The egg of the pompilid is always attached to the abdomen, on the back or on the sides, and when the victim is caught upside down, which is rare (Agenia), on the ventral side. As to the larva which comes out of it and which attaches itself exactly at the same point, it is protected by the host, which folds its legs and avoids any movement capable of wounding it. The same observation applies also to all ectoparasites.
I believe that Ferton has caught an inkling of the reason for this singular arrangement. Ee-turning to Nemesia badia, the victim of the wandering Pompilus, "Never," he says, "is the egg or the larva touched. The spider avoids it with its spiny leg as though it were a painfid wound." Whatever be the impression produced by it, it seems that the spider avoids rendering this in> pression more acute by touching the organism fixed to its side. He acts like a wounded person who refrains from touching his wounds, in order not to make them sorer. It is a phenomenon of differential sensibility.
Eabaud also attributes to differential sensibility the instinctive fear which spiders show in regard to their aggressors. But here there are two phenomena of different kinds : the flight or the defense of the spider on which the predator shapes his attack, and the complete inertia of the victim when it is just going to be captured. The first of these phenomena seems to have its origin in experience and the acquirement of heredity, but the eataleptic state which characterizes the second is a consequence of differential sensibility. I add that this state singularly facilitates the task of the pompilid, which can sting its victim with certainty and even place its egg npon it when it has lost the instinctive habit of stinging.