Pith-flecks are minute brown specks or patches found in the wood-layers of many trees, and consist of dead parenchymatous thick-walled cells, reminding one of the structure of pith. They are explained as due to the borings of minute insects, Diptera or Beetles, the larvae of which pierce the cortex and phloem and bore their way into the cambium. The latter then occludes the tunnels by filling them up with cells, and continuing its wood-forming activity gradually buries them deeper and deeper in the wood. Such pith-flecks are common in Willow, Birch, Alder, Sorbus, etc. It is possible that they may be due to other causes also in other trees.