220 OUR HOUSEHOLD INSECTS 



suppositions, one would imagine that there would be 

 at least as much to be obtained out-of-doors as inside, 

 and probably a good deal more. 



Such species as hibernate in the house, like Pollenia 

 rudiSj no doubt come in for the sake of shelter from 

 winter's cold. These gradually accumulate, instead of 

 suddenly coming in a swarm. An instance has been 

 recorded of two other species of the same genus swarm- 

 ing in the same building, to the exclusion of others, 

 for seven successive years. The suggested explanation 

 was that\ there were certain conditions which facili- 

 tated the entrance of the flies, but rendered their exit 

 difficult. 



