; so that 

 although it contains a large family of insects it remains 

 considerably smaller than when tenanted by only one, the 

 legitimate occupant. It is frequently possible to detect 

 from the outside the presence of these inquihnes ; there 

 is a minute swelling lighter than the ground colour, on the 

 surface of the gall over the spot where the invader's cell 

 is situated, and the slight scar caused by the puncture for 

 oviposition may also be traced. Thus the one gall becomes 

 the nursery of at least two difl'erent kinds of insects, which 

 are distinct enough to be referred to separate genera. 



But this is not all ; wherever a strange insect intrudes, 

 it may be the means of introducing with itself any 

 parasites which are dependent upon it, and thus a third 

 member is imported into the society, and besides this, the 

 legitimate inhabitant may also entice parasites of its own 

 which will most likely be of different species from the 

 others, and thus a fourth member may be added to the 

 community. These parasites (Fig. 2, b), truly so called, 



will very largely consist of members of the family 

 Cliiilriiliilir, whicli subsist upon the eggs and larvse of 

 insects, their eggs being often laid within those of other 

 insects, the contents being devoured by the parasite. It 

 stands to reason, therefore, that this tribe of parasites is 

 composed of very minute species ; but what they lack in 

 size they make up in brilliancy, and, with their polished 

 metallic green and golden skin, they may claim as a group 

 a foremost rank amongst insects in splendour of adorn- 

 ment 



To summarize what has been said and to endeavour to 

 put into realistic form the chief incidents in gall-life : we 

 have seen how, in the first place, a dull-coloured gall-fly, 

 looking round amongst the trees for a suitable spot in 

 which to prompt Nature to rear an asylum for her young, 

 punctures a minute hole in an oak bud, and lays therem 

 an egg, thus giving rise to the abnormal growth which is 

 to be put to the double use of nursery and provisions. 

 But the developing gall is soon spied by a far smaller 

 being, a gorgeous warrior-like egg-parasite, which is also 

 prompted by the anxieties of maternity, and is surveying 

 the country round to discover just such a growth as this, 

 which she instinctively knows contains the object of her 

 quest. She has a long ovipositor with which she 

 punctures the gall and reaches its contained egg, and in 

 this she lays her own and then departs, having frustrated 

 the intention of her predecessor and determined that out 

 of that gall shall proceed, not a gall-fly, but a chalcid. 

 But meanwhile another being, of quite a different character, 

 dull-coloured and sombre like the first arrival, is also 

 prowling about, ready to take a mean advantage of the 

 juicy store which the vigour of the oak has thx-own out in 

 responsL! to the tiny puncture that was the first link in the 

 chain of events. As soon as the growing gall is detected, 

 she alights upon it, and piercing it again and again, 

 deposits in each spot an egg, she on her part determining 

 that that same gall shall bring forth, not mie solitary gall- 

 fly, but a whole family, though of a different sort from 

 what was originally intended. But her intentions, again, 

 are to a large extent doomed to disappointment, for other 

 members of the glistening warrior-tribe of parasites are in 

 the air, and only await the conclusion of her labours to 

 advance to the attack, and mtroduce their own little eggs 

 here and there, ringing the death-knell of a gall-fly at each 

 puncture. And thus in due time there shall issue from 

 that one marble-like gall, not its legitimate occupant, but 

 su3h of the inquilines as escape the second invasion, and 

 a whole stock of brilliant freebooters, each burrowing its 

 way through the substance of the gall into the open air. 

 Thus the old and deserted gall will stand on its stem after 

 the winter, not pierced with a single hole as should have 

 been its fate, but riddled with minuter punctures on all 

 sides, thus bearing mute testimony to the tragedies of 

 which it has been the scene. But even this result is not 

 a foregone conclusion, and the struggle for existence may 

 not be over at the stage indicated, for there are insectivo- 

 rous birds also on the alert, and the tomtit may pounce 

 down on the ill-fated gall while it is still thronged with 

 life, tear it to pieces, and devour all its inhabitants, 

 leaving only the fragments of its walls standing like the 

 ruins of a bombarded castle which has been attacked by 

 a hostile force and has sutiered the massacre of its entire 

 garrison. 



But the relations of our Kollari gall with insect life are 

 hardly yet exhausted. If large numbers of these galls are 

 collected in winter and kept in some receptacle so that 

 the insects that emerge from them can be observed and 

 collected, a most miscellaneous assemblage will in all pro- 

 bability be obtained. If the particulars of one such 



