[4] Bacon-beetles.—Translator’s Note. [↑]
[5] The Horseshoe Bat.—Translator’s Note. [↑]
[6] Lazaro Spallanzani (1729–1799), the great Italian naturalist.—Translator’s Note. [↑]
Chapter xx
THE MODERN THEORY OF INSTINCT
The larvæ of the various Hunting Wasps require their prey to be incapable of movement, so that there may be no resistance on the victim’s part, which would be a source of danger to the fragile egg and, later, to the grub. Moreover, for all its lethargy, it must still be alive; for the grub would refuse to feed on a corpse. The fare provided must be fresh meat and not preserved stuff. I have already laid stress on these two antagonistic conditions, immobility and life, and enlarged on them so fully that I need hardly dwell upon them for a second time. I have shown how the Wasp realizes them by the medium of a paralysis which destroys movement and leaves the organic principle of life intact. With a skill which our most famous vivisectors would envy, the insect drives its poisoned sting into the nerve-centres, the seat of muscular incitation. The operator confines herself to one stroke of the lancet, or else gives two, three or more, according to the structure [[355]]of the particular nervous system and to the number and grouping of the ganglia. The course of the sting is determined by the exact anatomy of the victim.
The particular prey of the Hairy Ammophila is a caterpillar, each of whose nerve-centres, which are distant one from the other and to a certain extent independent in their action, occupies a different segment of the insect. This caterpillar, who is a very lively customer, cannot be stored in the cell, with the Wasp’s egg upon his flank, until he has lost all his power of motion. One movement of his body would crush that egg against the wall of the cell.
Now the paralysis of one segment would not mean that the next was also rendered incapable of movement, because of the comparative independence of the seats of innervation. It is necessary, therefore, that all the segments, or at least the most important, be operated on, one after the other, from the first to the last. The course which the Ammophila adopts is that which the most experienced of physiologists would recommend: her sting is transferred from one segment to the next, nine separate times over.
She does better than that. The victim’s head is still unscathed, the mandibles are at [[356]]work: they might easily, as the insect is borne along, grip some bit of straw in the ground and successfully resist this forcible removal; the brain, the primary nervous centre, might provoke a stubborn contest, which would be very awkward with so heavy a burden. It is well that these hitches should be avoided. The caterpillar, therefore, must be reduced to a state of torpor which will deprive him of the least inclination for self-defence. The Ammophila succeeds in effecting this by munching his head. She takes good care not to use her needle: she is no clumsy bungler and knows quite well that to inflict a mortal wound on the cervical ganglia would mean killing the caterpillar then and there, the very thing to be avoided. She merely squeezes the brain between her mandibles, calculating every pinch; and, each time, she stops to ascertain the effect produced, for there is a nice point to be achieved, a certain degree of torpor that must not be exceeded, lest death should supervene. In this way the requisite lethargy is obtained, a somnolence in which all volition is lost. And now the caterpillar, incapable of resistance, incapable of wishing to resist, is seized by the nape of the neck and dragged to the nest. Comment would mar the eloquence of such facts as these. [[357]]