Part 12

These crinoids live, for the most part, on diatoms, small crustaceans, and other tiny sea creatures which they first paralyze with poison from the tentacles which line the grooves of the arms through which food is carried to the mouth.

Cave-Dwelling Birds

True creature of night is the guacharo, or “oil bird”, of northern South America. It is reddish-brown, about the size of a barnyard hen. Excessive layers of fat built up about its abdomen formerly were valued highly by natives for eating purposes, resulting in the slaughter of countless thousands every year. The guacharo spends its days a half mile or more deep in the interior of mountain caves. Here it roosts and builds its nests in crevices high in the rock walls. It leaves in groups of twenty to thirty shortly after dusk and apparently spends the whole night foraging for food, sometimes covering as much as 200 miles.

Like the cave bat, it seems to have no difficulty finding its way in absolute darkness. An explanation of this ability, acoustic orientation, has been reported by Dr. Donald R. Griffin of Cornell University. The birds apparently are guided by echos of specific sharp “clicking” sounds which they make.

“The individual click,” Dr. Griffin explains, “consists of a very few sound waves having a frequency of about 7,000 cycles per second. The duration of each click is about a millisecond (1,000th of a second). The clicks were loud enough to be audible easily about 200 yards inside the cave. Except for their lower frequency, these sounds are very similar to those used by insectivorous bats for their acoustic orientation.

“The external ear canals of three captive birds were plugged with cotton. They then became disoriented when flying in the dark. They collided with every object they encountered. Before and immediately after this treatment they flew about in a small dark room avoiding all collisions with the walls.”

Their best known habitat is the guacharo cave in Venezuela’s Humboldt National Park, where they are rigidly protected. Most of them nest in a vast subterranean hall more than a half mile long and a hundred feet high. Here more than a thousand of the birds greet the intruder instantly with a wave of awesome and deafening shrieks.

“With the advent of dusk,” reports Dr. Eugenio de Bellard Pietri—Venezuelan cave explorer, “the birds come out in compact groups but before the exodus a preliminary flight is held by a few as if to make sure that night is falling. Soon they return to the depths of their somber mansion, evidently to give the flock the all clear signal. Late in the evening there is not a single adult specimen left in the cave. The flight of these birds is silent and cannot easily be detected.”

Where Snails Become Flowers

The lowly snail reaches an apotheosis—rivalling flowers and butterflies as an expression of nature’s artistry—in Cuban forests. Delicate sunrise tints of pink, blue, violet, green and yellow make the shells of two or three genera of tree-dwelling mollusks like rare jewels. Most conspicuous are snails of the genus Polymita, confined to the Oriente province. Here they cover some trees so completely that the effect is like that of a tree of flowers. Only upon close observation can one detect that the blossoms are shells.

The animals live for the most part on a fungus that grows on the bark. The colors of the shells are affected by various chemical constituents of the bark, notably tannic acid, and serve as warning to other creatures. In taste the snails are very bitter and no bird will intentionally attack them. The color serves notice that only a disgusting mouthful is to be had.

Two of the most beautiful of these shell forms were recently discovered by Dr. Paul Bartsch, former Smithsonian curator of mollusks. Fragile, translucent, colored as delicately as the loveliest of orchids, these particular snails are the fairies of the mollusk world in the unconscious artistry with which they have constructed their moving palaces. One, a hitherto unknown species, has a remarkable combination of pale orange, orange buff, deeper orange and flame color—all shading delicately into each other. The color effect is such as one might find rarely in rose petals. Another has a blending of ivory, olive green, lemon yellow and orange.

Termites That Eat Lead

On Barro Colorado island in the Panama Canal Zone the Smithsonian Institution maintains an “experimental cemetery.” It consists of rows of upright posts which look like gravestones, half buried in the soil. The purpose is to test the propensities of the island’s 42 species of termites—just about man’s most persistent and expensive enemy in the tropics—to eat different kinds of wood impregnated with different kinds of repellants and poisons. To date approximately 35,000 tests have been made. The longer the work is continued the more Dr. James Zetek, former director of the station, is impressed with the contrariness and ingenuity of the blind, ant-like insects which achieve sub-human acmes of engineering ability, and whose appetites are marvelous.

