﻿Grossenbacher: Medullary spots and their cause 229 



At first the larva has but one hook and well-developed girdles 

 of plates which increase in number per segment from the first to 

 the last: the first segment having one, and the last or twelfth, from 

 seven to nine. This is also characteristic of the Prunus miner as 

 given in my former paper. The second hook eventually appears 

 on the left of the first. It is very small and lies in the curve of the 

 large one as is characteristic of an Agromyza larva. The length of 

 the larva increases from about three millimeters to twenty milli- 

 meters before emerging from the bark to pupate in the ground. A 

 girdle of small spines is developed on the second segment. They 

 project caudad. Before the end of larval life the number of plate 

 girdles is much reduced; the skin becomes thicker and wrinkles 

 crosswise. The pupa is three to four millimeters long, barrel- 

 shaped, and has the spiracles at the ends on the dorsal side. 



Some of Nielsen's figures of the causal insect were later re- 

 produced by Tubeuf* in an article on medullary spots. He made 

 the interesting observation, also frequently noted by the writer, 

 that the miners are present only in such trees of a species which 

 have active cambium throughout most of the growing season. In 

 the autumn they were found most numerous in trees growing in 

 wet places. He secured only the larval stages of the miners. 



Greenef recently gave an account of the pupation of a single 

 larva of the river birch miner discussed by Brown and described 

 the adult that emerged from it. The fly was thought to be 

 Agromyza pruinosa, and the opinion is expressed that possibly it 

 is the only species in America producing medullary spots. The 

 latter, of course, is a premature and very unlikely assumption even 

 as regards the dipterous insects that mine in the cambium of woody 

 plants; besides it has long since been shown to be incorrect as 

 regards cambium miners in general. At least one other cambium 

 miner is a lepidopterous insect (Opostega nonstrigella) . Owing 

 to the fact that our knowledge of the subject is so very fragmentary 

 and incomplete it is evident that we are not in position to generalize 

 as yet. In fact if Greene's statement on page 474 of the above 



* Tubeuf. K. von. Uber die Zellgange der Birke und an. 

 Naturwiss. Zeit. Forst- und Landwirt. 6: 235-241- ipoS. 



