Should one insist and explain that the larvæ require not preserved food which could never have the properties of flesh still palpitating, but prey yet alive, so to say, in spite of complete absence of motion, the learned Congress, after ripe consideration, will fall back upon paralysis: “Yes, of course; the creature has to be paralysed without being killed.” There is but one means of arriving at this [[70]]result, namely, to injure, cut, and destroy the nervous system of the insect in one or more skilfully chosen points.
If the question be thus left in hands unfamiliar with the secrets of a delicate anatomy it will not have advanced far. What is the arrangement of this nervous system which must be paralysed without killing the insect? First, where is it? In the head no doubt and along the back, like the brain and spinal marrow in the superior animals. “A grave mistake!” our congress would reply; the insect is so to say an animal reversed, which walks on its back—that is, it has the spinal marrow below instead of above, all along breast and stomach; therefore on the lower surface alone can the operation to paralyse the insect be performed.
This difficulty removed, a far graver one presents itself. Armed with his scalpel, the anatomist can direct its point where he will in spite of obstacles which he may have to set aside. The Hymenopteron has no choice. Its victim is a solidly cuirassed beetle, its lancet a dart, extremely delicate, which the horny mail would certainly turn aside. Only certain points are vulnerable to the frail tool, namely, the joints, protected simply by a membrane with no power of resistance. But the joints of the limbs, although vulnerable, do not in the least fulfil the necessary conditions, for through these the utmost that could be obtained is local paralysis, not one affecting the whole organism of motion. Without any prolonged struggle, without repeated operations, which, if too numerous, might endanger the victim’s life, the Hymenopteron has, if possible, to abolish all [[71]]motive power at one blow. Therefore she must direct her dart at the nervous centres, the source of the power of motion whence radiate the nerves running up to the various organs of movement. Now these sources of locomotion, these nervous centres, consist of a certain number of ganglia, more numerous in the larva, less so in the perfect insect, and arranged on the median line of the under surface in a string of beads more or less distant and connected by a double ribbon of nervous tissue. In all insects which have reached the perfect state the ganglia called thoracic, i.e. those furnishing nerves to wings and feet and governing their movements, are three in number. Here are the points to be struck: if their action can be in any way destroyed, the possibility of movement is destroyed also.
Two ways of reaching these motive centres offer themselves to the feeble dart of the Hymenopteron; one, the joint between neck and corslet; the other the spot where the latter joins the continuation of the thorax, between the first and second pair of feet. The way through the neck does not answer; it is too far from the ganglia, which lie near the base of the feet which they animate. The blow must be dealt at the other spot, and through that only. Thus would an academy decide where Claude Bernards illuminated the question by their profound science. And it is precisely there, between the first and second pairs of feet on the median line of the under surface, that the Cerceris plunges her lancet. By what learned intelligence must she be inspired!
To choose as the spot in which to plant her sting the one vulnerable point, the point which only a [[72]]physiologist versed in the anatomy of insects could determine beforehand is by no means enough; the Hymenopteron has a far greater difficulty to overcome, and she overcomes it with a mastery which fills one with amazement. We said that the nervous centres controlling the organs of motion in an insect are three. These are more or less distant from each other, but sometimes, though rarely, near together. They possess a certain independence of action, so that an injury to one does not cause, at all events immediately, more than paralysis of members connected with it, while the other ganglia and their corresponding members are not affected by it. To reach these three sources of motion one after the other, the second farther off than the first, and the last farther still, and by a single way, between the first and second pairs of feet, seems impossible for the sting, which is too short, and besides, so difficult to aim well in such conditions. True, certain Coleoptera have the three ganglia of the thorax almost touching, and others have the two last completely united, soldered, smelted together. It is also recognised that in proportion as the different nervous centres combine and centralise, the characteristic functions of animality become more perfect, and also, alas, more vulnerable. Those Coleoptera with centres of motion so near that they touch or even gather into one mass, and so are made part of each other, would be instantly paralysed by one sting; or if several were needed, at all events the ganglia to be paralysed are all collected under the point of the dart.
Now which are the Coleoptera so specially easy [[73]]to paralyse? That is the question. The lofty science of a Claude Bernard, floating in the fundamental generalities of organisation and life, is no longer enough for us; it is unable to inform and guide us in this entomological selection. I appeal to every physiologist under whose eye these lines may fall. Without having recourse to his book-shelves, could he name the Coleoptera where such a nervous centralisation is found, and even with the help of his library, could he instantly lay his hand on the information wanted? The truth is, we are entering on the minute details of the specialist; the highway is quitted for a path known to few.
