insect-jaws is very great, as many people have the misfortune to know. From before the time of Pharaoh, the biting insects have been a scourge to the farmer. Their voracity is awful; and, when urged by hunger, few substances can withstand their jaws. In countless hosts they ravage a country, and blast it as with the curse of Jehovah. It is stated that some caterpillars eat three to four times their weight of food every day. That beasts like the elephant and tiger are not voracious in proportion to their size is matter for congratulation. Fortunately, however, such ravenous insects eat only in the larval state; for most moths and butterflies exist on love, and none take any more substantial nourishment than the honey of flowers.
The common fly has minute teeth. Many grubs live within trunks of trees, gnawing immense galleries, and subsisting on the wood. The termites, or "white ants," devour whole houses, leaving only a shell fair to all external appearance, but crumbling at a touch. "At Tonnay-Charente the termites, having gnawed away the props of a dining-room without its being perceived, the flooring collapsed during a party, and the entertainer and his guests sank through." The larva of the giant sirex gnaws burrows in lead.
Teeth of mollusks and articulates are usually horny—that is, hardened skin, like the crust of the lobster and beetle. Sometimes they
Fig. 6.—Narwhal, or Sea-unicorn.
are calcareous and sometimes siliceous. Those of the vertebrates are complex in structure and substance, and are regarded as the only true teeth. They consist of dentine or ivory, of which there are several kinds. In mammals and the highest reptiles, the dentine is surrounded by a sort of bone called cement; and the surface exposed to wear is capped or otherwise protected by enamel, the hardest of animal tissues. Teeth do not belong to the bony skeleton, but are developed by the lining membrane of the mouth, which, like the lining of the whole food-canal, is only a continuation of the skin. Hence teeth are classed with other skin appendages, as the nails and hair.
The teeth of fishes are extremely various in number, form, struc-
