228 



THE POPULAR SCIENCE MONTHLY. 



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. For- 

 tunately, 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 Ton- 

 nay-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, hard- 

 ened 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- 



