SOME ENEMIES OF THE TREES 109 



mies. For example, who has not observed the 

 thick clusters of slender twigs called "thunder 

 besoms," or "old witch's broom"? Strange 

 tales used to be told concerning them. But now 

 the witch that makes them has been caught red 

 handed. She is a tiny fly, whom Mother Na- 

 ture has cunningly taught to pierce a twig and 

 lay her eggs in the pith. In this snug nursery 

 hosts of little grubs hatch in the warm spring 

 sunshine. While they live and feast inside the 

 branch, it puts forth a cluster of little deformed 

 shoots or "brooms" — the sign manual of a fly 

 nursery. 



The familiar "oak-apples" are leaves which 

 have fallen victims to the gall-fly. She puts an 

 egg into each middle vein, and by a queer proc- 

 ess of Nature's own, turns it into a brittle shell, 

 which gradually fills and swells with a greenish- 

 white spongy substance. In the center of this 

 is a hard, pea-like kernel, containing a tiny 

 white grub. Oak apples turn brown and fall 

 with the leaves, and in the spring the little grub 

 is transformed into a gall-fly and eats its way 

 out. Occasionally the gall-fly pierces a hole in 

 a branch or twig and deposits her egg. Then 

 woe to the victim I The wood becomes diseased 

 and changes color and presently a queer lump 

 or knob is formed. In the very heart of this 



