386 THE BIOLOGY OF INSECTS 



that the cells close to the larva collapse, leaving a cavity 

 which becomes the chamber of the gall. The grub's salivary 

 secretion, largely poured out on the cambium, hastens the 

 change of the starch of the plant-cells into sugar ; thus the 

 formative cells are incited to rapid multiplication and the 

 resultant growth of the tissue leads to the formation of a 

 gall. Direct action on the cambium does not, however, 

 always occur ; " any actively growing tissue will respond '* 

 to the gall insect's presence and the stimulation may work 

 *' on tissue at a considerable distance from the centre of 

 application." It remains doubtful why each gall should 

 have its specific form and texture, though mechanical con- 

 siderations may afford some clue to the problem. Cosens 

 suggests as a factor the direction in which stimulation is 

 applied so that a gall of spherical form may be expected 

 as the result of influence " about equally distributed in all 

 directions." 



An especially remarkable feature in gall-formation is 

 presented by the differences in position, shape, and texture 

 observable in galls produced at different seasons by alter- 

 nating generations of the same species of gall-fly. These 

 seasonal forms of insects of the same kind differ markedly 

 from one another, so that there has been established a 

 wonderful reciprocal rhythm of growth and change between 

 the insects and the trees. The alternating seasonal genera- 

 tions of many common British gall-flies are described by 

 H. Adler and C. R. Straton (1894). The grubs which are 

 found in early summer within the '' oak-apple " (Plate XV, A) 

 develop after pupation into a gall-fly known as Teras 

 terminalis with winged males and wingless females. After 

 pairing the females go underground and lay their eggs in 

 the oak- roots, whereon, after the hatching of the larvae 

 large root-galls (Plate XV, B), are formed in which the grubs 

 live and grow through the winter, being finally transformed 

 into adults, which being all females and all wingless, 

 were formerly regarded as belonging to a genus and species 

 distinct from those of the oak-apple gall-flies and known as 

 Biorhiza aptera. It has now been shown that these virgin 



