Chapter XI. The Method of The Ammophilae
Jean-Henri Fabre More Hunting Wasp
Thread-waisted Sphecid Wasp (Ammophila Procera)
My readers may differ in appraising the comparative value of the trifling discoveries which entomology owes to my labours. The geologist, the recorder of forms, will prefer the hypermetamorphosis of the Oil-beetles (2), the development of the Anthrax (3) or larval dimorphism; the embryogenist, searching into the mysteries of the egg, will have some esteem for my enquiries into the egg-laying habits of the Osmia (4) ; the philosopher, racking his brain over the nature of instinct, will award the palm to the operations of the Hunting Wasps. I agree with the philosopher. Without hesitation, I would abandon all the rest of my entomological baggage for this discovery, which happens to be the earliest in date and that of which I have the fondest memories. Nowhere do I find a more brilliant, more lucid, more eloquent proof of the intuitive wisdom of instinct; nowhere does the theory of evolution suffer a more obstinate check.
Darwin, a true judge, made no mistake about it (5). He greatly dreaded the problem of the instincts. My first results in particular left him very anxious. If he had known the tactics of the Hairy Ammophila, the Mantis-hunting Tachytes, the Bee-eating Philanthus, the Calicurgi and other marauders, his anxiety, I believe, would have ended in a frank admission that he was unable to squeeze instinct into the mould of his formula. Alas, the philosopher of Down quitted this world when the discussion, with experiments to support it, had barely begun: a method superior to any argument! The little that I had published at that time left him with still some hope of an explanation. In his eyes, instinct was always an acquired habit. The predatory Wasps killed their prey at first by stabbing it at random, here and there, in the softest parts. By degrees they found the spot where the sting was most effectual; and the habit once formed became a true instinct. Transitions from one method of operation to the other, intermediary changes, sufficed to bolster up these sweeping assertions. In a letter of the 16th of April, 1881, he asks G.J. Romanes to consider the problem:
„I do not know,“ he says "whether you will discuss in your book on the mind of animals any of the more complex and wonderful instincts. It is unsatisfactory work, as there can be no fossilised instincts, and the sole guide is their state in other members of the same order, and mere PROBABILITY.
„But if you do discuss any (and it will perhaps be expected of you), I should think that you could not select a better case than that of the sand-wasps which paralyse their prey as described by Fabre in his wonderful paper in the "Anales des sciences naturelles,“ and since amplified in his admirable „Souvenirs…“
I thank you, O illustrious master, for your eulogistic expressions, proving the keen interest which you took in my studies of instinct, no ungrateful task — far from it — when we tackle it as it should be tackled: from the front, with the aid of facts, and not from the flank, with the aid of arguments. Arguments are here out of place, if we wish to maintain our position in the light. Besides, where would they lead us? To evoking the instincts of bygone ages, which have not been preserved by fossilization? Any such appeal to the dim and distant past is quite unnecessary, if we wish for variations of instinct, leading by degrees, according to you, from one instinct to another; the present world offers us plenty.
Each operator has her particular method, her particular kind of game, her particular points of attack and tricks of fence; but in the midst of this variety of talents we observe, immutable and predominant, the perfect accordance of the surgery with the victim's organization and the larva's needs. The art of one will not explain the art of another, no less exact in the delicacy of its rules. Each operator has her own tactics, which tolerate no apprenticeship. The Ammophila, the Scolia, the Philanthus and the others all tell us the same thing: none can leave descendants if she be not from the outset the skilful paralyser or slayer that she is to-day. The „almost“ is impracticable when the future of the race is at stake. What would have become of the first-born mammal but for its perfect instinct of suckling?
And then, to suppose the impossible: a Wasp discovers by chance the operative method which will be the saving attribute of her race. How are we to admit that this fortuitous act, to which the mother has vouchsafed no more attention than to her other less fortunate attempts, could leave a profound trace behind it and be faithfully transmitted by heredity? Is it not going beyond reason, going beyond the little that is known to us as certain, if we grant to atavism this strange power, of which our present world knows no instance? There is a good deal to be said for this point of view, my revered master! But, once more, arguments are here out of place; there is room only for facts, of which I will resume the recital.
