﻿Grossenbacher: Medullary spots and their cause 237 



fact that longitudinal resistance or compression has been elimi- 

 nated while in case of the others only the lateral pressure has been 

 reduced. Neither diameter of a mine is increased by subsequent 

 growth as suggested by Greene but all available evidence indi- 

 cates that a considerable decrease occurs in both diameters during 

 the occlusion of a cambium mine. 



It appears that ordinarily the radial diameter of a channel is 

 but little greater than that of the miner while the tangential diame- 

 ter may be several times that of the larva. The principal reasons 

 for this seem to be that the portion of the cambium most desired 

 by the larvae for food is a sheath only a few cells in thickness and 

 that they feed with the lateral sides toward the bark and wood. 

 In that position the hooks with which the cell-walls are ripped 

 open also lend themselves more readily to foraging a wide path in 

 the plane of the cambium mantle than at right angles to it. The 

 tangential widths of mines made by larvae of a certain size seem 

 therefore to depend upon the relation of the movement and feeding 

 impulses dominating them. When the cambial cells about a miner 

 are in just the right stage to be most agreeable for food and the 

 larva is hungry, and besides has no special desire to travel, it makes 

 a very wide path ; while if the cells encountered afford less suitable 

 food or the insect is dominated by an impulse for movement rather 

 than for feeding, the mine is made only large enough to permit its 

 passage. That is, the mines have a greater tangential diameter 

 at times chiefly because the larvae feed more voraciously and 

 browse more widely at some times than at others. Compare the 

 two medullary spots of FiG. 14: the lower resulted from a mine 

 made in early summer, and the upper from one made in mid- 

 summer. Both appeared to have been made by the same larva, 

 the lower one while the insect was younger and smaller. 



The irregular mass of cells making up the tissue that occludes 

 or fills up a channel left by a miner has the general appearance of 

 pith. Like pith these occlusion masses come to serve as places 

 for storing elaborated food. The chief difference between the 

 appearance of sections from the two places lies in the greater 

 variation in the size and shape of the medullary spot cells, as 

 shown especially in FiG. 9. Sections of material collected in sum- 

 mer show that in general the cells occluding mines do not lignify 



