On this subject we should first listen to Ferton, who more than any one else has studied the habits of the pompilids. After having observed that the size of the victims captured by each species cannot exceed certain limits, sometimes very large, the distinguished observer adds:
It is not only necessary that the pompilid should hunt for a spider which fills certain conditions of size, but it is necessary also that it should have a special kind of life which adapts it to the wasp's method of chase. The pompilid which takes customarily earth-inhabiting spiders will not attack those which build webs . . . and, reciprocally, one which chases spiders living in the free air will neglect those which live under the earth.
Let us give some examples of this. Let us take the above-ground spiders. Evagetes laboriosus hunts exclusively the wandering Lycosas, the jumpers (wolf-spiders), and does not always succeed in following them, because it is short-sighted. The six-spotted Pompilus catches the thomisid spiders which lie in wait among the flowers. The thick-headed Pompilus attacks the epeirid spiders in their orb-webs, and the apical Pompilus, studied by Fabre, knows how to extract the seges-trid spider from its silken funnel. The same specialization exists with the earth-inhabiting spiders. The ringed Calicurgus, the banded Pompilus, and the somber Pompilus pursue the burrowing lycosid spiders. The first makes the spider issue from its burrow, and Fabre supposes that "in the walls of a narrow burrow the wasp cannot direct its sting with the necessary precision," but, more audacious or more expert, the two other species do not hesitate to attack the victim in the depths of its burrow. Not less brave and still wiser are the pompilids which hunt the Mygales, for the burrow of these spiders is closed with a door and the spider within often grasps this, so that it is necessary to use some ruse to force it. The wandering Pompilus and the yellow-bellied Planiceps, which hunt the species of the genus Memesia, have no other tool than their appendages with which to do this, while to reach the Corsican Mygale the plicate Pompilus has an especial adaptation. "Its flat head, armed with an elongated hood, is," says Ferton, "a powerful lever which gives support to the mandibles when they have succeeded in getting under the edge of the door of the spider."
In order to hunt successfully, the pompilids have been obliged to conform in details to the habits of their adversaries. With certain species studied by Ferton these adaptive habits are of the greatest interest. The acrobatic Pompilus chases a very nimble theridiid spider, namely Li-thyphcmtus corollatus, which joins together tufts of grass with silken threads over which it darts away in flight with an acrobatic quickness, but the Pompilus is not less accomplished in this exercise. With rapidity it follows the frail threads, and when, as a stratagem, the spider lets itself fall on the soil, the Pompilus jumps after it. Here is something still better : When the wandering Pompilus chases Nemesia badia in the spring, it paralyzes the creature at the bottom of its hole, which is a simple shaft, but in the autumn it must manúuver a bit, for the shaft is then flanked with a lateral branch in which it can hide, and is fortified with a door. Then the Pompilus, says Ferton, carries away the doors "which cover the two orifices" and seeks to make the spider come out to repair the damage. It sticks its abdomen into one of the holes, but scarcely has it done so when it draws it out quickly and rests on the ground with its eyes turned toward the second orifice, its wings raised and vibrating, ready to dart upon the spider. Then it goes to the second door and does the same thing, and then begins again at the first. At other times it contents itself by giving certain knocks at one of the orifices, stopping and watching the second opening of the nest. Firrally the spider jumps quickly out of the hole, and flees with all its agility, but the hunter is quicker still and captures it within a few centimeters, and stings it. The yellow-bellied Planiceps has not yet arrived at this perfection, and enters one of the branches and loses the spider, which escapes by the other. All of the pompilids are not equally crafty, and some, although very few, hunt all kinds of spiders indifferently, and remain in the primitive condition in which, without doubt, all of the members of the family once lived. This is the case, according to Ferton, with Vachal's Pompilus, and in the United States, according to the Peckhams, with Pompilus marginatus.