In investigations of this kind it is necessary to know how to concentrate one's effort and to follow the actions of a species in their slightest details. No discipline is less harsh, for the life of these animals is full of captivating incidents and certain of these incidents have a scientific bearing. I followed in 1900, at Luc-sur-Mer, the goings and comings of a predatory wasp, Philanthus triangulum, which hunts bees and stores them in cells at the bottom of its long burrow. At that time the sand-dunes of Luc extended up the breaches in the cliffs, and the Philanthus wasps nested there by the thousands, to the great injury of the bee-keepers of the neighborhood. Every Philanthus which returned to the nest carrying a bee, first opened its closed burrow, then penetrated it to place its prey, then some minutes later returned to the orifice to scratch the sand together and close the burrow again. This cleaning up done, the insect rests for a longer or shorter time in the interior of its home, after which it reappears at the entrance, which it opens and where it shows its yellow face before taking flight. In every case, before starting out on another hunt, it comes out of the burrow and completely closes the orifice with sand, which it moves about with its hind legs. This is always done by the Philanthi of the sand-dunes, but on the other hand some of the Philanthus wasps nest in the vertical walls of the breach, and these never close the holes. It is not incapacity which causes them to neglect this work, for they know just as well as the others how to close their burrows every evening, or even during the day when a robber wasp tries to get in. It is necessary to attribute the striking differences which exist between the Philanthi of the dunes and their brothers of the breach to the difference in the places of nesting. The soil of the one being argillaceous and compact, the insect in penetrating the burrow with its large victims does not injure the walls of its home like the Philanthus of the sand, and therefore has no need of repairing the walls, nor the door, in the way that the other does. On the other hand, the walls of the breach being vertical, the Philanthus would not be able without great difficulty to close its gallery at the moment of leaving, and it abstains from this task which is always performed with great punctuality by its neighbors of the dune. From all of this evidence, our bee-hunter can modify its customary habits and adapt itself to the conditions of the place where it lives.