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."[229]
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,[230] 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.