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Fabre got a file of pine processionary caterpillars onto the rim of a large palm vase until the circle closed, so each caterpillar followed the silk thread and the one ahead of it. With food a short distance away, they kept circling for about a week, sometimes in the cold, until fatigue and disorder broke the chain. Following the leader's trail, normally a way home, became a closed loop that no individual could detect.
Insect instinct performs complex tasks flawlessly in its normal sequence, but it cannot adapt when one step is disrupted. When Fabre moved a Sphex wasp's paralyzed cricket a few inches from the burrow while the wasp was inside checking it, the wasp dragged the cricket back and inspected the burrow again, as many times as he repeated the trick. A mason bee whose cell he pierced went on filling and sealing it as if nothing had happened. Each act is triggered by finishing the previous one, not by understanding the goal.
Solitary hunting wasps such as Cerceris, Sphex and Ammophila sting their prey precisely in its nerve centres. This paralyzes the prey without killing it, and the number of stings matches the prey's nervous anatomy. The larva's food therefore stays motionless but alive and fresh for weeks, so the mother wasp can provision offspring she will never see.
Fabre argued against Darwin's gradual evolution of instinct. His case was that a wasp's paralysing technique only works if it is complete. A sting that is slightly misplaced leaves the prey able to fight back or leaves it dead and rotting, so the larva perishes. He concluded that the intermediate steps natural selection would require could not have survived.
Fabre's harmas, a patch of wasteland he turned into an open-air laboratory, stands for a method: watch the living creature in its own setting for years and test it with small, cheap interventions. He preferred this to dissecting specimens in a laboratory. Behaviour, timing and life history show up only through patient repeated observation of the living animal, and anatomy alone would miss them.