I will first invoke testimony which cannot be impugned,-that of Fabre, an intense believer in the changelessness of instincts. The venerable biologist says :
Pure instinct, if it'existed alone, would leave the insect unarmed in the perpetual conflict of circumstances. ... A guide is necessary. . . . This guide the insect possesses to a very evident degree. It is the second domain of its psychology. In this domain it is conscious and can improve by experience. Not daring to call this aptitude rudimentary intelligence, which is too advanced a title, I will call it discernment.
Of this aptitude the '1 Souvenirs Entomologiques ' ' show us many examples.
Many predatory Hymenoptera capture and paralyze for their young different kinds of prey, choosing, however, species of the same group. With certain forms this taste is extremely localized, and this is notably observed with the wasps of the genus Sphex, notably with our old acquaintance the yellow-winged sphex. This species captures only crickets, and would it be capable of changing its habit in case of emergency and taking prey of another kind 1 "To prove such facts," says Fabre, "would be of prime importance, for it would teach us whether the inspirations of instinct are absolutely immovable or vary within certain limits." But the indefatigable biologist of Serignan has had the "good fortune to note a case of this kind." Upon the banks of the Rhone he saw this sphex "attack, not a cricket, but a common grasshopper." This observation has been doubted by Kohl after an examination of Fabre ?s figures, and he attributes the observation to another species of wasp,-Sphex maxillosus. I think, however, that Fabre had studied the yellow-winged sphex too often to be deceived in this instance; but if Kohl is really right, it is Sphex maxillosus which can modify its customary habits, because, according to Ferton and Kohl, it is not accustomed to hunt crickets, and Picard has often seen it attack victims of this kind.
We have known for centuries that the sacred scarab makes little balls of the excrement of the horse or of the mule and that it rolls these little balls to a hole where it buries itself with them. But what we did not know, and what Fabre himself did not know for a long time, is the relation of this practice to the conservation of the species. This relation is, in fact, indirect. The excrement of the little ball serves exclusively for the nourishment of the female; but where is the egg? And what will nourish the larva when it is born ? After a long career of biological research of the most careful kind, Fabre solved this problem. For its progeny the beetle chooses sheep dung, finer and more oily than that of the horse, and makes a round ball of it. Then it makes a little hole in this ball and places there its large, ovoid egg, resembling a bead of amber, and covers the hole carefuly, modeling the ball into the shape of a pear and transforming its narrow end into an egg chamber. Upon the stony plateaus where this insect almost always works it is very difficult to dig a hole and make a comfortable nest. Therefore the round ball is made on the surface and rolled to the spot where the beetle is going to bury itself with it to watch over the hatching of the young.
In the cages where Fabre raised the insect in soil which he brought in, and when, very rarely, the free beetle found a soft soil, the method was quite different. The beetle dug its hole, gathered its sheep dung, and then enclosed itself in order to prepare the little ball into its pear-shaped form. Thus the ball-roller modified its instinctive habit under fortuitous conditions when it found itself near soft earth in which it could make its nest rapidly. In other words, it discerned the favorable character of the soil and adapted itself to these conditions.1