[To face p. 208.
AMMOPHILA HIRSUTA HUNTING FOR CATERPILLARS; AMMOPHILA SABULOSA ON THE WING
The fidelity of memory shown here is striking. The insect, belated at its work, puts off completing it until the morrow. It passes neither evening nor night in the new-made abode, but departs after marking the entrance with a small stone. The spot is no more familiar to it than any other, for like Sphex occitanica the Ammophila lodges her family here and there as she may chance to wander. The creature came here by chance, like the soil, and dug [[209]]the burrow, and now departs. Whither? Who knows? Perhaps to the flowers near, to lick up by the last gleam of day a drop of sugary liquid at the bottom of their cups, just as a miner after labouring in his dark gallery seeks the consolation of his bottle when evening comes. The Ammophila may be enticed farther and farther by the inviting blossoms. Evening, night, and morning pass, and now she must return to her burrow and complete her task,—return after all her windings and wanderings in the chase that morning, and the flight from flower to flower, and the libations of the previous evening. That a wasp should return to the nest and a bee to the hive does not surprise me; these are permanent abodes, and the ways back are known by long practice, but the Ammophila, who has to return after so long an absence, has no aid from acquaintance with the locality. Her shaft is in a place which she visited yesterday, perhaps for the first time, and must find again to-day when quite beyond her bearings, and, moreover, when she is encumbered by heavy prey. Yet this exploit of topographical memory is accomplished, and sometimes with a precision which left me amazed. The insect made straight for the burrow as if long used to every path in the neighbourhood; but at other times there would be long visitation and repeated searches.
If the difficulty become serious, the prey, which is an embarrassing load in a hurried exploration, is laid in some obvious place, on a tuft of thyme or grass, where it can be easily seen when wanted. Freed from this burden the Ammophila resumes an active search. As she hunted about I have [[210]]traced with a pencil the track made by her. The result was a labyrinth of lines, with curves and sudden angles, now returning inward and now branching outward—knots and meshes and repeated intersections—a maze, showing how perplexed and astray was the insect.
The shaft found and the stone lifted, she must return to the prey, not without some uncertainty when comings and goings have been too many. Although it was left in a place obvious enough, the Ammophila often seems at a loss when the time comes to drag it home; at least, if there be a very long search for the burrow, one sees her suddenly stop and go back to the caterpillar, feel it and give it a little bite, as if to make sure that it is her very own game and property, hurrying back to seek for the burrow, but returning a second time if needful, or even a third, to visit her prey. I incline to believe that these repeated visits are made to refresh her memory as to where she left it.
This is what happens in very complex cases, but generally the insect returns without difficulty to the spot whither its vagrant life may have led it. For guide it has that local memory whose marvellous feats I shall later have occasion to relate. As for me, in order to return next day to the burrow hidden under the little flat stone, I dared not trust to my memory, but had to use notes, sketches, to take my bearings, and stick in pegs—in short, a whole array of geometry.
The temporary closing of the burrow with a flat stone as practised by A. sabulosa and A. holosericea appears unknown to the two other species; at least I [[211]]never saw their homes protected by a covering. This is natural in the case of A. hirsuta, for, I believe, this species hunts the prey first and then burrows near the place of capture. As provender can therefore be at once stored it is useless to take any trouble about a cover. As for A. holosericea, I suspect there is another reason for not using any temporary door. While the two others only put one caterpillar in each cell, she puts as many as five, but much smaller ones. Just as we ourselves neglect to shut a door where some one is constantly passing to and fro, perhaps this Ammophila neglects to place a stone on a well which she will go down at least five times within a short space of time. All four lay up caterpillars of moths for their larvæ. A. holosericea chooses, though not exclusively, those slender, long caterpillars known as Loopers. They move as a compass might by opening and closing alternately, whence their expressive French name of Measurers. The same burrow includes provisions of varied colours—a proof that this Ammophila hunts all kinds of Loopers so long as they are small, for she herself is but feeble and the larva cannot eat much, in spite of the five heads of game set before it. If Loopers fail, the Hymenopteron falls back on other caterpillars equally small. Rolled up from the effect of the sting which paralysed them, all five are heaped in the cell; the top one bears the egg for which the provender is destined.
The three other Ammophilæ give but one caterpillar to each cell. True—size makes up for this; the game selected is corpulent, plump, amply sufficing the grub’s appetite. For instance, I have [[212]]taken out of the mandibles of A. holosericea a caterpillar fifteen times her own weight—fifteen times!—an enormous sum if you consider what an expenditure of strength it implies to drag such game by the nape of its neck over the endless difficulties of the ground. No other Hymenopteron tried in the scales with its prey has shown me a like disproportion between spoiler and capture. The almost endless variety of colouring in the provender exhumed from the burrows or recognised in the grasp of the various species also proves that the three have no preference, but seize the first caterpillar met with, provided it be neither too large nor too small, and belongs to the moths. The commonest prey are those gray caterpillars which infest the plant at the junction of a root and stem just below the soil.
That which governs the whole history of the Ammophila, and more especially attracted my attention, was the way in which the insect masters its prey and plunges it into the harmless state required for the safety of the larva. The prey, a caterpillar, is very differently organised from the victims which we have hitherto seen sacrificed—Buprestids, Weevils, Grasshoppers, and Ephippigers. It is composed of a series of segments or rings set end to end, the three first bearing the true feet which will be those of the future butterfly; others bear membranous or false feet special to the caterpillar and not represented in the butterfly; others again are without limbs. Each ring has its ganglion, the source of feeling and movement, so that the nerve system comprehends twelve distinct centres well separated from each other, [[213]]without counting the œsophageal ganglion placed under the skull, and which may be compared to the brain.
We are here a long way from the nerve centralisation of the Weevil and Buprestis that lends itself so readily to general paralysis by a single stab; very far too from the thoracic ganglia which the Sphex wounds successively to put a stop to the movements of her crickets. Instead of a single centralised point—instead of three nerve centres—the caterpillar has twelve, separated one from another by the length of a segment and arranged in a ventral chain along the median line of the body. Moreover, as is the rule among lower animals, where the same organ is very often repeated and loses power by diffusion, these various nervous centres are largely independent of each other, each animating its own segment, and are but slightly disturbed by disorder in neighbouring ones. Let one segment lose motion and feeling, yet those uninjured will none the less remain long capable of both. These facts suffice to show the high interest attaching to the murderous proceedings of the Hymenopteron with regard to its prey.