Hitherto I had but one means of studying the operative methods of the spoilers: to surprise the Wasp in possession of her capture, to rob her of her prey and immediately to give her in exchange a similar prey, but a living one. This method of substitution is an excellent expedient. Its only defect — a very grave one — is that it subjects observation to very uncertain chances. There is little prospect of meeting the insect dragging its victim along; and, in the second place, should good fortune suddenly smile upon you, preoccupied as you are with other matters you have not the substitute at hand. If we provide ourselves with the necessary head of game in advance, the huntress is not there. We avoid one reef to founder on another. Moreover, these unlooked for observations, made sometimes on the public highway, the worst of laboratories, are only half-satisfactory. In the case of swiftly-enacted scenes, which it is not in our power to renew again and again until perfect conviction is reached, we always fear lest we may not have seen accurately, may not have seen everything.
A method which could be controlled at will would offer the best guarantees, above all if employed at home, under comfortable conditions, favourable to precision. I wished, therefore, to see my insects at work on the actual table at which I am writing their history. Here very few of their secrets would escape me. This wish of mine was an old one. As a beginner, I made some experiments under glass with the Great Cerceris (C. tuberculata) and the Yellow-winged Sphex. Neither of them responded to my desires. The refusal of each to attack respectively her Cleonus or her Cricket discouraged further progress in this direction. I was wrong to abandon my attempts so soon. Now, very long afterwards, the idea occurs to me to place under glass the Bee-eating Philanthus, whom I sometimes surprise in the open engaged in forcing a bee to disgorge her honey. The captive massacres her bees in such a spirited fashion that the old hope revives stronger than ever. I contemplate reviewing all the wielders of the stiletto and forcing each to reveal her tactics.
I was obliged to abate these ambitions considerably. I had some successes and many more failures. I will tell you of the former. My insect-cage is a spacious dome of wire-gauze resting on a bed of sand. Here I keep in reserve the captives of my hunting-expeditions. I feed them on honey, placed in little drops on spikes of lavender, on heads of thistle, or field eryngo, or globe-thistle, according to the season. Most of my prisoners do well on this diet and seem scarcely affected by their internment; others pine away and die in two or three days. These victims of despair nearly always throw me back, because of the difficulty of obtaining the necessary prey at short notice.
Indeed it entails no small trouble to secure in the nick of time the game demanded by the huntress who has recently fallen a captive to my net. As assistant-purveyors I have a few small schoolboys, who, released from the tedium of their declensions and conjugations, set out, on leaving the classroom, to inspect the greenswards and beat the bushes in the neighbourhood on my behalf. The gros sou, the penny-piece, if you please, stimulates their zeal; but with misadventurous results! What I need to-day is Crickets. The band sallies forth and returns with not a single Cricket, but numbers of Ephippigers, for which I asked the day before yesterday and which I no longer need, my Languedocian Sphex being dead. General surprise at this sudden change of market. My young scatterbrains find it hard to understand that the beast which was so precious two days ago is now of no value whatever. When, owing to the chances of my net, a renewed demand for the Ephippiger sets in, then they will bring me the Cricket, the despised Cricket.
Such a trade could never hold out if now and again my speculators were not encouraged by some success. At the moment when urgent necessity is sending up prices, one of them brings me a magnificent Gad-fly intended for the Bembex. For two hours, when the sun was at its height, he kept watch on the threshing-floor hard by, waiting for the blood-sucker, in order to catch him on the buttocks of the Mules which trot round and round trampling the corn. This gallant fellow shall have his gros sou and a slice of bread and jam as well. A second, no less fortunate, has found a fat Spider, the Epeira, for whom my Pompili are waiting. To the two sous of this fortunate youth I add a little picture for his missal. Thus are my purveyors kept going; and, after all, their help would be very inadequate if I did not take upon myself the main burden of these wearisome quests.
Once in possession of the requisite prey, I transfer the huntress from my warehouse, the wire-gauze cage, to a bell-glass varying in capacity from one to three or four litres (6), according to the size and habits of the combatants; I place the victim in the arena; I expose the bell-glass to the direct rays of the sun, without which condition the executioner as a rule declines to operate; I arm myself with patience and await events.
We will begin with the Hairy Ammophila, my neighbour. Year after year, when April comes, I see her in considerable numbers, very busy on the paths in my enclosure. Until June I see her digging her burrows and searching for the Grey Worm, to be placed in the meat-cellar. Her tactics are the most complex that I know and more than any other deserves to be thoroughly studied. To capture the cunning vivisector, to release her and catch her again I find an easy matter for the best part of a month; she works outside my door.
