100 THE CANADIAN ENTOMOLOGIST. 



there seemed to be a depression and ridge like the arcs of a great circle 

 cutting each other at right angles and dividing the sac into quarters. 

 The " sweat drops " stood out upon the ptilinum like little beads as it 

 expanded and relaxed gently. 



Suddenly the ptilinum apparently withdrew into the aperture of the 

 gall, being almost out of sight, and then it as suddenly swelled out ; at 

 the same time there was a change in the position of the " sweat drops," 

 and they were larger tlian before. 



After this great relaxation and emergence the fly came out rapidly 

 until the eyes were visible. The ptilinum extended far out above the 

 eyes, overshadowing them. The appearance of the visible part of the fly 

 at this period reminded one strongly of a back view of a head that is 

 bald, the eyes of the fly taking the place of the ears. Under the mag- 

 nifier the eyes were distinctly purplisli. 



After the head and part of the body were out of the gall the 

 relaxations of the sac were shown by depressions. These depressions 

 were across the top of the sac — laterally — and were four in number. 

 There were also three depressions at the back — horizontally. Some- 

 times the fly did not seem to gain much in an effort after a relaxation, 

 and at other times the amount of emergence was quite noticeable. 

 When the mouth-parts became visible they were seen to open and the 

 ptilinum to relax simultaneously with each effort. When the fly had 

 nearly emerged the efforts became greater, and the ptilinum sank very 

 much at each relaxation. 



After a final struggle the fly crawled slowly out upon the gall and 

 rested. The ptilinum partially collapsed, relieving the fly of its top-heavy 

 appearance, and gradually, and at the same time almost unnoticeably, it 

 was all seemingly absorbed, and a very natural looking fly was before us. 

 We noticed the emergence of a number of these insects, with the same 

 results, as far as we could determine. 



The disappearance of the ptilinum at the final stage was almost a 

 mystery. It would seem to be gradually disappearing, and yet, all at 

 once, it was gone and there was not the slightest trace of it. Perhaps a 

 stronger microscope would have revealed more. 



We opened some of the galls before the flies emerged to see how 

 heavy a door they had to open. We found the pupa in a small excavation 

 at the centre of the gall, and a circular channel, less than two millimeters 

 in diameter, leading from it to the outside of the gall, only the mere skin 

 of the enlarged stem serving as a covering. 



