INSECTS AND OTHER ORGANISMS 385 



years from caterpillars which have been obliged to " feed 

 up " and pupate on an inadequate supply of foliage late 

 in the season. 



Oak-trees are of special interest in a discussion on the 

 relations between insects and plants, because they have, to 

 a greater extent than any other tree or shrub, become 

 adapted to the presence of certain insects, so that by the 

 formation of galls they provide these insects with food at 

 the least possible expense and disadvantage to their own 

 health and well-being. A gall is an abnormal plant-growth, 

 incited by the influence of an insect on the plant's formative 

 tissue, which develops under the chemical or tactile stimu- 

 lation in such a way that the insect is provided with shelter 

 and food without the necessity of destroying large quantities 

 of foliage or wood. Most by far of the galls on an oak are 

 due to the small Hymenoptera of the family Cynipidae, 

 generally known as gall-flies. Every one knows the sub- 

 spherical *' oak-apples," the smooth, woody " marble 

 galls," the succulent '' cherry- galls," and the round, 

 flattened '' spangle-galls," on or under the leaves, while 

 many- chambered hard galls are to be found on the roots. 

 These are only a few of the multiplicity of growths on oak 

 due to various Cynipidae. Though galls are abnormal from 

 the purely botanical point of view, each has a definite 

 specific form, so that the particular causative insect can be 

 determined from the result of its action on the plant. It^ 

 was formerly thought that the stimulation leading to gall- 

 growth must be due to the puncture of the plant tissues by 

 the female fly's ovipositor, or to some irritant fluid injected 

 when the egg is laid. But the inciting cause for cynipid 

 galls is now determined as the feeding and digestive functions 

 of the larva. There will be no growth of a gall if the egg 

 be removed ; growth begins after the hatching of the larva 

 and then lars^a and gall grow together. A. Cosens (191 2) 

 has elucidated the physiological processes accompanying 

 gall-formation. The egg is often laid in or near the plant's 

 formative tissue (cambium), and on this the grub when 

 hatched begins to feed, sucking out the cell- contents, so 



2 c 



