248 Forestry Quarterly. 



downward again, occasionally turns once more, and finally after 

 it has taken on a shorter and stouter form, bores through the 

 bark, forces itself slowly out through the narrow opening and 

 falls to the ground. 



The length of the tunnels varies considerably and seems to be 

 determined by the amount of available food which the larva finds. 

 In slow-growing mountain ash stems the greatest observed dis- 

 tance up and down was i meter, while in a succulent willow 

 sprout the distance was from 20-30cm. In the latter space, how- 

 ever, were found three turning points. 



Frequently, several larvae were found in the same slender 

 shoot. For example, in a small stem of mountain ash less than 

 I cm. in diameter were found four larvae at the same time. 

 Their paths frequently crossed. 



Kienitz observed that in a cross section of a stem, the large pith 

 flecks were always found near the outer edge of the growth ring, 

 the smaller ones in the early wood. (Fig. 5.) From this he 

 concluded that there is only one generation a season, since if there 

 were two generations of the insects, more than one layer or 

 region of broad spots would be found in the same growth ring. 

 He also concluded that the larvae in mountain ash, various species 

 of willows, and Betula verrucosa were of the same species. 



In the tunneling process the larva does not completely devour 

 the cells but seems only to tear them open, presumably using for 

 the purposes the hooks on the end of its mouth (Fig. 3), after 

 which it appears to suck up the cell contents. The torn cell walls, 

 which are still soft, are compressed by the advancing larva and in 

 this way a narrow passage is formed. These tunnels, as well as 

 the larva which is of the same color as the cambium cells, are at 

 first hard to see since the decomposition of the destroyed cells has 

 not begun and the radial diameter of the passages is very small. 



Since cambial activity continues undisturbed on both sides of 

 a mine, while at the same time cell formation in the mine itself 

 ceases, the radial diameter of the tunnel increases after the larva 

 has passed. Consequently, pith flecks are larger in rapid-grow- 

 ing sprouts than in slow-growing shoots. In time, the cambium 

 layer (Fig. 6, a) bridges over the mine and afterward produces 

 normal xylem and phloem. 



In the meantime, some of the uninjured cells on the cortical 

 side of the channel become rounded and increase in size in cross- 



