6 PITH-EAY FLECKS IX WOOD. 



eggs singly, apparently in the young branches of the tree. Kienitz, 

 unfortunately, failed to observe the manner of entrance into the 

 cambial layer, but found the young larvae in infested trunks as early 

 as May, eating passages in the cambium and in the soft tissue of the 

 inner bark. At first these passages are narrow, onty a few cells 

 broad, and lead downward. As the larvae proceed downward they 

 grow but shghtly in thickness, though considerably in length. Finally 

 each one becomes 2 or 3 cm. long. The passage, Kienitz says, grad- 

 ually widens as the insect grows in size. It spreads out laterally as 

 the larva turns as far to left or right as its worm-shaped body will 

 permit. In a radial direction the wound is always slight, since the 

 larva attacks onl}- the growing cells of the cambium. After it has 

 proceeded a certain distance downward, the larva abruptly turns, 

 and, without forming a loop, starts upward. The size of the passage 

 is increased onty shghtly at this turning point. 



The position of the turning point in the tree A^aries. Often it is at 

 the dividing point between root and stem, or else in a lateral root. 

 Sometimes the larva turns but once or twice, while m other cases it 

 may turn a number of times. Finally it assumes a somewhat shorter 

 and more compact fonoi, bores through the bark, emerges from the 

 narrow opening, and faUs to the ground. Kienitz did not observe 

 the adult form. 



Investigations of the msects mvolved are being carried on by the 

 Bureau of Entomology through its branch of forest insects. 



ORIGIN OF PITH FLECKS. 



The present investigation confirms the conclusion of Eaenitz, pub- 

 lished in 1883, that pith flecks are caused by the larva of a dipterous 

 msect living in the cambium durmg the gro-v^dng season. Specimens 

 of river birch were examined as early in the season as April 25. 

 Growth had then begun, as shown by the ease with which the bark 

 peeled from the wood. At this time the larvse were burrowing ver- 

 tically downward m the cambium and had already reached a pomt 

 about 6 feet above the ground. They were 1| cm. long, 1 mm. in 

 width, and pure white, except for the dark mouthparts. 



To determine the manner in which the larvae obtained entrance into 

 the cambium, a number of miaes were traced back to their source. 

 This could be done with reasonable care, because at this early date 

 growth had not progressed far enough to cover up or obliterate the 

 mines. The}" had their ongm invariably in the upper part of the 

 tree's crown and on branches of about 5 years' growth. In such cases 

 the mine could be traced to a suppressed lateral twig. At the point 

 of attachment of this to the maia branch, and always on the side 

 toward the groxmd, a dark coloration could usually be detected 

 extending from the outer bark into the cambium. This appeared 



