286 



KNOWLEDGE. 



[October 1, 1894. 



thereafter eujoys the exchisive attention of its foster parents. 

 Did the cuckoo require to build a nest and rear its young 

 in the ordinary way, it is doubtful whether the latter 

 would be sutBciently advanced to take their departure 

 along with the other birds when the time for leaving our 

 shores arrived. 



The most perfect galls are made by the Hymenoptera, 

 though those of the Diptera are perhaps more numerous. 

 The Hemiptera, I epidoptera, Coleoptera, and several other 

 orders include gall-making species. A species of beetle or 

 gall-weevil forms pseudo-galls on the willow ; the leaves 

 of the lime, poplar, elm and many other plants are galled 

 by plant lice ; the curiously shaped Chinese galls are also 

 formed by one of the aphides (Aphis Chinensis). These 

 grow on a species of Ehus and are of commercial im- 

 portance on account of the high percentage of tannin 

 they contain. Coccus galls on the Australian Eucalypti 

 constitute what is known as vegetable coral ; the male 

 insect, as Mr. Froggat tells us in his recent paper, 

 occupies a separate gall from the female. The larvie of 

 Lepidoptera are more frequently leaf-miners and leaf- 

 rollers than gall-makers, but several moths have been 

 reported as bearing this character ; one forms a fusiform 

 gall on a creeping plant in New Zealand ; a lepi- 

 dopterous gall from Patagonia has also been described by 

 Mr. Cameron which closely resembles the oak marble gall 

 of Cijnijis kolhiri. Mite-galls in the form of little tufts of 

 hair, or thickened and blistered leaves, duo to species of 

 Phytopus, are of very frequent occurrence. ]\Iites are not 

 properly insects, but belong to the Arachnida ; galls of 

 theirs may be seen on the stitch wort, on the meadow 

 grass and several others. Examples of " worm-galls " 

 occur on some of the hawkweeds and on species of 

 Planfago. Although mite and aphis galls are of common 

 occurrence, they are not so perfect nor are their forms so 

 characteristic as those made by Hymenoptera and Diptera ; 

 they are generally more or less open. This is also the 

 case wiih many dipterous galls ; those of the Hymen- 

 optera, on the other hand, are completely closed. 



Two families of hasects stand out pre-eminently as gall- 

 makers — the Cynipid;!' among the Hymenoptera, and the 

 CecidomyidiE, a family of Diptera ; the former, alongside of 

 which we may place the allied Tenthredinidre, or saw-tlies, 

 are the gall-wasps or true gall-Hies, as distinguished from 

 the Cecidomyias, which are the gall-midges or gall-gnats. 

 The females in both groups are provided with a slender 

 ovipositor ; with this the punctures are made in vegetable 

 tissues and the eggs introduced. How the presence of 

 these should stimulate the tissues into abnormal growth 

 is a question which has attracted a considerable amount 

 of attention, and can hardly be said to be finally settled 

 yet. It was formerly supposed that along with the egg 

 "the insect also injected some secretion into the wound, 

 which acted as an irritant. This view gained plausil)ility 

 from the ovipositor being merely a modification of the 

 wasp's sting. Latterly, however, the irritant properties of 

 the secretion have been questioned ; it has been shown 

 in many cases that the formation of the gall does not 

 commence until after the larvic are hatched, and Dr. Adler 

 holds that the formation of galls must be attributed 

 almost entirely to the action of the larvse in feeding, aided 

 possibly by their salivary secretions. That this is the 

 true explanation of the lormation of a large number of 

 galls admits of no doubt ; but if Prof. Trail's assertion 

 that the galls of C^ynips and Tenthredo are fully formed 

 before the eggs they contain are hatched be true, then 

 their formation must be due to some other cause than the 

 activity of the larva\ 



Not only do different gall-makers select different plantu, 



but they very often confine themselves to particular' parts 

 of the plant, such as the leaf, young shoots, stems, tloral 

 organs, or roots. The galls of the oak, which are exceed- 

 ingly numerous, are mainly the work of Cynipidw. C, 

 hillari produces the marble galls often used to ornament 

 rustic work ; the oak-apple or sponge is caused by Andricus 

 terminalis : the currant gall by Spat/n'ijaxtei- hoccnrum : 

 Xt'uniteru.t leittuuhais gives rise to the button galls or 

 oak spangles on the under sides of the leaves ; the catkins 

 are galled by Andricus ncmJtus : Dryophnnta scntrtlaris 

 makes the succulent cherry-galls ; and the large galls on 

 the root of the oak are occasioned by Aphihithiix mdicis. 



The species of gall-fly are very numerous, and as each 

 gall-maker follows its own pattern, their galls assume a 

 great variety of shapes, as their common names indicate. 

 But the same thing has happened here as was formerly 

 found to be the case with parasitic leaf fungi ; in a number 

 of instances, two forms, which were at first described as 

 distinct species, turn out to be only different phases in the 

 life-cycle of one and the same species ; and as the 

 alternating generations produce totally different galls, we 

 have the latter exhibiting the phenomenon of dimorphism. 

 The currant-galls of the oak, which appear in spring, and 

 the oak spangles, which arc formed towards the end of 

 summer, are believed to be related in this way. The brood 

 which emerges from the former consists of both males and 

 females ; the autumnal galls yield nothing but females. 

 Among twelve thousand specimens reared from the Devon- 

 shire gall, Mr. F. Smith found not a single male ; and 

 different observers relate a similar experience with other 

 galls. There can be no doubt, therefore, that the gall-tlies 

 resemble the aphides in their power of multiplying both 

 sexually and asexually. Before these facts were brought 

 to light, it was impossible to explain how the fly which 

 emerges from the currant-gall ia June succeeded in 

 surviving the winter and producing new galls in the spring, 

 but it now appears that the spangle galls are carried down 

 to the earth with the falling leaves, where they remain 

 throtighout the winter months, and from the impregnated 

 eggs they contain, in due time the female brood emerges, 

 which deposit their unimpregnated eggs in the currant- 

 galls of spring. So far as known, this dimorphism is 

 confined to the galls of Cynipida^ 



Irregular, egg-shaped swellings of a bright red colour 

 on the leaves of willows are caused by a very common saw- 

 fly, Ncmiitm i/KllicoIii : but a much more familiar example 

 is the mossy gall, or bedeguar of the wild rose. This gall, 

 which is produced by a Cynipid yBhodites rose(e), is an 

 arrested leaf-shoot ; only the vascular fibres of the leaves 

 are developed, and it is to these the gall owes its mossy 

 appearance. The russet and carmine tints of these fibres 

 give the gall a very beautiful appearance. Internally it 

 consists of a number of cells massed together, in each of 

 whicli is a larva. Galls of this description which are made 

 up of a number of compartments are termed polythalamous ; 

 those having but a single larval chamber are mono- 

 thalamous. When inquilines take possession of a many- 

 chambered gall they often convert it into a monothalamous 

 one, besides distorting its shape. Galls at first are solid, 

 but as the larva eats away the interior they become hollow. 

 The grubs of many gall-flies complete their metamorphoses 

 within the gall, from which they only emerge after attain- 

 ing the winged condition ; others again, when full fed, 

 pierce a hole in the gall, through which they descend to 

 the ground, where they pass the pupa stage of their 

 existence. In the highly developed galls of tlie true 

 gall-flies, as many as seven different layers or tissues may 

 be distinguished ; the central mass constituting the food 

 of the larviB, and the outer layers being protective in 



