Medullary Spots in Wood. 245 



Later, R. Hartig^ made use of the pith flecks in describing 

 woods, and Kienitz- utiHzed them in a wood key which he made 

 up for lecture purposes. Kienitz early came to the conclusion 

 that the spots were not normal developments and during the years 

 1881-1882 made some investigations of the cause of the for- 

 mations. The results were published^^ in 1883. 



He discovered that the pith flecks in species of Salix, Sorbus 

 and Betula were tunnels — filled by new cells — of an insect larva 

 which obtained its nourishment from the cells of the cambium 

 and new growth at the time of the formation of the growth rings. 

 This verified the hypothesis advanced at an earlier date and 

 referred to above. 



Kienitz observed the development of the larvae in small stems 

 of mountain ash and willow (Sali.v rubra Huds., S. viminalis L., 

 and S. caprea L.) Unfortunately, he was unable to grow the 

 insect to maturity or obtain the imago. He submitted some of 

 the larvae to Dr. Gerstacker, who identified them as dipterous 

 insects. 



Kienitz concluded that the insect winters in the pupal state in 

 the ground and that the mature insect lays its eggs singly on or in 

 the young sprouts or any tender stem of the woody plant. In 

 May or June the eggs hatch and young larva (Fig. 2) eats a path 

 or tunnel into the cambial zone. In the beginning the tunnel is 

 very narrow corresponding to the relative size of the larval body. 

 The larva grows little in thickness but considerably in length, 

 finally becoming 2-3 cm. long. The tunnel which remains very 

 narrow radially widens in a peripheral direction, since the larva 

 eats to its right and left. Only the cells of the cambium and of 

 the very latest formed wood are eaten, even by the full grown 

 larvae. 



In all cases observed, the tunnels were directed downward at 

 first. When the larva has penetrated to a certain depth, it turns 

 about without building a loop, making the tunnel only a little 

 wider at this turning point. The location of this turning point 

 varies, often being at the root collar or perchance, in a lateral 

 root. After turning back it follows the old tunnel for a short 

 distance, usually about the length of its body, and then starts off 

 in an oblique direction so that the two tunnels make an acute 

 angle with each other. (Fig. 4.) The larva, as a rule, does not 

 go very far upwards but reverses itself and eats with its head 



