Part 35

Ellis's Visits to Madagascar, 57.

Atkinson's Siberia, 472.

XI.

PARASITES.

Vast as is this round world on which we live, its surface is not nearly large enough for all the living creatures which are ordained to inhabit it. Multitudes of animals do not walk on the ground, or swim in the waters, or fly in the air, but find the scene of their abode on or in the bodies of other animals. Multitudes of plants do not grow out of the soil, but attach themselves to other plants, and draw their sustenance and support thence. Nay, there are parasites upon parasites, and this, according to Hood, in an infinitely descending series.

"Great fleas have little fleas Upon their backs to bite 'em; And little fleas have lesser fleas; And so ad infinitum."

Perhaps the poet's imagination ran a little ahead of his science here; but the idea of an infinite succession of parasites, like nests of pill-boxes, is surely a funny one. There is nothing funny, however, in the thought "that even man," who was made in the image of God, "bears about in his vital organs various forms of loathsome creatures, which riot on his fluids, and consume the very substance of his tissues while ensconced where no efforts of his can dislodge them, no application destroy them. So it is; and few physical facts are better calculated to humble man, and stain the pride of his glory, than to feel that he may at any moment be nourishing a horrid tape-worm in his alimentary canal, or that his muscles may be filled with millions of microscopic trichinæ.

I will not dwell on these; though, if I were writing a book of pure science, there is a wondrous array of facts of the most striking and interesting character, connected with the structure, the metamorphoses, and the habits, of the Entozoic Worms, which I might present to my readers. It is more pleasant to consider other facts, perhaps not less marvellous, which, as they do not come quite so home to our personal feelings, will not excite horror and disgust in our minds.

The economy of creation is remarkable. He who, by His divine manipulation converted five loaves and two small fishes into a hearty meal for five thousand men, besides women and children, and who could, with the same ease have made them a hundred times as much, said, when the meal was over, "Gather up the fragments, that nothing be lost." And, when He spread the earth with life, though His resources were infinite, He ordained that one object, itself healthfully enjoying life, and fulfilling its own proper ends of being, should be a microcosm, on which another range of life should find its sphere, and on which it should disport, as on an independent world. I have often admired, in the gorgeous tropical forests, what a wilderness of vegetation a single tree supports; what numbers of orchids and wild pines spring out of the forks, what creepers and lianes hang and twine about its branches, what elegant ferns cluster on the horizontal limbs, what snake-like cacti creep from bough to bough, what mosses, and jungermanniæ crowd in every crevice, what many-coloured lichens stud the rugged bark! And then animal life is swarming in all this great field of parasitic vegetation. Reptiles and birds, snails and slugs, insects and millepedes, and spiders and worms nestle by thousands in such prolific situations, so that a great old tropical tree, one of the giant figs or cotton-trees, is a very museum in itself.

And in my wanderings along the sea-edge here at home how often have I been amazed at the diverse population, plant and animal, which crowds a single oar-weed, or tangle! The stem fringed with delicate red-weeds, as the minute Rhodymeniæ, and Polysyphoniæ, and Callithamnia; the tortuous roots studded with Anemones, with Flustræ and Lepraliæ, and multitudes of other Polyzoa, with tiny Polypes of many kinds, with Barnacles and Limpets, and sheltering small Crustacea, and Mites, and Annelids by scores.

Mr Darwin has an interesting passage on this subject, evoked by the profusion of parasitic life on the long sea-weed of Cape Horn (Macrocystis). "The number of living creatures" he remarks, "whose existence intimately depends on the Kelp is wonderful. A great volume might be written, describing the inhabitants of one of these beds of sea-weed. Almost all the leaves, excepting those that float on the surface, are so thickly incrusted with corallines as to be of a white colour. We find exquisitely delicate structures, some inhabited by simple hydra-like polypi, others by more organised kinds, and beautiful compound Ascidiæ. On the leaves also, various patelliform shells, Trochi, uncovered molluscs, and some bivalves are attached. Innumerable crustacea frequent every part of the plant. On shaking the great entangled roots, a pile of small fish, shells, cuttle-fish, crabs of all orders, sea-eggs, star-fish, beautiful Holuthuriæ, Planariæ, and crawling nereidous animals of a multitude of forms, all fall out together. Often as I recurred to a branch of the kelp, I never failed to discover animals of new and curious structure. In Chiloe, where the kelp does not thrive very well, the numerous shells, corallines, and crustacea are absent; but there yet remain a few of the Flustraceæ, and some compound Ascidiæ; the latter, however, are of different species from those in Terra del Fuego: we here see the fucus possessing a wider range than the animals which use it as an abode. I can only compare these great aquatic forests of the southern hemisphere, with the terrestrial ones in the intertropical regions.

