The Encyclopedia Americana (1920)/Land Crabs
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LAND CRABS. Almost all shore-crabs will withstand exposure to the air for some hours without suffering injury, and many of them are regularly so exposed at ebb tide. It is not surprising, then, that some of them have wandered far from the seashore into the fresh-water streams and the moist woods and mountain forests, or even largely forsaken the vicinity of water and established themselves on dry hillsides. The land crabs, par excellence, belong to the family Gecarcinidæ, in which the carapace is thick and hard, strongly arched above, rounded, bent downward and truncate in front, and vaulted in the branchial region. The branchial chamber is spacious and lined by a spongy membrane which retains moisture for a long time and thus keeps the gills damp. Six or seven genera and several species inhabit the warmer regions of both hemispheres. One species (Gecorcinus ruriola) is common on many of the West Indian islands, is excessively abundant on some of the Bahamas and occurs in southern Florida. They abound at the eastern end of Cuba, and were a source of immense annoyance to the inexperienced soldiers of the United States army in the campaign of 1898 against Santiago de Cuba. On Jamaica and the other larger islands they inhabit a zone in the hills from one to three or four miles from the shore, where they live in burrows beneath stones or under the roots of trees, from which they emerge at night to seek their chiefly vegetable food. They are large, robust creatures with extremely powerful claws, which a pugnacious disposition leads them to use freely. Mating takes place in the spring, and during the rainy season in May a common impulse causes them to migrate in vast armies to the sea, where they bathe and deposit their eggs, which are washed up and buried in the sand by the waves. By resident naturalists and travelers who have observed it, this animal's march to the sea is described as a most impressive sight. Most of the males form an advance guard which is followed in two or three days by the females and remaining males. They are said to proceed in a direct line and to climb over, instead of passing around, every obstacle, even houses. After discharging the eggs, bathing and resting, the crabs return to the hills and in midsummer close their burrows with leaves, grass, etc., and retire to their inmost recesses to moult. At this time, while in the soft-shelled state, they are in high favor for the table, and are dug out of their burrows in large numbers for market. The crabs of this family possess well-developed gills, but in addition the gill-chambers are modified for air-breathing, the lining being thick and well supplied with blood-vessels, and divided by a fold of membrane so as to leave a sort of pocket in the upper part. The Brazilian mangrove-crab shows this especially well.
On Christmas Island, in the Indian Ocean, the soil of the high lands is in some places honeycombed with the burrows of a land-crab (Geocarcoidea calandi) that feeds on leaves which are dragged into the burrows. “From their enormous numbers,” says a student of them, “they must play a great part in the destruction of decaying vegetable matter, and its incorporation into the soil.” Once a year, in the rainy season, they migrate to the sea for breeding purposes.
Another family of crabs, the land-hermit crabs Cœnobitidæ), are found on all tropical coasts, and spend most of their lives inland, often climbing bushes and trees. To this family belong the large cocoanut, or robber, crabs of tropical islands.
Even more perfectly adapted to terrestrial life are the many species of land isopods called wood-lice (q.v.); and many of the river-crabs (Thelphusidæ), and of the crayfish, are more terrestrial than aquatic in habit. A well-known species of Thelphusa in Japan is sometimes met with on mountains at an elevation of 2,000 feet or more above the sea-shore. Consult Kingsley, ‘Standard Natural History’ (Vol. II); Proceedings of the United States National Museum (1899); Calman, ‘Life of Crustacea’ (New York 1911).