298 THE CANADIAN ENTOMOLOGIST. 



From all galls collected there were about an equal number of males and 

 females which emerged. On May ist I examined a large number of galls 

 along the Poudre, and found at this time most of the flies had pupated ; 

 some few hid already escaped, a few were still larvae, and in one case an 

 adult female was found in the gall, having just completed its transforma- 

 tion to the adult stage, and was ready to emerge. 



It is interesting to note how the adult fly escapes from the gall, or 

 rather how it makes provision for its escape. If galls are examined 

 during the winter months many of them will be seen to contain a small 

 round smooth hole, usually near one end. If these galls are opened, the 

 larvae will be found at the end of a burrow leading to this opening, but 

 securely sealed from the outside by a plug, made from bits of wood 

 chewed off by the larvse in the process of making the hole. It is a 

 wonderful instinct that guides these larvae in making this hole, which the 

 adult could not possibly make itself, and were not means for its escape 

 from the cell prearranged by the larvae, death would be the inevitable 

 result. 



While examining galls I noticed that there were many that did not 

 have this hole by means of which the adult could escape, so proceeded to 

 determine the cause for this condition of affairs. A large number of these 

 galls were cut open, and not in a single case was a Euura larva found 

 within. There was, however, in nearly all of these the larva of a little 

 hymenopterous parasite belonging to the Chalcididse family. A number 

 of these galls were placed in a breeding cage by themselves, where the 

 little parasites soon emerged. The only explanation that I could give for 

 this condition was : That this parasite had worked upon the Euura in the 

 fall of the year, before the former had attained its growth and provided 

 means for its escape as an adult the following spring by gnawing the usual 

 hole in side of gall. The parasitized larva, being unable to withstand the 

 attack of its little enemy, perished, whereas the destroyer lived in the gall 

 in comfort throughout the winter months, and after completing its 

 transformations in the spring escaped by means of a small hole made with 

 its tiny strong mandibles. These little escape holes were plainly seen, 

 freshly gnawed in many of the galls from which the little parasites were 

 emerging. 



Two Cecydomiid flies were also found in this breeding cage, but came 

 from different galls, much resembling the others, but monothalamous 

 instead of polythalamous. 



