﻿Grossenbacher: Medullary spots and their cause 233 



was so frequently found to be in groups because of the large number 

 of flies present as compared to the few lenticels in which oviposition 

 could occur. Both in nature and in the cages the deposition of the 

 eggs seemed to have been accomplished by the insertion of the 

 ovipositor through the lenticel; as may be inferred from Fig. 4. 

 This is a photograph of a shoot from one of the breeding jars 

 after removing the periderm from a portion of it. 



The larvae always emerged from the end of the egg turned 

 away from the lenticel and fed first in the cortex just under the 

 periderm, mining parallel with the surface of the bark from half a 

 millimeter to three millimeters; then straight in toward the 

 cambium, where they continued their mines up or down depend- 

 ing upon how the eggs had been inserted. Most of the eggs 

 were inserted so that their long axes coincided with that of the 

 plant. In cases where the eggs had been inserted at an angle with 

 the long axis the larvae always turned in the direction of the lesser 

 angle with the stem, irrespective of whether it was up or down 

 the plant. The direction of the first mine therefore depends upon 

 oviposition and perhaps the direction of the long axes of the 

 cambium cells encountered by the larva and is not uniformly up 

 or down as suggested by Kienitz, Greene, and others. For 

 example, in the case of those shown in the above cited figure there 

 is a group of two eggs from which the larvae would have mined first 

 toward the distal end of the shoot, while from the group of three 

 they would have mined toward the base of the plant first. Some 

 larvae were found which had mined as far as four decimeters in 

 the cambium in one direction ; others had mined only about a deci- 

 meter and then reversed the course and mined in the opposite direc- 

 tion, after having veered to one side from the old course. The 

 emerging larvae were about 2-2.5 nim. long and not more than 

 0.2 mm. in diameter. They were comparatively numerous: four 

 empty egg cases were found at lenticels of a piece of a P. avium twig 

 which was only about six millimeters in diameter and three deci- 

 meters long. The larvae were collected into alcohol. 



The egg stage of these cambium miners is very short, apparently 

 less than three days; while the larval stage lasts at least eleven 

 months. The pupal stage lasts perhaps about three weeks, and 

 the flies apparently oviposit within two days after emerging from 

 the puparia. 



