Stem-Borers — Galls 303 



sects eat away the wood with their strong mandibles 

 and pass the fragments through their food-canals as 

 they bore their way onwards. Like many of the root- 

 feeders the wood-boring grubs are, as a rule, long- 

 lived ; the well-known caterpillar of the "goat" 

 moth takes over three years to reach its full size. In 

 most cases the wood-boring habit is confined to the 

 larval stage, the perfect insect living in the open air. 

 The female Moth lays her eggs on the bark, leaving 

 the grub when hatched to bore its own way into the 

 wood ; but the Sawfly pierces the stem with her 

 ovipositor so as to place the egg where the new-born 

 larva will find itself amid congenial surroundings. 



Galls. — The vegetable-feeding insects, whose habits 

 we have briefly surveyed, necessarily mutilate the 

 plants on which they live, riddling the leaves, eating 

 away the roots, or tunnelling through the wood. Some 

 insects, however, have the power of modifying the 

 growth of the plant which is to afford them food. 

 Around the spot where the female insect has laid her 

 egg an excrescence or gall is formed, within which the 

 grub shelters and feeds, the main mass of the plant 

 remaining unaffected. The production of galls there- 

 fore seems calculated to supply the wants of the 

 insect with as little damage as possible to the plant. 

 Galls are of the most various shapes, smooth and 

 spherical, like fruits : the well-known oak-apple for 

 example, flat and round like buttons, or spreading out 

 in leaf-like rosettes. One or more chambers may be 

 found within each gall, containing the growing grub. 

 The precise cause of the formation of galls has long 

 been a subject of discussion among naturalists. Galls 

 are known to be due to the activity of the meristem-cells 

 of the plant under some stimulus, which has been 

 variously ascribed to the mechanical irritation caused 

 by egg-laying or to chemical irritation from a fluid 



