"At the right moment" they invincibly obey some sort of mysterious and inflexible prescription. Without apprenticeship, they perform the very actions required, and blindly accomplish their destiny.
Then, the moment having passed, the instincts "disappear and do not reawaken. A few days more or less modify the talents, and what the young insect knew the adult has often forgotten." [(8/4.)]
Among the Lycosae, at the moment of exodus, a sudden instinct is evolved which a few hours later disappears never to return. It is the climbing instinct, unknown to the adult spider, and soon forgotten by the emancipated young, who are destined to roam upon the face of the earth. But the young Lycosae, anxious to leave the maternal home and to travel, become suddenly ardent climbers and aeronauts, each releasing a long, light thread which serves it as parachute. The voyage accomplished, no trace of this ingenuity is left. Suddenly acquired, the climbing instinct no less suddenly disappears. [(8/5.)]
The great historiographer of instinct has thrown a wonderful light, by his beautiful experiments relating to the nidification of the mason-bee, upon the indissoluble succession of its different phases; the lineal concatenation, the inevitable and necessary order which presides over each of these nervous discharges of which the total series constitutes, properly speaking, a mode of action.
The mason-bee continues to build upon the ready-completed nest presented to her. She obstinately insists upon provisioning a cell already duly filled with the quantity of honey required by the larva, because, in this case as in the other, the impulse which incites her to build or to provision the nest has not yet been exhausted.
On the other hand, if we empty the little cup of its contents when she has filled it she will not recommence her labours. "The process of provisioning being complete, the secret impulse which urged her to collect her honey is no longer active. The insect therefore ceases to store her honey, and, in spite of this accident, lays her egg in the empty cell, thus leaving the future nursling without nourishment." [(8/6.)]
In the case of the Pelopaeus, Fabre calls our attention to one of the most instructive physiological spectacles that can be imagined.
While the mason-bee does not notice that her cell has been emptied, the Pelopaeus cannot perceive that the tricks of the experimenter have resulted in the disappearance of her progeny; and she "continues to store away spiders for a germ that no longer exists; she perseveres untiringly in her useless hunting, as though the future of her larva depended on it; she amasses provisions which will feed no one; more, she pushes aberration to the extent of plastering even the place where her nest was if we remove it, giving the last strokes of the trowel to an imaginary building, and putting her seals upon empty nothing." [(8/7.)]
From these facts, and others, no less celebrated, which show "the inability of insects to escape from the routine of their customs and their habitual labours," Fabre derives so many proofs of their lack of intelligence.
The Epeïra fasciata is incapable of replacing a single radial thread in the geometrical structure of its web, when broken; it recommences the entire web every evening, and weaves it at one stretch with the most beautiful mastery, as though merely amusing itself.