Certain pompilids make no nest. Aporus bicolor deposits its prey in a crevice, and Evagetes laboriosus in a shell or in a cleft in the ground; others, like the buffoon Calicurgus, scratch some bits of mortar out of old walls to make their hiding-places, or, like the banded Pompilus, dig only slightly; but most of them are adroit miners which dig a hole at the bottom of which they arrange a chamber for the victim.
Having been acquired progressively, the mining art can be lost the same way. It is notably lost with those pompilids which chase the earth-inhabiting spiders, and who use as a nest the burrow of the victim. This is the habit of the wandering Pompilus, hunter of the Nemesias, and which, when one offers it a free-living spider, paralyzes it, digs a hole, but no longer knows how to arrange it, and abandons the job. Ferton says :
Thus, P. vagans, accustomed to nest in the burrow of its victim, has become incapable of digging a nest when she wants one. She is, however, still a digger, since she knows how to dig . . . and it is not the tools nor even the habit of digging which are lacking. Just as an organ becomes atrophied from want of use, an unused instinct becomes atrophied after a while even when the tools which it used have not become modified.
There are also among the pompilids mason species which construct the nest with bits of plastered earth. Our Pseudagenia punctum builds in this way a nice cellule in the form of a little tunnel, and the Peckhams state that the nest of Pseudagenia bombycina is composed of sixteen contiguous cells. To understand the origin and development of this new industry, we must study the manner in which other pompilids arrange for the closing of their nests. Most of them content themselves with sweeping sand over it and scratching it about with their legs. Some, like Pompilus quadripunctatus studied by Ferton, puddle the closure with the end of the abdomen. Evagetes laboriosus closes the entrance "with a thick barricade of little stones, with bits of earth, and with debris of twigs and leaves," and Agenia variegata with balls of spider-web which it compresses into a compact tissue. Agenia struct or comes nearer the mason species, nesting in a shell, says Ferton; she closes the orifice "with a stopper of finely plastered earth" which she fortifies with a barricade of stones and bits of earth. Sometimes she makes several cells, like the preceding species, but instead of separating them with grains of quartz or limestone she employs materials similar to those of the stopper.
Pompilus scelestus is one of the rare burrowing species which prepare the burrow before leaving for the chase. Most of the others capture their victim first, place it on a tuft or in a hollow, sometimes even hiding it under a layer of sand, and then devote themselves to the preparation of the nesting-hole. The transition between these two very different methods is shown us by species which make no nest, which leave their prey in the hole of the victim, and by diggers like Pompilus viaticus and P. cingulatus, which build two cells in the same nest or put two victims in each cell. The momentary abandonment of the prey is not without inconvenience, for it leaves a chance for robbers. Nicolas (1888) reports that certain Pompilus viaticus will themselves be deceived indefinitely when they leave their victims while they make their burrows, while others very quickly become alive to the situation, like the yellow-winged sphex in Fabre's experiments. Pompilus pulcher employs another method. Ferton has seen it hide its prey under a light layer of earth and proceed immediately to hunt for a convenient spot in which to dig its hole. When this is half dug the Pompilus returns to its victim, carries it to the hole, and enters the hole with it. It has not, however, finished the digging. "1 have seen it, '9 says Ferton, "come out from time to time to push away the debris. . . . The wasp is thus enclosed with its spider while finishing its task, which may last half an hour.19 For the Pompilus, the fear of the robber is the beginning of wisdom.