"Yet if in any country a forest was destroyed, I do not believe nearly so many species of animals would perish as would here, from the destruction of the kelp. Amidst the leaves of this plant numerous species of fish live, which nowhere else could find food or shelter: with their destruction the many cormorants and other fishing birds, the otters, seals, and porpoises, would soon perish also; and lastly, the Fuegian savage, the miserable lord of this miserable land, would redouble his cannibal feast, decrease in numbers, and perhaps cease to exist."

I have alluded to the epiphytic plants which are so abundant in the tropics, and which add so greatly to the gorgeousness of the forests there. The most remarkable, or, at all events, the best known, of these are the Orchideæ, to which, as I have already had occasion more than once to speak of them, I shall do little more than refer here. These establish themselves in the forks, upon the greater limbs, and even in the roughnesses of the bark of the trunk, adhering by their long, interlaced roots, which look like knotted whip-cord, and forming their bunches of psuedo-bulbs, whence their succulent, thick, but elegant leaves project,--a great tuft of verdure; and their fantastic flower-scapes wave in the air or droop with their weight of gorgeous bloom. Thus they derive their nourishment from the humid atmosphere alone, being dependent on the friendly tree only for support and elevation. Humidity seems essential to the vigour of these and most other forms of parasitic vegetation. In the deep shady, gloomy forests of Java, which constitute the zone of vegetation around the base of the mountains, these plants abound, where the air is heavy and damp with the vapours that cannot ascend, and where the density of the foliage is almost frightful; where heat, moisture, and a most extraordinarily deep and rich vegetable soil combine to produce wood of a fungus-like softness, and an inconceivable abundance of twining plants and epiphytes. In those forests, more especially where huge fig-trees constitute the principal part of the timber, intermingled with the most tropical forms of vegetation, such as Sterculiaceæ, Sapindaceæ, and Artocarpeæ, tufts of Orchideæ attain a vast size and luxuriance, in company with Aroideous and Zinziberaceous plants. In Demerara, Mr Henchman found masses of Oncidium altissimum and Maxillaria Parkeri of wide dimensions, and so densely growing as to defy any attempt at intrusion; and on the Spanish main he saw the Epidendrum known as the "Spread Eagle" clasping enormous trees, and covering them from the top to the bottom.

The fig-trees, which are among the most gigantic of the tropical forest-trees, and which support an immense profusion of epiphytes, are themselves frequently parasitic and epiphyte in their early condition. It is not uncommon in Jamaica to see a network of roots partially embracing the trunk of some great tree, far up its column, and gradually creeping round and downward. I have seen an old wall so covered, presenting a very curious spectacle. The roots of a wide-spreading fig growing out of the summit of the wall, had spread over its perpendicular surface, down to the earth, all in the same plane, clinging to the wall; the chief roots were as thick as a man's leg, but subordinate roots had proceeded from one to another, anastomosing in all directions (if I may use such a term), so as to make a most elaborate network of a multitude of meshes of various angular forms and sizes. These cross-roots were at each extremity united with the larger roots, and looked as if the whole network had been skilfully carved out of one solid plank of wood, by cutting out the areas or meshes, and rounding the component bars; the very bark that covered the whole was continuous, where the roots united, as if they had been always integrally one.

The only mode in which I can account for this singular phenomenon is the following hypothesis:--The seed of the tree was originally deposited on the summit of the wall, beneath the eaves. As it germinated, the roots ran down towards the earth, some perpendicularly, some diagonally; but all creeping along the surface of the wall, no roots having shot out from its perpendicular. As these roots increased, they sent out side rootlets, which, still running on the face of the wall, by and by came in contact with another of the primary roots. Then, instead of creeping over it, as the roots of other trees would have done, the soft tip of the rootlet actually united with the substance of the root at the point of contact, the fibres of the two becoming interlaced, and their united surfaces gradually becoming covered with a common bark. The repetition of this process had produced the very curious wooden net which I have attempted to describe.

A still more remarkable example of this parasitic mode of growth I have seen in the same island. By the side of a mountain road was a large fig-tree, the base of whose trunk was about thirty feet from the ground. Thence it reared itself up pillar-like towards the heavens, and spread abroad its vast horizontal array of branches across the road. From the same point there descended to the earth a hollow cone of roots, interwoven and anastomosed, especially at the upper parts, in the same manner as those of the boiling-house wall, but forming towards the bottom only three or four flattened irregular columns. Into the area inclosed by this network of roots a person might enter, for it was about six feet wide, and, looking up, behold the base of the trunk eight or ten yards above his head.