I have still to obtain the Grey Worm. This means a repetition of the disappointments which I had before, when, to find a caterpillar, I was obliged to watch the Ammophila while hunting and to be guided by her hints, as the truffle-hunter is guided by the scent of his Dog. A patient exploration of the harmas, one tuft of thyme after another, does not give me a single worm. My rivals in this search are finding their game at every moment; I cannot find it even once. Yet one more reason for bowing to the superiority of the insect in the management of her affairs. My band of schoolboys get to work in the surrounding fields. Nothing, always nothing! I in my turn explore the outer world; and for ten days the pursuit of a caterpillar torments me till I lose my power of sleep. Then, at last, victory! At the foot of a sunny wall, under the budding rosettes of the panicled centaury, I find a fair supply of the precious Grey Worm or its equivalent.
Behold the worm and the Ammophila face to face beneath the bell-glass. Usually the attack is prompt enough. The caterpillar is grabbed by the neck with the mandibles, wide, curved pincers capable of embracing the greater part of the living cylinder. The creature thus seized twists and turns and sometimes, with a blow of its tail, sends the assailant rolling to a distance. The latter is unconcerned and thrusts her sting thrice in rapid succession into the thorax, beginning with the third segment and ending with the first, where the weapon is driven home with greater determination than elsewhere.
The caterpillar is then released. The Ammophila stamps on the ground; with her quivering tarsi she taps the cardboard on which the bell-glass stands; she lies down flat, drags herself along, gets up again, flattens herself once more. The wings jerk convulsively. From time to time the insect places its mandibles and forehead on the ground, then rears high upon its hind-legs as though to turn head over heels. In all this I see a manifestation of delight. We rub our hands when rejoicing at a success; the Ammophila is celebrating her triumph over the monster in her own fashion. During this fit of delirious joy, what is the wounded caterpillar doing? It can no longer walk; but all the part behind the thorax struggles violently, curling and uncurling when the Ammophila sets a foot upon it. The mandibles open and shut menacingly.
SECOND ACT. — When the operation is resumed, the caterpillar is seized by the back. From front to rear, in order, all the segments are stung on the ventral surface, except the three operated on. All serious danger is averted by the stabs of the first act; therefore, the Wasp is now able to work upon her patient without the haste displayed at the outset. Deliberately and methodically she drives in her lancet, withdraws it, selects the spot, stabs it and begins again, passing from segment to segment, taking care, each time, to lay hold of the back a little more to the rear, in order to bring the segment to be paralysed within reach of the needle. For the second time, the caterpillar is released. It is absolutely inert, except the mandibles, which are still capable of biting.
THIRD ACT. — The Ammophila clasps the paralysed victim between her legs; with the hooks of her mandibles she seizes the back of its neck, at the base of the first thoracic segment. For nearly ten minutes she munches this weak spot, which lies close to the cerebral nerve-centres. The pincers squeeze suddenly but at intervals and methodically, as though the manipulator wished each time to judge of the effect produced; the squeezes are repeated until I am tired of trying to count them. When they cease, the caterpillar's mandibles are motionless. Then comes the transportation of the carcase, a detail which is not relevant in this place.
I have set forth the complete tragedy, as it is fairly often enacted, but not always. The insect is not a machine, unvarying in the effect of its mechanism; it is allowed a certain latitude, enabling it to cope with the eventualities of the moment. Any one expecting to see the incidents of the struggle unfolding themselves exactly as I have described will risk disappointment. Special instances occur — they are even numerous — which are more or less at variance with the general rule. It will be well to mention the more important, in order to put future observers on their guard.
Not infrequently the first act, that of paralysing the thorax, is restricted to two thrusts of the sting instead of three, or even to one, which is then delivered in the foremost segment. This, it would seem, from the persistency with which the Ammophila inflicts it, is the most important prick of all. Is it unreasonable to suppose that the operator, when she begins by pricking the thorax, intends to subdue her capture and to make it incapable of injuring her, or even of disturbing her when the moment comes for the delicate and protracted surgery of the second act? This idea seems to me highly admissible; and then, instead of three dagger-thrusts, why not two only, why not merely one, if this would suffice for the time being? The amount of vigour displayed by the caterpillar must be taken into consideration. Be this as it may, the segments spared in the first act are stabbed in the second. I have sometimes even seen the three thoracic segments stung twice over: at the beginning of the attack and again when the Wasp returned to her vanquished prey.
