THE LIMNORIA (Limnoria lignorum). This isopod crusta cean, which has other names as the wood flea, sand flea, gribble, and boring gribble, is the prin cipal one of several s im il a r forms that attack woods when in sea-water. The Limnoria is much smaller than the shipworm, but it usually occurs in larger numbers and in some localities is almost equally destructive.
The Limnoria is found along the Atlantic Coast from Nova Scotia to Florida. It exists sparingly in Long Island Sound; but is abundant along the Coast of Massachusetts, and very destruc tive in the Bay of Fundy. It is also active in the north Pacific, as in Puget Sound and the Straits of San Juan de Fuca. It is said to exist upon the coast of Great Britain and in other European waters.
The body of the Limnoria is grayish in color, and sometimes resembles the color of the wet wood so much that it is difficult to distinguish it. The Limnoria can swim, creep backward and forward, as well as jump backward by means of its tail. When touched, it rolls itself into a ball, and in this particular, as well as in general appearance, it resembles the common sow-bug.
The Limnoria differs from the shipworm in that quite certainly it is a vegetarian. The shipworm is sustained, at least for the most part, by microscopic life drawn from the sea water, but the Lim noria devours wood. Its tunnel affords both food and shelter.
Limnoria are plentiful in some regions in the North where shipworms can exist but spar ingly because of the cold. Limnoria require pure sea water and are seldom found in the comparatively fresh waters encountered near the rivers.
Character of Excavation.The Limnoria attacks the wood by means of its mandibles or jaws. It prefers wet wood and succeeds in making a very clean-cut excavation.
The work of the Limnoria differs from that of a shipworm in that its tunnels terminate on the surface of the wood where they can be plainly seen, whereas those of the shipworm are for the most part concealed within the wood. The body of the shipworm cannot emerge from the wood within which it has located, while that of the Limnoria can pass freely in and out. The Limnoria frequently works in conjunction with the shipworm. It attacks the surface, while the shipworm takes away from the interior of the woodwork.
The numberless, smooth, clean-cut galleries are close together and the partitions that separate them are so thin that they can not long resist the action of the waves. Later, the partitions are either washed away by the waves, or they decay. Fresh sur faces are then exposed and these are destroyed in the same man ner. Layer after layer is removed until the timber is destroyed. The Limnoria can penetrate knots, but sometimes avoids them, when such hard portions stand out in relief as the other parts are (lest royed.
The Limnoria is very small, but notwith standing this fact, it is very destructive. The multitude of these woodborers compensates for their size. Each may be assumed to be from one-sixth to one-fourth inch in length and about one-sixteenth of an inch in diameter. The tunnels are about one-half of an inch in length and about one-tenth of an inch in diameter.
The Limnoria does not work as rapidly as the shipworm. The number of individual workers must in this case be taken as a measure of the rapidity of destruction. The number of tunnels is more important than their depth. The thickness of a piece of timber may be reduced from one-fourth of an inch to as much as an inch in a year. Much wood used in marine constructions is in the form of piles that are necessarily exposed on all sides. The effective diameters of such pieces are, therefore, reduced twice as rapidly as indicated by the figures noted.
The depredations of Limnoria are confined to a limited distance above and below the low-water mark. Where the variations of the tides are extensive, as in the vicinity of the Bay of Fundy, the range of the Limnoria is correspondingly great. It has been found, although rarely, at a depth of forty feet.