The explanation of this curious phenomenon depends upon the tendency just mentioned. On this site once stood a large tree of some other species, probably a cotton-tree (Eriodendron), or some other soft-timbered kind. The little scarlet berry of a Fig-tree was carried by some vagrant Banana-bird or Pigeon to its boughs, and there devoured. After the little truant had finished his morsel, he perhaps wiped his beak against the rough bark of the trunk, beside the branch on which he was seated. Some of the minute seeds, enveloped in mucilage, were thus left on the tree, which the rain presently washed down into the broad concavity of the forks, where, among moss and rotten leaves, it soon germinated and grew. The roots gradually crept down the trunk of the supporting tree, closely clinging to its bark, and by their interlacement at length formed a living case, enveloping it on every side, and penetrating the earth around its base. The growth of these, and also of the inclosed tree, daily induced a tighter and tighter pressure on the latter, which at length arrived at such a degree as to stop the circulation of the sap between the bark and the wood. Death, of course, was the result, and speedy decay reduced the supporting tree to a heap of mouldering dust: while the parasite, now able to maintain its own position by its hollow cone of roots, increased in size and strength, and overtopped its fellows of the forest;--a tree standing upon stilts.

A few years ago I was struck with the appearance of an East Indian species of the same genus in one of the conservatories at Kew. Three shoots had run up the wall, clinging so close, that the leaves looked as if they were actually glued to the bricks, one over the other, in the most regular manner. Yet, on examination, I saw that the leaves did not adhere at all; the only support was that of the tiny rootlets which proceeded laterally from each stem, which the leaves concealed. The appearance of the whole was so curious, with the pale growing bud peeping out from beneath the topmost leaf, that I was greatly attracted by it. The base of the plant was in a pot, but the attendant informed me that this connexion was about to be cut off, by severing each shoot at the point where it first seized the wall. The leaves above this point, by their superior size and vigour, shewed that the plant was already independent of its pot, and that it was capable of supporting itself, like a proper air-plant, by imbibition from the atmosphere alone, needing nothing more than support in its upright position, which it obtained from the wall by its clinging aerial rootlets.

Every one who has wandered in a primeval forest of the tropics, whether in the eastern or the western hemisphere, has been struck by the inconceivable profusion of the climbers and twiners with which the trees are laced together. They are found from the thickness of a warship's cable to that of pack-thread; the stronger ones often uncouthly twisted together, and binding tree to tree. They are of the orders Malpighaceæ, Apocyaneæ, Asclepiadeæ, Bignoniaceæ, &c., and often are adorned with the most brilliant flowers.

I have before cited descriptions of these wonderful lianes, as they occur in the forests of South America; my readers may like to peruse Sir Emerson Tennent's graphic sketch of those of Ceylon:--

"It is the trees of older and loftier growth that exhibit the rank luxuriance of these wonderful epiphytes in the most striking manner. They are tormented by climbing plants of such extraordinary dimensions that many of them exceed in diameter the girth of a man; and these gigantic appendages are to be seen surmounting the tallest trees in the forest, grasping their stems in firm convolutions, and then flinging their monstrous tendrils over the larger limbs till they reach the top, whence they descend to the ground in huge festoons, and, after including another and another tree in their successive toils, they once more ascend to the summit, and wind the whole into a maze of living network as massy as if formed by the cable of a line-of-battle ship. When, by and by, the trees on which this singular fabric has become suspended give way under its weight, or sink by their own decay, the fallen trunk speedily disappears, while the convolutions of climbers continue to grow on, exhibiting one of the most marvellous and peculiar living mounds of confusion that it is possible to fancy. Frequently one of these creepers may be seen holding by one extremity the summit of a tall tree, and grasping with the other an object at some distance near the earth, between which it is strained as tight and straight as if hauled over a block. In all probability the young tendril had been originally fixed in this position by the wind, and retained in it till it had gained its maturity, where it has the appearance of having been artificially arranged as if to support a falling tree."

Leaving the vegetable world, we may find some very curious examples of parasitism among Insects. Every one who has paid the slightest attention to this class of animals is aware that there are slender flies called Ichneumons, whose grubs are hatched and reared in the bodies of other insects. Many of these have the ovipositor greatly lengthened, and projecting like a very slender needle from the extremity of the abdomen. In some species, this needle-like organ is three or four times the entire length of the body; and this great longitude is intended to reach the pupæ of wasps and similar insects which inhabit deep holes. The needle itself is well worthy of study. It is not simple, but composed of two pieces forming a sheath, which open and reveal a central finer filament, furnished at its tip (in Pimpla manifestator, for example) with saw-like teeth. With this instrument, which possesses great elasticity and flexibility, the insect works, as a carpenter with his brad-awl, boring through the clay, with which the wasp has closed up the hole that contains her grub, until the tip of the ovipositor reaches the soft body of the insect. Into this it pierces, and deposits an egg, and is withdrawn. The slight puncture is scarcely felt by the grub, which continues to eat and grow; the inserted egg, however, presently hatches, and produces the ichneumon-grub, which begins to feed on the fat of the wasp-grub, instinctively avoiding the vital parts, until the latter has attained nearly its full size, and is ready to pass into the pupa state; when, its vigour being gone, it fails to accomplish the metamorphosis, the insidious intruder, now also full grown, taking its place, and by and by issuing from the hole a perfect Ichneumon.