The Ammophila's triumphant transports beside her wounded and writhing victim are also subject to exceptions. Sometimes, without releasing its prey for a moment, the insect proceeds from the thorax to the next segments and completes its operation in a single spell. The joyous entr'acte does not take place; the convulsive movements of the wings and the acrobatic postures are suppressed.
The rule is paralysis of all the segments, however many, in regular order from front to back, including even the anal segment if this boast of legs. By a fairly frequent exception the last two or three segments are spared. Another exception, but a very rare one, of which I have observed only a single instance, consists in the inversion of the dagger-thrusts of the second act, the thrusts being delivered from back to front. The caterpillar is then seized by its hinder extremity; and the Ammophila, progressing towards the head, stings in reverse order, passing from the succeeding to the preceding segment, including the thorax already stabbed. This reversal of the usual tactics I am inclined to attribute to negligence on the insect's part. Negligence or not, the inverted method has the same final result as the direct method: the paralysis of all the segments.
Lastly, the compression of the neck by the mandibulary pincers, the munching of the weak spot between the base of the skull and the first segment of the thorax, is sometimes practised and sometimes neglected. If the caterpillar's jaws open and threaten, the Ammophila stills them by biting the neck; if they are already growing quiescent, she refrains. Without being indispensable, this operation is useful at the moment of carting the prey. The caterpillar, too heavy to be carried on the wing, is dragged, head first, between the Ammophila's legs. If the mandibles are working, the least clumsiness may render them dangerous to the carrier, who is exposed to their bite without any means of defence.
Moreover, once on the way, thickets of grass are traversed in which the Grey Worm can seize a blade and offer a desperate resistance to the traction. Nor is this all. The Ammophila does not as a rule trouble about her burrow, or at least does not complete it, until she has caught her caterpillar. During the mining-operations, the game is laid somewhere high up, out of reach of the Ants, on some tuft of grass, or the twigs of a shrub, whither the huntress, from time to time, stopping her well-sinking, hastens to see if her quarry is still there. For her this is a means of refreshing her memory of the spot where she has laid it, often at some distance from the burrow, and of preventing attempts at robbery. When the moment comes for removing the game from its hiding-place, the difficulty would be insurmountable were the worm, gripping the shrub with all the might of its jaws, to anchor itself there. Hence inertia of the powerful hooks, which are the paralysed creature's sole means of resistance, becomes essential during the carting. The Ammophila obtains it by compressing the cerebral ganglia, by munching the neck. The inertia is temporary; it wears off sooner or later; but by this time the carcase is in the cell and the egg, prudently laid at a distance on the ventral surface of the worm, has nothing to fear from the caterpillar's grapnels. No comparison is permissible between the methodical squeezes of the Ammophila benumbing the cephalic nerve-centres and the brutal manipulations of the Philanthus emptying the crop of her Bee. The huntress of Grey Worms induces a temporary torpor of the mandibles; the ravisher of Bees makes them eject their honey. No one gifted with the least perspicacity will confound the two operations.
For the moment we will not dwell any longer on the method of the Hairy Ammophila; we will see instead how her kinswomen behave. After protracted refusals the Sandy Ammophila (A. sabulosa, FAB.),on whom I experimented in September, ended by accepting the proffered prey, a powerful caterpillar as thick as a lead-pencil. The surgical method did not differ from that employed by the Hairy Ammophila when operating on her Grey Worm in one spell. All the segments, excepting the last three, were stung from front to back, beginning with the prothorax. This single success with a simplified method left me in ignorance of the accessory manoeuvres, which I do not doubt must more or less closely recall those of the preceding species.
I am all the more inclined to accept these secondary manoeuvres, not as yet recorded — the transports of triumph and the compressions of the neck — inasmuch as I see them practised upon the Looper caterpillars, which differ so greatly from the others in external structure, exactly as I have described them in the case of the Grey Worm, which is of the ordinary form. Two species, the Silky Ammophila (A. holoserica, FAB.) and Jules' Ammophila (a), affect this curious prey, which moves with the stride of a pair of compasses. The first, often renewed under glass during the greater part of August, has always refused my offers; the second, her contemporary, has, on the contrary, promptly accepted them.