Among Barro Colorado’s termites are some extraordinary bugs indeed. One, for example, eats lead. It gnaws its way through the lead sheathings on cables. This is not because it likes a lead diet. Lead, in fact, is indigestible and the insects starve to death. But their appetites are so insatiable that the little creatures just keep on gnawing, in the hope that there will be wood on the other side.

This particular insect is known by the scientific name of coptotermes niger. It has been known to eat through a concrete floor nearly five inches thick—again not because of any particular liking for concrete but because of the expectation of coming eventually to digestible wood. The feat was made possible because the sand used in making the concrete contained many fragments of sea shells which were dissolved by a powerful chemical excreted by the insects.

It is very difficult to dispose of termites by poison—that is, permanently. Races have risen here, for example, which seem to thrive on arsenic. The insect lives on the cellulose in wood. This must be digested by certain intestinal bacteria in the digestive tract. If these microörganisms can be poisoned the termite starves. At first at least 99 percent of the bacteria succumb to heavy doses of arsenic. This means that 99 percent of the termites are killed. But always there are a few exceptionally tough bacteria with a high resistance to the poison. Their descendants in a few generations apparently become almost entirely resistant. With their help a new race of termites comes into existence.

Ordinarily termites attack only dead or dying wood. Some of them, however, carry fungi around with them to kill their own wood. The Canal Zone insects can dispose of living trees. Dr. Zetek tells of one attempt to establish an avocado plantation. He warned against it. When the trees had reached the fruit-bearing stage and seemed healthy he was ridiculed for his warnings. Branches were heavy with avocados and there was promise of a record crop. He shook his head when shown the flourishing orchard. “The poor trees,” he remarked. “They know they are going to die. They are just making one last mighty effort to preserve their species by producing plenty of fruit and seeds.” He secured the orchard owner’s permission to chop down one tree. The whole inside, he found, was riddled with termite galleries. This tree and all the others in the orchard were dead within a year.

The Plant That Eats Animals

There are life-and-death battles in the microscopic world between tiny shelled animals and flesh-devouring fungi. The phenomenon can be compared to that of a tree catching and eating big turtles.

When a culture of diseased plant roots is made, there soon appear great numbers of microscopic plants and animals—bacteria, fungi, amoebae, nematodes and other life forms. Immediately the struggle for survival starts. The animals try to eat the plants and the plants attempt to devour the animals.

Among the animal forms which appear are vast numbers of creatures known as rhizopods. Practically unknown except to specialists, these microscopic creatures play an important part in the economy of life. They are probably the best-equipped of all the new arrivals to survive, since their soft bodies are covered with relatively heavy shells.

Some years ago Dr. Charles Dreschler of the U.S. Department of Agriculture reported the existence of predaceous meat-eating fungi—parasitic forms of plant life—which literally lassoed such unprotected animals as amoebae and thread-like nematodes and proceeded to devour them at leisure by the process of infiltrating their bodies. It would appear that the armored rhizopods are completely protected from these ferocious plants.

But the animal has one weak spot in its defense. It must get its mouth outside its shell in order to eat. Apparently the most inviting forage at hand is the innocent-appearing fungus. The rhizopod proceeds to suck at it with movements which Dr. Dreschler describes as similar to “sucking an egg.”

The rhizopod mouth is small. Once it has sucked in any of the fungus its fate is sealed, for, explains Dr. Dreschler, “to such undiscriminating voracity the fungus responds by rapidly proliferating from the partly ingested portion a bulbous outgrowth slightly larger than the mouth, so that the rhizopod is held securely.”

The unfortunate shelled animal is like a fish caught on a hook. It struggles vainly to get away. It rushes, but the fungus simply lets out the line until the rhizopod is brought to an abrupt stop and can be hauled in. The line is a filament connecting the body of the fungus with the bulb in the animal’s mouth.

Once its prey is secure, the fungus proceeds to send out growths from the bulb through the creature’s flesh, literally eating it alive. Very rarely, like a hooked fish, a rhizopod is able to break away.

In the course of its life, a single one of these thread-like fungi will capture many of the shelled animals, lining them up securely mouth-to-mouth on both sides of itself. It absorbs their substance at its leisure. Other predaceous fungi have definite external organs for capturing their prey. This particular species, however, has no external appendages and appears completely inert and innocent until it is stimulated to action by the sucking of the rhizopod.