How often has the enthusiastic young entomologist been subjected to sore disappointment by the parasitic habits of these Ichneumonidæ! He has obtained some fine caterpillar, a great rarity, and by dint of much searching of his Westwood or his Stainton, feels quite certain that it is the larva of some much-prized butterfly. He ascertains its leaf-food; which it eats promisingly; all goes on encouragingly. Surely it cannot be far from the pupa state now! When some morning he is horrified to behold, instead of the chrysalis, a host of filthy little grubs eating their way out of the skin of his beautiful caterpillar, or covering its remains with their tiny yellow cocoons.

Some of these parasites are so minute that their young are hatched and reared in the eggs of other insects. Bonnet found that the egg of a butterfly, itself no bigger than the head of a minikin pin, was inhabited by several of the stranger grubs; for out of twenty such eggs, he says, "a prodigious quantity" of the grubs were evolved.

A very interesting tribe of insects, so diverse from all other known forms as to constitute an order among themselves, that of the Strepsiptera, passes its youth in the bodies of certain wild bees. Mr Kirby's account of his first detection of one of these, though often quoted, is so interesting that I must cite it afresh. "I had previously observed," he remarks, "upon bees something that I took to be a kind of mite (Acarus), which appeared to be immovably fixed just at the inosculations of the dorsal segments of the abdomen. At length, finding three or four upon an Andræna nigroænea, I determined not to lose the opportunity of taking one off to examine and describe; but what was my astonishment when, upon my attempting to disengage it with a pin, I drew forth from the body of the bee a white fleshy larva, a quarter of an inch in length, the head of which I had mistaken for an acarus (bee louse)! After I had examined one specimen, I attempted to extract a second; and the reader may imagine how greatly my astonishment was increased, when, after I had drawn it out but a little way, I saw its skin burst, and a head as black as ink, with large staring eyes and antennæ, consisting of two branches, break forth, and move itself quickly from side to side. It looked like a little imp of darkness just emerged from the infernal regions. My eagerness to set free from its confinement this extraordinary animal may be easily conjectured. Indeed, I was impatient to become better acquainted with so singular a creature. When it was completely disengaged, and I had secured it from making its escape, I set myself to examine it as accurately as possible; and I found, after a careful inquiry, that I had got a nondescript, whose very class seemed dubious."

Mr Newman, in an essay of much value, has shewn that the larvæ of this tribe of insects are born alive, that they attach themselves to the abdomens of wild bees, nestling among the hair, and that they are thus introduced into the nest of the bee. Here it is somewhat uncertain how they are sustained at first, for at this time the bee-grubs are not hatched; probably they remain without food for some days, or devour a portion of the pollen and honey stored up. As soon, however, as the bee-grub is hatched, the Stylops-larva undergoes a metamorphosis, sheds its six legs, and becomes a footless maggot; it pierces the soft skin of the bee-grub, and feeds on its juices, till its maturity, as the Ichneumon on the body of the caterpillar.

When the perfect bee emerges in the following spring, it bears the full-grown Stylops, protruding from the rings of its abdomen. The latter is in pupa, all the organs being distinct and separate, but wrapped together, and inclosed in separate pellicles; very soon, it emerges, as described by Kirby, and escapes, leaving a great unsightly cavity in the body of the bee. This is the male: the female never escapes, but lays its eggs on the bee in which it has been reared, and then dies.

In the spring we frequently see among herbage a great uncouth beetle of a dark blue-black hue, with short wing-cases and long, heavy body, which discharges drops of yellow fluid when handled, and is therefore called the Oil-beetle (Melöe proscarabæus). The early stages of this beetle have much affinity with those of the Stylops. The beetle lays a number of yellow eggs in a hole in the earth; these produce little active six-footed larvæ, resembling lice, which crawl to the summit of dandelion and other flowers in the sunshine, and await the visit of a bee. On the arrival of one, the active grub immediately clings to its body, and is carried to the nest, not, however, to introduce itself parasitically into the body of the bee-grub, but to feed on the provision which the parent bee has stored up for its own young. Thus it becomes very fat, and grows to a size much larger than that of the full-grown bee-grub, having early dropped its six long clinging legs, which, having performed their proper function in catching hold of a bee, are no longer needed. It changes to a perfect beetle in autumn, lies in the bee's nest all the winter, and emerges in the spring.