It is, then, certain that some pompilids do not hunt, do not paralyze, do not nest, and yet assure the future of their progeny by ravaging others of the benefit of their work. How can such an inversion in the ordinary habits of the family be brought about?
Beginning with 1890, Ferton observed that Pompilus pulcher and P. viaticus carry the paralyzed victim to the nest which they make, and in 1901 that Salius vachali enters sometimes in an effort to rob them of their prey ; and these struggles and the thefts which follow appear to occur more frequently still with Pompilus rufipes which lives in colonies in the warm sand-pits. Ferton notes :
They are a colony of crafty rogues, always in motion, rummaging about, often hunting for the spiders of the neighborhood, entering the holes which they find in their desire to hunt the proprietor. If they succeed in stealing a victim, they bury it, if a new thief does not interpose and oviposit on it. Sometimes these thefts are the cause of very lively fights.
However, this species is not frankly a parasite, but is only inclined to robbery. Every day Ferton saw it hunt and oviposit honestly in the same locality.
What is accident with P. rufipes becomes the rule in a related species, P. pectinipes. From the numerous and patient observations carried on at Bonifacio by Ferton, it seems that this species examines the ground with its antennae in order to discover the nests of other pompilids, and opens a discovered burrow, penetrates it, detaches the egg fixed upon the victim, substitutes its own, and then closes the violated burrow. This Pompilus is closely related to P. rufipes and P. argyrolepis whose nests it exploits. This is, says Ferton, a P. rufipes which has taken on the habit of theft, and has therefore lost the habit of nesting.
And so by successive steps we are led to the habits which characterize Ceropales. We cannot do better than to approve the opinion of J. Perez upon the origin of parasitism with the pompilids and the parasitic bees : " Instinct in general is like an organ, subject to variation; it is even more variable than an organ. 'The cessation of laborious habits," once acquired by heredity, is ordinarily "followed secondarily by the atrophy of the working structures," for all change of habits modifies more or less the physiological condition and reacts, therefore, upon the organism. Bor-dage (1912) does not believe "that it suffices that a female predatory Hymenopter should proclaim . . . her right to idleness" in borrowing a neighboring nest, in order that this habit should become hereditary, and he invokes an internal modification at the same time physiological and organic. This modification surely exists, but it has been produced by the change of habit. One cannot admit, on the other hand, with Marchai (1890) that the parasitism of our Hymenoptera results primitively from a modification of structure of the organs, for Pompilus pectinipes differs in no respect from related non-parasitic species, and the oviscapt of Ceropales seems, indeed, to be the product, not the cause, of parasitism. Here is verified the truth of the old biological adage, "Function creates the organ, but the organ lends itself to functions."
It appears from this long exposition that in all or in part of their instincts certain species of the family do not yet present any perfect specialization. These ambiguous forms are evidently more malleable than the others in the psychological domain, and one conceives that analogous forms by apprenticeship and heredity have been able to produce new psychic types by orientating their actions in different ways.
Thus the comparative psychology of the pompilids appears to us full of instruction, for it makes evident the possible ways which they have probably followed in their psychic evolution. We have noted in these direction many stages, not always sufficiently to give an exact view of the different possibilities, but do not forget that the family includes almost a thousand species and that fifty at most of these have been the object of biological research. What rich and suggestive fields are reserved to the zoologist in studies of the pompilids of the tropics! And how fertile should this comparative method prove to clear up the past in the light of the present !