I find the necessary documents in the fine work of M. E. Blanchard (Annales des Sciences Naturelles, 3me série, tome v.) on the nervous system of Coleoptera. There I find that this centralisation of nerve power belongs especially to the Scarabæus, but most of these are too large; the Cerceris could neither attack nor carry them away; besides, many live in filth, where the cleanly Hymenopteron could not go to seek them. Motive centres very close together are also found among the Histers, which live on impurity, amid the smell of decay, and again that will not do; also in the Scolytus, which is too small, and finally in Buprestids and Weevils.
What unexpected light amid the pristine obscurities of the problem! Amid the immense number of the Coleoptera which the Cerceris seem able to prey upon, two groups alone, Weevils and Buprestids, fulfil the indispensable conditions. They live far from decay and dung, which perhaps cause invincible repugnance in this dainty Cerceris; they are [[74]]of most varied size, proportioned to that of their different captors, which may thus choose according to their convenience. They are far more vulnerable than all the others at the one point where the sting of the Hymenopteron can penetrate successfully, for at that point, all easily accessible to the dart, crowd the motor centres of feet and wings. At this point the three thoracic ganglia of Weevils lie very close, the hind two are contiguous. At that same spot in the Buprestids the second and third are welded in one large mass a little distance from the first. And as it is precisely Buprestids and Weevils which are hunted, to the absolute exclusion of all other game, by the eight species of Cerceris, whose food stores of Coleoptera have been ascertained, a certain internal likeness, namely, in centralisation of the nervous system must be the explanation why there are heaped in the dens of various Cerceris victims, outwardly so unlike.
In this choice, upon which even transcendent knowledge could not improve, such an assembly of difficulties is splendidly resolved, that one asks if one be not the dupe of some involuntary illusion, and if preconceived theories have not obscured the reality of facts, in short, whether the pen has not described imaginary marvels. A scientific result is only solidly established when confirmed by experiments repeated in every possible way. Now let us submit to experimental proof the physiological operation taught us by Cerceris tuberculata. If it be possible to obtain artificially what the Hymenopteron obtained by her sting, i.e. abolition of movement, and long preservation of the victim in a [[75]]state of perfect freshness; if it be possible to bring about this wonder with the Coleoptera hunted by the Cerceris, or with those possessing a like nervous centralisation, while one fails with those whose ganglia are far apart, one must admit, however exacting one may be in the matter of proof, that the Hymenopteron possesses in the unconscious inspirations of instinct the resources of sublime science. Let us see then what experiment shows. The manner of operation is very simple. With a needle, or, better still, with the point of a fine steel pen, we must introduce a tiny drop of some corrosive liquid into the thoracic motive centres, pricking the insect slightly at the jointing of the prothorax behind the first pair of feet. The liquid which I use is ammonia, but it is evident that any other liquid whose action is equally strong would produce the same results. The metal pen being charged with ammonia as it might be with a droplet of ink, I give the prick. The effects thus obtained differ enormously, according to whether the experiment be made upon species with thoracic ganglia near together or upon those where these same ganglia are far apart. With regard to the first category, my experiments were made on Scarabæus, S. sacer and S. longicollis; on a bronze Buprestis; and on weevils, especially that Cleonus hunted by the heroine of these observations. In the second category I have experimented on Caraboidea, Carabus, Procrustes, Chlœnius, Sphodrus, Nebria; Longicornia, Saperda, and Lamia; on Melasomes; Blaps, Scaurus, and Asida.
Among the Scarabæus class, the Buprestids, and the [[76]]Weevils, the effect is instantaneous. Every movement stops suddenly, without any convulsion, as soon as the fatal drop has touched the nerve centres. The sting of the Cerceris does not produce prompter extinction. Nothing can be more striking than this sudden immobility in a vigorous Scarabæus sacer, but the likeness between the effects produced by the dart of the Cerceris and the steel pen charged with ammonia does not stop here. Scarabæids, Buprestids, and Weevils artificially stung, in spite of their complete immobility, preserve for three weeks, one month, or even two, the perfect flexibility of every joint and the normal freshness of the interior organs. With them defecation takes place on the first days as in the normal condition, and movement can be excited by the Voltaic current. In a word, they behave exactly as do Coleoptera sacrificed by the Cerceris. There is complete identity between the state into which she plunges her victims and that produced at will by injecting ammonia into the nerve centres of the thorax. Now, as it is impossible to attribute the perfect preservation of the insect during so long a time to the drop injected, one must altogether reject the notion of an antiseptic fluid, and grant that in spite of utter immobility the creature is not really dead. A spark of life exists, keeping the organs for some time in normal freshness, but dying out by degrees and leaving them at last subject to corruption. Moreover, the ammonia in some cases produces extinction of movement in the feet only, and then the deleterious action of the fluid having doubtless not extended far enough, the antennæ preserve some mobility, [[77]]and one sees that the creature, even a month after inoculation, draws them back quickly at the least touch—an evident proof that life has not completely abandoned the inert body. This movement is not rare with Weevils wounded by the Cerceris.