One of the most remarkable cases of commensalism in nature has been found by Dr. Melbourne Ward, Australian zoologist in a degenerate type of barnacle which makes its way through the thin shell of one of the Barrier Reef crabs. It wanders through the blood stream of the crab and finally comes to the surface where it forms a little sac for itself. Here it metamorphoses into another form and sends long, thread-like filaments into every part of its host’s body. In some respects it is like a cancer among higher animals, except that in this case the malignant growth is that of an individual animal of another species. It lives off the food eaten by the crab but never kills nor apparently seriously injures its host. The one notable effect, for which there is no adequate explanation, is that it changes a male crab into a female.
The soldier crabs are beachdwellers, about two inches long. They march across the hard sand in perfect order, as if they were under the control of leaders. No “officers”, however, have been observed. When approached, they burrow rapidly in waves, like a regiment of infantry. First the front rank disappears in the sand, followed in order by those behind. The regiment disappears completely in a very short time.
The soldier crabs can hardly be driven into the water. When Dr. Ward succeeded in pushing a few of them off the shore they were set upon by ferocious small fish which rapidly devoured them. Realization of this danger apparently is instinctive in the animals.
Some of the land-dwelling crabs of the mud flats dig very intricate burrows with labyrinthine cross and side galleries. Some species live in a communal life. Each crab has its own burrow, but from each there is a passage into a large central hall which seems to be a community gathering place. Other species are intensely individualistic. Each excavates an elaborate labyrinth in the mud, considers this its own home, and vigorously defends it.
During courtship some of these mud crabs perform dances like the courtship dances of birds. The male of one variety, after attracting a mate by his dancing, picks her up bodily in one of his nippers and carries her away. Another variety of sand crab seems to have perfected an engineering technique which still evades human skill—that of building a burrow in soft, dry sand. These burrows are about two inches in diameter. The crab is able in some mysterious fashion to compress the soft sand into a solid substance with its nippers.
In precision of instinctive behavior, Dr. Ward found, these Great Barrier crabs come quite close to the spiders, their distant relatives.
Ticks With Noses in Their Legs
Ticks, remote spider relatives, smell with their front legs. When these legs are amputated the tick shows no reaction to odors. It cannot smell blood but will feed on any sort of liquid sucked through a warm, moist membrane like the skin. Presumably such a tick in nature recognizes an animal as a proper source of food by smell, while a combination of warmth and moisture from the skin gives a stimulus for feeding.
The Fourth Realm of Life
There is a wind-tossed green-grey ocean between earth and sky. It is a sea on stilts, the world’s fourth realm of life. There are plants and animals of the land, of the water, and of the air—and there are plants and animals of the canopy of the rain forest, a thousand-mile-wide broken belt around the world. It covers several million square miles—the jungles of South America extending northward into southern Mexico, the basins of the Niger and the Congo, strips of southern India and Ceylon, much of New Guinea. Life is rather sparse in the perpetual, drenched twilight of the jungle floor. It is abundant in the treetops, the habitat of fantastic, and still largely unknown, plants, mammals, birds, snakes, toads, frogs and insects. These might be compared to the flora and fauna of an as yet unexplored continent.