268 INSECT LIFE. 



close to the face of the cliffs inhabited by them. 

 The openings to their burrows are just large enough 

 for a bee to enter, but a short distance from the open- 

 ing the burrow is enlarged so that a bee can turn 

 about in it easily. This feature and the small size of 

 the opening distinguish the burrows of Halictus from 

 those of other common cliff-dwellers. In the sides of 

 this comparatively large burrow there are many 

 small openings leading into cells, in each of which is 

 placed a supply of food and an ^g^. The walls of 

 these cells are glazed like the surface of pottery. It 

 is said that several females unite in making the larger 

 burrow, after which each female makes passages ex- 

 tending sidewise from this main burrow or public 

 corridor to her own cells. If this is true a cliff in- 

 habited by Halictus may be compared to a city com- 

 posed of apartment houses. 



Certain cliff-dwelling bees, which are much larger 

 than Halictus, resembling the honey-bee in size, con- 

 ceal the entrance to their burrows by building over 

 each a tube which is bent downward. 



Some solitary wasps and many species of digger- 

 wasps are cliff-dwellers. The nests of these can be 

 recognized by the fact that they are provisioned with 

 insects or spiders. 



THE MINERS OF THE PLAINS. 



Although the sides of sandy cliffs afford the min- 

 ing insects the best of conditions for building their 

 nests, both as regards the economy of labor and in 

 protection from drenching rains, there are many spe- 

 cies that prefer to mine in level ground. The fol- 

 lowing are among the more common of these : — 



