1 907-1908.] Galls, Gall-makers, and Cuckoo Flies. 33 



tortuous galleries in the substance of the leaf; and not less 

 remarkable is the knowledge of vegetable pathology, physi- 

 ology, and embryology involved in the instinct displayed by 

 the various species of gall-makers. Their eggs as a rule must 

 be placed in the cambium layer, where cell-division is actively 

 going on, and the unerring instinct with which the gall-maker 

 singles out the proper layer of microscopic cells in the rudi- 

 mentary bud or undeveloped leaf is one of those fine adjust- 

 ments of means to ends wherein nature excels the highest 

 achievements of art. 



Formerly it was believed that along with the egg the insect 

 also injected a secretion which by its irritant action excited 

 abnormal growth in the cells of the cambium layer. Analogy 

 favoured this idea, the organ with which the puncture is made 

 beiug homologous with the sting of the wasp. Eecent investi- 

 gations, however, have rendered it doubtful if the secretion 

 possesses any irritant properties at all ; it seems in many cases 

 to serve rather as a simple antiseptic dressing to close up the 

 wound. Further, it has been found that in many instances 

 the formation of the gall does not begin until after the eggs 

 are hatched ; the larvae of the coccus insect itself, for example, 

 attacks the plant directly, and by the irritation thus caused a 

 kind of gall is formed. The galls made by the mites and by 

 the aphides or plant-lice also originate in this way. From 

 such considerations Dr Adler and others have been led to 

 ascribe the formation of galls mainly, if not entirely, to the 

 activity of the larvae themselves, aided to a certain extent by 

 their salivary secretions. The latter are supposed to contain 

 some kind of ferment or enzyme, which diffuses into adjacent 

 cells and stimulates their development in a special way : this 

 supposition is necessary, since those cells which are actually 

 injured generally die. On the other hand, Professor Trail 

 states that the galls of Cynips and Tenthredo are fully formed 

 before the eggs they contain are hatched. Further investiga- 

 tions on the subject appear to be required. 



A considerable number of galls present a superficial re- 

 semblance to cherries, berries, or other fruits. Now, a fruit, 

 we know, is produced by the development of the seed-vessel 

 resulting from the stimulus imparted by the application of the 

 pollen-grain to the stigma in fertilisation. The egg or larva 



VOL. VI. c 



