Timber both in its growing and converted states is subject to the attacks of worms and insects; when these exist in large numbers they remove so much of the wood as seriously to impair the strength of any structure depending upon the timber, and in some cases they destroy the balks altogether.
It will not be necessary to describe these worms and insects in detail. But a brief notice of a few of the most important, gleaned chiefly from the works of Tredgold and Britton, may be useful.
The Teredo navalis is the most common enemy to timber used in submarine work.
It is found in warm and cold climates, and in nearly every English port. It avoids fresh water and prefers clear water to that which is muddy. This is one reason why wood placed at the mouth of a river, or in turbid water, is not so liable to be attacked as when it is in clear salt water.
The Teredo is first deposited upon the timber in the shape of an egg, from which in time emerges a small worm; this worm soon becomes larger, and commences its depredations.
Furnished with a shelly substance in its head, shaped like an auger, it bores into the wood, chiefly with the grain; at the same time it lines the hole it makes with a thin coating of carbonate of lime, and closes the opening with two small lids.
As the work of the Teredo advances its size increases. Worms two feet long and 3/4 inch in diameter have been found at Sheerness, and even larger ones are stated to exist.
The Teredo penetrates nearly all kinds of timber, but is most successful in fir.
The general opinion seems to be that the boring is mechanical, but some authorities think that it is done or assisted chemically by the aid of an acid secretion.
The Xylophaga dorsalis is of the same family as the Teredo, not so common, but more destructive; it bores in all directions (some say only with the grain), and does not line its hole with shell.
The Limnoria terebrans is a marine insect, resembling in appearance a very small woodlouse.
It is very abundant in British (salt) waters, and makes up for its diminutive size by the numbers in which it attacks timber : "as many as twenty thousand will appear on the surface of a piece of pile only 12 inches square." 2
Mr. Stevenson found that Memel timber was destroyed by the Limnoria at the Bell Rock at the rate of about 1 inch inwards per annum. At Lowestoft, piles were eaten at the rate of 3 inches inwards per annum.
This insect prefers soft woods, avoids knots, but will attack all woods except teak and greenheart.
"The Limnoria almost always works just under neap tides. It cannot live in fresh water (or under the sand), and whilst it is destroying the surface of a pile, the Teredo is attacking the interior."
The Tanais vittatus, a species of the same family as the Limnoria, in appearance like a very small caterpillar with enormous foreclaws, was found by Mr. Hurst in beech piles.
The Chelura terebrans, or wood-boring shrimp, is also an inhabitant of British seas. It tunnels close below the surface of timber, the waves wash away the thin covering of the tunnel, and then the shrimp drives another below, so that the timber is removed in successive flakes.
"The Limnoria will exist in comparatively foul water if salt, but the Chelura must have sea water comparatively pure, hence the former is most frequently found in harbours and the latter along the sea coast.
"The Limnoria and Chdura terebrans do not attack wood more than a few inches above high water or neap tides."1
The Lycoris fucata is the enemy of the Teredo.
A little worm with legs, something like a centipede, it lives in the mud, crawls up the pile inhabited by the Teredo, enters the tunnel in which it is ensconced, eats the Teredo, enlarges the entrance to the tunnel, and then lives in it.
A great many different plans have been tried in order to protect timber in marine works from the ravages of worms.
Broad-head scupper nails driven in close together rust into a mass, and so form a good protection, but the process is expensive.
Creosoting by Bethell's process when properly carried out is quite successful (see p. 394) but no other chemical process answers.