I present Jules' Ammophila with a slender, brownish Looper which I caught on the jasmine. The attack is not slow in coming. The caterpillar is grabbed by the neck: lively contortions of the victim, which rolls the aggressor over and drags her along, now uppermost, now undermost in the struggle. First the thorax is stung, in its three rings, from back to front. The sting lingers longest near the throat, in the first segment. This done, the Ammophila releases her victim and proceeds to stamp her tarsi, to polish her wings, to stretch herself. Again I observe the acrobatic postures, the forehead touching the ground, the hinder part of the body raised. This mimic triumph is the same as that of the huntress of the Grey Worm. Then the Looper is once more seized. Despite its contortions, which are not in the least abated by the three wounds in the thorax, it is stung from front to back in each segment still unwounded, no matter how many, whether supplied with legs or not. I expected to see the sting refrain more or less in the long interval which separates the true legs in front from the pro-legs at the back (7): segments devoid of organs of defence or locomotion did not seem to me to deserve conscientious surgery. I was mistaken: not a segment of the Looper is spared, not even the last ones. It is true that these, being eminently capable of catching hold with their false legs, would be dangerous later were the Wasp to neglect them.
I observe, however, that the lancet works more rapidly in the second part of the operation than in the first, either because the caterpillar, half subjugated by the triple wound at the outset, is easier to reach with the sting, or because the segments more remote from the head are rendered harmless with a smaller injection of poison. Nowhere do we see repeated the care expended upon paralysing the thorax, still less the insistent attention to the first segment. On returning to her Looper after the entr'acte devoted to the joys of success, the Ammophila stabs so swiftly that, on one occasion, I saw her obliged to begin all over again. Lightly stung along its whole length, the victim still struggles. Without hesitation, the operator unsheathes her scalpel for the second time and operates on the Looper afresh, with the exception of the thorax, which was already sufficiently anaesthetized. This done, all is in order; there is no more movement.
After the stiletto the hooks of the mandibles rarely fail to intervene. Long and curved, they nibble at the paralysed victim's neck, sometimes from above, sometimes from below. It is a repetition of what the Hairy Ammophila showed us: the same sudden squeezes of the pincers, with rather long intervals between. These intervals, these measured bites and the insect's watchful attitude have every appearance of telling us that the operator is noting the effect produced before giving a fresh pinch of the nippers.
It will be seen how valuable is the evidence of Jules' Ammophila: it tells us that the immolaters of Looper caterpillars and those of ordinary caterpillars follow precisely the same method; that victims displaying very dissimilar external structure do not entail any modification of the operative tactics so long as the internal organization remains the same. The number, arrangement and degree of mutual independence of the nerve-centres guide the sting; the anatomy of the game, rather than its form, controls the huntress' tactics.
Let me mention, before I dismiss the subject, a superb example of this marvellous anatomical discrimination. I once took from between the legs of a Hairy Ammophila, which had just paralysed it, a caterpillar of Dicranura vinula. What a strange capture compared with the ordinary caterpillar! Bridling in thick folds beneath its pink neckerchief, its fore-part raised in a sphinx-like attitude, its hinder-part slowly waving two long caudal threads, the curious animal is no caterpillar to the schoolboy who brings it to me, nor to the man who comes upon it while cutting his bundle of osiers; but it is a caterpillar to the Ammophila, who treats it accordingly. I explore the queer creature's segments with the point of a needle. All are insensitive; all therefore have been stung.
1. See in the first volume of the „Souvenirs entomologiques“ what I mean by this denomination.
1. For these Sand-wasps, cf. „The Hunting Wasps“: chapters 13 and 18 to 20.
2. The chapter treating of this subject has not yet been translated into English and will appear in a later volume.
3. Cf. „The Life of the Fly“: chapter 2.
4. Cf. „Bramble-bees and Others“: chapter 4.
5. Charles Robert Darwin, born the 12th of February, 1809, at Shrewsbury, died at Down, in Kent, on the 19th of April, 1882. For an account of certain experiments which the author conducted on his behalf, cf. „The Mason-bees“: chapter 4.
6. 1 3/4 to 5 or 7 pints.
7. Fleshy legs found on the abdominal segments of caterpillars and certain other larvae.