﻿228 Grossenbacher: Medullary spots and their cause 



above cited paper before giving the life history of the Ribes miner 

 (Opostega nonstrigella) . A little later Record* published a very 

 similar review of the same literature and added some observations 

 on the occurrence of medullary spots in other hosts. Another 

 review of the early literature was given in a later paper by Brownf 

 in which he also records additional hosts and gives some details 

 regarding the mines and miners. Five very excellent plates give 

 interesting information about the mines. However, in his dis- 

 cussion of the occlusion of the channels made by the insects, occurs 

 an error that should not be left unmentioned: "The larva destroys 

 only those cells in its immediate path through the inner bark. . . . 

 As the cambium layer moves outward radially, the passage left 

 by the larva increases in diameter. For this reason, when growth 

 is very rapid, the pith-fleck spots are larger than when it is slow." 

 The error in these statements may be readily shown by quoting 

 from a few lines below out of the same paper: "The healing process 

 proceeds mainly from the bast pith-ray cells in the bark." This 

 shows that Brown's observations were correct and that the chan- 

 nels once made are not moved outward radially by further growth 

 of cells on the wood-side of the wound. In other words, it is 

 unlikely that subsequent growth could widen a channel made by a 

 cambium miner and it is very apparent that the mines are made 

 narrower by the enlargement of the normal cells on both sides. 

 Plate 4 of his paper shows how the channels are filled mainly by 

 proliferations from the rays on the bark side. 



The first to determine the most important stages in the life 

 history of a cambium miner was Nielsen.^ He studied the larva 

 in all its stages, the pupa as well as the imago, though the egg was 

 not found. The miner proved to be the larva of a fly, Agromyza 

 carbonaria. He found that the larvae fed in the cambium until 

 autumn and then entered the ground, where they passed the winter 

 as pupae. The adult flies emerge in May. 



* Record, S. J. Pith flecks or medullary spots in wood. For. Quar. 9: 244-251. 



t Brown. H. P. Pith-ray flecks in wood. Forest Ser. U. S. Dept. Agr. Cir. 

 215: 5-15- Pl- 1-5- 1913. 



X Nielsen, J. C. Uber die Entwicklung von Agromyza carbonaria Zett., der 

 Urheber der " Markflecken." Zool. Anzeiger 29: 221-222. 1905; Zoologische 

 Studien iiber die Markflecke. Zool. Jahrb. 33: 725-737. 1906. 



