Many experiments followed these; but we are indebted to Alexander von Humboldt for the first precise account of this very curious fish. The celebrated naturalist read to the Institute of France an important memoir upon the electrical eel from Bonpland's observations, the substance of which we shall give here.
Plate XXVII.—Fishing for Eel (Gymnotus) on the Orinoco.
In traversing the Llanas of the province of Caracas, in order to embark at San Fernando de Apure on his voyage up the Orinoco, M. Bonpland stopped at Calabozo. The object of this sojourn was to investigate the history of the gymnotus, great numbers of which are found in the neighbourhood. After three days' residence in Calabozo some Indians conducted them to the Cano de Bera, a muddy and stagnant basin, but surrounded by rich vegetation, in which Clusia rosea, Hymenœa courbaril, some grand Indian figs, and some magnificent flowering odoriferous mimosas, were pre-eminent. They were much surprised when informed that it would be necessary to take thirty half-wild horses from the neighbouring savannahs in order to fish for the gymnotus.
The idea of this fishing, called in the language of the country embarbascar con caballos (intoxicating by means of horses), is very odd. The word barbasco indicates the roots of the Lacquinia, or any other poisonous plant, by contact of which a body of water acquires the property of killing, or, at least, of intoxicating or stupefying the fishes. These come to the surface when they have been poisoned in this manner. The horses chasing them here and there in a marsh has, it seems, the same effect upon the alarmed fishes. While our hosts were explaining to us this strange mode of fishing, the troop of horses and mules had arrived, and the Indians had made a sort of battue, pressing the horses on all sides, and forcing them into the marsh. The Indians, armed with long canes and harpoons, placed themselves round the basin, some of them mounting the trees, whose branches hung over the water, and by their cries, and still more by their canes, preventing the horses from landing again. The eels, stunned by the noise, defended themselves by repeated discharges of their batteries. For a long time it seemed as if they would be victorious over the horses. Some of the mules especially, being almost stifled by the frequency and force of the shock, disappeared under the water, and some of the horses, in spite of the watchfulness of the Indians, regained the bank, where, overcome by the shocks they had undergone, they stretched themselves at their whole length. The picture presented was now indescribable. Groups of Indians surrounded the basin; the horses with bristling mane, terror and grief in their eyes, trying to escape from the storm which had surprised them; the eels, yellow and livid, looking like great aquatic serpents swimming on the surface of the water, and chasing their enemies, were objects at once appalling and picturesque. In less than five minutes two horses were drowned. An eel, more than five feet long, glided under one horse, discharged its apparatus through its whole extent, attacking at once the heart, the viscera, and the plexus of the nerves of the animal, probably benumbing and finally drowning it.
When the struggle had endured a quarter of an hour, the mules and horses appeared less frightened, the manes became more erect, the eyes expressed less terror, the eels shunned in place of attacking them, at the same time approaching the bank, when they were easily taken by throwing little harpoons at them attached to long cords, the harpoon, sometimes hooking two at a time, being landed by means of the long cord. They were drawn ashore without being able to communicate any shock.
Having landed the eels, they were transported to little pools dug in the soil, and filled with fresh water; but such is the terror they inspire, that none of the people of the country would release them from the harpoon—a task which the travellers had to perform themselves, and receive the first shock, which was not slight—the most energetic surpassing in force that communicated by a Leyden jar completely charged. The gymnotus surpasses in size and strength all the other electric fishes. Humboldt saw them five feet three inches long. They vary in colour according to age, and the nature of the muddy water in which they live. Beneath, the head is of a fine yellow colour mixed with red; the mouth is large, and furnished with small teeth arranged in many rows.
The gymnotus makes its shock felt in any part of its body which is touched, but the excitement is greater when touched under the belly, and in the pectoral fin. The gymnotus gives the most frightful shocks without the least muscular movement in the fins, in the head, or any other part of the body. The shock, indeed, depends upon the will of the animal, and in this respect differs from a Leyden jar, which is discharged by communicating with two opposite poles. It happens sometimes that a gymnotus, seriously wounded, only gives a very weak shock, but if, thinking it exhausted, it is touched fearlessly and at once, its discharge is terrible. Indeed, the phenomena depend so much upon the will of the animal, that, according to Von Humboldt, if it is touched by two metallic rods, the shock is communicated sometimes by one, sometimes by the other wand, though their extremities are close together.
The experiments already related in connection with the torpedo were repeated here. If we place ourselves upon an isolated support, and take hold of a metallic rod, a shock is received; but no shock is received, on the other hand, if the fish is touched with a glass rod, or one covered with wax. Humboldt and Bonpland repeated this experiment many times, with decisive results. The electric organ has been carefully described by these observers. The organs extend from under the tail, occupying nearly one-half of the thickness. It is divided into four longitudinal bundles of muscles, the upper ones large, the two smaller below, and against the base of the anal fin. Each bundle consists of many parallel membranous plates, placed closely together and very nearly horizontal. These plates abut in one part on the skin, in another, on the mean vertical plane of the fish. They are united to each other by an infinity of smaller plates, placed either vertically or transversely. The smaller prismatic and transversal canal, intercepted by these two orders of plates, are filled with gelatinous matter. All this organic apparatus receives many nerves, and presents, in many respects, an arrangement nearly analogous to that of the torpedoes.