The first wasp that we saw was just alighting with a medium-sized green caterpillar near a partly closed nest. When disturbed she flew away, but soon returned, dropped her prey half an inch from the nest, proceeded to clear the opening, ran inside to see that all was right, and then backed in with the caterpillar. Emerging after a few minutes, she placed a small pebble in the doorway, which was thus partly closed, and flew away. She brought three more caterpillars at intervals of thirty minutes, and then, after wedging a pebble into the neck of the opening, she began to fill it in solidly, scratching in dirt and packing in lumps of earth which were brought in her mandibles. We did not allow her to complete this operation, as it would have made excavation more difficult, but caught her and dug out the nest. The tunnel ran down obliquely for five inches, being two inches below the surface at the pocket. In it we found a wasp larva, which was at least three days old, and four caterpillars. There were no signs of the banqueting which must have already taken place. We carried this larva home with us, and it ate the caterpillars up clean, finishing with a fifth which we supplied from another nest, and going into its cocoon on September sixteenth. The caterpillars all wriggled about on the slightest stimulation, and remained in this lively state until they were eaten. They belonged to four different species.
In a second nest to which food was being carried, we found four caterpillars and a larva about three days old, all the conditions being like those in the other example. Evidently the larva had been fed from day to day, since four or five days must have elapsed since the making of the nest.
Westwood states that Ammophila, when she has captured her prey, walks backward, dragging it after her;[1] but in all the cases that came under our notice she went forward, the caterpillar being grasped near the anterior end, in her mandibles, and either lifted above the ground or allowed to drag a little if long and heavy. It is usually held venter up, but in one case, in which the wasp, while carrying it to her nest, frequently laid it down and picked it up again, it was held with the venter down or up indifferently.
The all-important lesson that Fabre draws from his study of the Ammophiles is that they are inspired by automatically perfect instincts, which can never have varied to any appreciable extent from the beginning of time. He argues that deviation from the regular rule would mean extinction. For example, if the wasp should sting ever so little to one side of the median line the prey would be imperfectly paralyzed and the egg would consequently be destroyed; or a sting in the wrong place might cause the death of the caterpillar and thus the death of the wasp larva, which, he thinks, can be nourished only by perfectly fresh food.
The conclusions that we draw from the study of this genus differ from these in the most striking manner. The one preëminent, unmistakable, and ever present fact is variability. Variability in every particular,—in the shape of the nest and the manner of digging it, in the condition of the nest (whether closed or open) when left temporarily, in the method of stinging the prey, in the degree of malaxation, in the manner of carrying the victim, in the way of closing the nest, and last, and most important of all, in the condition produced in the victims of the stinging, some of them dying and becoming “veritable cadavers,” to use an expressive term of Fabre’s, long before the larva is ready to begin on them, while others live long past the time at which they would have been attacked and destroyed if we had not interfered with the natural course of events. And all this variability we get from a study of nine wasps and fifteen caterpillars!
In his chapter on “Méthode des Ammophiles” Fabre says that each species has its own tactics, allowing no novitiate. “Not one could have left descendants if it were not the handy workman of to-day. Any little slip is impracticable when the future of the race depends upon it.” And yet we find that the prey may be stung so slightly that it can rear and wriggle violently or so severely that it dies almost at once, and in neither case is a break made in the generations of the Ammophiles, since in the former the egg or larva is so firmly fastened as to keep its hold, while in the latter the dead and decomposing caterpillar is eaten without dissatisfaction or injury.
Nor do we, in gathering evidence for the evolution of the instincts of these wasps, need to rely entirely upon our own observations. Fabre himself gives many facts which point in the same direction, but he draws a line between those actions which are the result of mechanical and unvarying instinct and those which come within the sphere of reason, and in relation to which the insect must consider, compare, and judge. Yet this line, even in the light of his own work, is so extremely variable, needing readjustment with every new species and often among the individuals of the same species, that it loses for others the meaning which it has for its author. He himself speaks of certain individuals of the genus Sphex which refuse to be duped when he withdraws their prey to a distance. These, he says, are the élite, the strong-headed ones, which are able to recognize the malice of the action and govern themselves accordingly, but these revolutionists, apt at progress, he goes on to say, are few in numbers. The others, the conservators of old usages and customs, are the majority, the crowd. Yes, but is it not always the strong-minded few that direct the destiny of a race?